Symposium SF13—From Actuators and Energy Harvesting Storage Systems to Living Machines

This talk will discuss recent progress in utilizing gallium-based liquid metals for energy harvesting and actuation. The promise of soft devices—wearable electronics, implantables, soft robotics, e-skins, sensors—has accelerated the demand for soft, flexible, and stretchable energy sources and actuators. This talk will discuss a completely soft and stretchable (> 400% strain) energy harvester based on variable-area electrical double layer capacitors. Electrical double layers form spontaneously at the interface between liquid metal electrodes and hydrogel encapsulants. They also have large capacitance (~40 μF cm^-2). Mechanically varying the interfacial contact area, and thus the capacitance, disrupts equilibrium and generates a driving force for charge movement through an external circuit. Because the design is entirely soft, it is possible to vary the area of the electrode itself. This new approach is attractive because the entirely soft device can work under water and can use various modes of mechanical input such as pressing, stretching, bending, and twisting. In addition, the talk will discuss the use of liquid metal as electrodes for more established modes of energy harvesting, including thermal (thermoelectric), mechanical (tribocharging), and electromagnetic. Finally, the talk will discuss the use of liquid metals for actuation via the use of interfacial tension. Liquid metals have the largest surface tension of any liquid at room temperature. The interfacial tension can be modulated significantly using electrochemical reactions. These changes in tension can generate forces to create actuators, muscles, and jumping droplets. This work is interesting because it provides a route to actuate devices using entirely soft materials that are controlled electrically.

Millimeter/centimeter-scale origami robots have recently been explored for biomedical applications due to their inherent shape-morphing capability. However, they mainly rely on passive or/and irreversible deformation that significantly hinders the clinic functions in an on-demand manner. Here, we report magnetically actuated crawling and swimming origami robots for effective locomotion and targeted drug delivery in severely confined spaces and aqueous environments. We design our robots based on the Kresling origami, whose thin shell structure 1) provides an internal cavity for drug storage, 2) permits torsion-induced contraction as the mechanism for crawling and controllable drug release, 3) serves as propellers that spin for propulsion to swim, 4) offers anisotropic stiffness to overcome the large resistance from the severely confined spaces in biomedical environments. These magnetic origami robots can potentially serve as minimally invasive devices for biomedical diagnoses and treatments.

Programmable shape-shifting materials can take different physical forms to achieve multifunctionality in a dynamic and controllable manner. By pre-programming and reprogramming molecular anisotropy in liquid crystal elastomers (LCEs) and their nanocomposites in the forms of films, fibers, and microparticles, we show shape morphing from 2D to 3D with combination of curvatures. By incorporating 1D and 2D nanomaterials (e.g. cellulose nanocrystals, carbon nanotubes, graphene and gold nanorods) in LCEs, we demonstrate tendon-like actuators and reprogrammable shape transformation in response to heat, light, magnetic field, electric field and pneumatic actuations for soft robots, 3D displays and thermal energy harvesting.

The biaxial strain produced by relaxor ferroelectric single crystals has has been used extensively to explore the control of magnetism at micron length scales, resulting in many novel applications of the magnetoelectric composite effect. Relaxor ferroelectric single crystals of ternary PIN-PMN-PT undergo electric field and stress driven phase transformations from a rhombohedral phase to an orthorhombic phase when cut, poled, and electrically loaded in the [011]c direction and mechanically loaded in the [001]c direction. At smaller loading levels there is a linear piezoelectric response. At higher loading levels, above the transformation threshold field, there is a considerably larger strain. The threshold field for the phase transformation is a function of composition and temperature. Experimental results of the composition and temperature dependence of the phase transformation threshold are presented. Over the past several years researchers have explored the effect of this strain field on various 2-D shapes with in-plane magnetic anisotropy including rectangular, elliptical, hollow, and full; and recently the effect of this biaxial strain on perpendicular magnetic anisotropy. Multiple examples of controlling magnetism at these length scales are presented. Controlling magnetism at the micron length scale without use of coils enables applications such as capture and release of individual superparamagnetic particles, one of the first steps in a new approach to cell sorting.

As the application fields of robots become increasingly complex and diverse, soft robotics has attracted a great deal of interest. Although conventional robots made of rigid materials have significantly improved productivity in structured environments (e.g. in industry), they are difficult to operate in unstructured environments because they consist of multiple links and joints. In addition, they are often heavy, tethered, and different in elastic modulus compared to the tissue of living organisms. In order to precisely and locally control motion while having mechanical properties similar to those of living things, many attempts are being made to use soft materials for robotics. Functional soft materials such as stimuli-responsive hydrogels, shape memory polymers, and liquid crystal elastomers have been viewed as potential candidate materials for soft robotics. Hydrogels bring several advantages due to their high water content and large elasticity. They can respond to various external stimuli via interactions between the polymer network and water, which can endow soft robots with controlled actuation. Moreover, hydrogels have biocompatibility if their polymer networks consist of non-toxic polymers, which allow soft robots to broaden into more biological applications. Despite these advantages, hydrogels have challenges in terms of mechanical strength, response time, and responsive area control. To overcome this intrinsic problem, a well-designed hydrogel actuator system with sufficiently strong mechanical properties and selective responsive sites is envisioned. In this study, we introduced a photothermal-driven hydrogel actuator system with enhanced mechanical properties while accomplishing a precise control of response areas. For this, we used poly(N-isopropylacrylamide) (PNIPAM) as a temperature-responsive polymer network and gold nanoparticle (AuNP) as a transducer from light to heat, bacterial cellulose nanofibrils (BCNF) as a supporting material for AuNPs and an interpenetrating polymer for hydrogel. The essence of our approach is to provide BCNFs with the ability to support gold nanoparticles while boosting the mechanical properties of hydrogel. Specifically, BCNFs are physically immobilized in a covalently bonded hydrogel mesh of PNIPAM, thus producing a hydrogel composite with a semi-interpenetrating network structure. This PNIPAM/BCNF hydrogel, which acts as an active layer, is combined with a non-thermoresponsive poly(acrylamide) passive layer to generate a thermal-expansion coefficient mismatch among the interface, eventually leading a bending motion. As an energy transducer, the AuNPs embedded in the BCNF absorb the light energy and transform it into thermal energy effectively as a result of a photothermal effect. The resulting hydrogel actuators exhibit enhanced mechanical properties. The actuation of bi-layered hydrogel could be easily programmed by local irradiation, allowing finger-like one-by-one bending motions. These results highlight that our hydrogel actuator system can have significant potential for the development of smart soft robots.

Dielectric elastomer actuators are attracting attention because they can be used for artificial muscles and soft robotics. However, they still remain challenge to demonstrate long-term stable actuators and high movement actuators because most soft materials constituting devices are vulnerable to under external damage during repeated operation and unrecoverable after being subjected to damage. To solve these problems, conductors was given self-healing capability. Some reports suggested technique for self-healing ionic gel but they resulted toxicity and complex fabrications caused by doped electrolytes, ions and macromolecular in order to increase ion conductivity and self-healing properties. In our study, we used non-toxic and bio-compatible gelatin, tannic acid and deep eutectic solvent. Our gelatin-tannic acid gels have good mechanical property and self-healing efficiency, and these gels satisfied the ink condition suitable for direct printing during a programmed electrohydrodynamic printing process. As a result, it was possible to desired patterns of gelatin conductors were successfully obtain easily, which strain sensors and electrode for dielectric elastomer actuators could be easily fabricated with desired patterns. Dielectric elastomer actuators are fabricated in this method exhibit high movements and self-healing efficiency of ~26.3% and ~ 96.8%.

The integration of functions in materials to gain macroscopic effects in response to environmental changes is an ongoing challenge in material science. When functions on different hierarchical levels are sequentially linked by combining a suitable polymer morphology in combination with a proper microstructuring of the macroscopic device, the processes occurring on the molecular level can be translated to the macroscopic level to induce directed movements in hydrogels.
Here we describe microporous hydrogel systems based on covalently cross-linked polymer networks, which are capable to respond to the change of pH, the presence of enzymes or a decrease of temperature with a macroscopic shape shift. The micro-porosity permitted swelling into the pores and thus reduced volumetric changes on a macroscopic level, which otherwise could interfere with the shape shift. The translation of the presence of protons on the molecular level to the macro level was achieved by implementing lysine-rich peptide molecules, which change their conformation into a b-hairpin structure due to the reduced electrostatic repulsion amongst deprotonated amino groups when the pH is increased.^[1] Coupled to this conformation change is the capability of the b-hairpin motifs to subsequently assemble into aggregates acting as reversible crosslinks, which are used as controlling units to fix a temporary macroscopic shape. When such controlling units are equipped with enzyme-specific cleavable bonds, a transformed shape is achieved by the release of internal stresses because of the enzymatic reaction.^[2] In this case the increase in entropy goes along with a swelling-supported stretching of polymer chains within the microarchitectured hydrogel. Vice versa, the directed formation of micelles in hydrogels based on covalently crosslinked oligo(ethylene glycol)-oligo(propylene glycol)-oligo(ethylene glycol) (OEG-OPG-OEG) segments upon heating is capable to fix a deformation. Upon cooling, the micelles dissociate again, the deformation is reversed and the permanent shape is obtained. In this way an inverse shape-memory effect (iSME) can be obtained.
In this presentation the challenges in the coupling of function are discussed and examples are provided for potential application of such multifunctional hydrogels ranging from sensors to scaffolds for tissue reconstruction.
References
Zewang You, Marc Behl, Stephan L. Grage, Jochen Bürck, Qian Zhao, Anne S. Ulrich, and Andreas Lendlein Biomacromolecules 2020 21 (2), 680-687.
Maria Balk, Marc Behl, Ulrich Nöchel, and Andreas Lendlein ACS Applied Materials & Interfaces 2021 13 (7), 8095-8101.
Lucile Tartivel, Anna M. Blocki, Steffen Braune, Friedrich Jung, Marc Behl, and Andreas Lendlein, Advanced Material and Interfaces 2021, 2101588 DOI: 10.1002/admi.202101588.

Artificial muscles have shown their value in various applications, especially in bio-inspired robotics with scales from large to small. Among many types of artificial muscles studied, shape memory alloy (SMA), has the advantage of creating high force with large deformation via simply heating and cooling. However, limited bandwidth of SMA and its corresponding slow actuation speed was troublesome due to remarkably sluggish cooling rate compared to the heating. While active cooling components such as heavy and bulky air or liquid pumps were used to increase the actuation speed, they depreciated the merit of SMA being lightweight and shape-conformable. Therefore, we focused on accelerating cooling performance under natural cooling by maximizing thermal dissipation of SMA. Specifically, Cu nanowire-forest with high thermal conductivity was directly grown on the surface of SMA which resulted with maximizing the surface-area of the SMA for increased thermal dissipation. Cu nanowire-forest grown SMA with acclerated cooling showed faster actuation compared to the bare SMA, and it was successfully applied to biomimetic with enhanced working speed.

Many stimuli-responsive polymeric systems and composites have been developed in recent years with growing interest in their potential for applications, including as remote-controlled actuators, intelligent soft robotics, biomedical devices, and in energy harvesting. Here, we describe aramid-based multi-stimuli responsive films that produce programmable movement driven by submolecular switching. Films are responsive to near infrared (NIR) light, humidity, solvent, and heating, e.g., less than 5 degrees above room temperature. Moreover, in tandem with poly (vinylidene fluoride) (PVDF), film deformation can generate electricity, which provides a pathway to low-grade thermal energy harvesting.
Aramid films contain a small amount of crosslinking dibenzocycloocta-1,5-diene (DBCOD) and are covalently bonded to a thin sheet of aligned carbon nanotubes (CNTs). Coordinated switching of submolecular DBCOD units from ‘twist-boat’ to ‘chair’ conformation upon thermal stimulation manifests as a macroscopic shape change. This design takes advantage of the self-assembly of DBCOD units along the CNT direction to generate anisotropic thermal contraction. As a result, this deformation can be pre-programmed with the selection of CNT/cut pattern. Additionally, this unique thermal contraction offers the distinct advantages of being inherently fast, repeatable, low-energy driven, and does not require the presence or control of moisture. While the polymer layer exhibits a large negative thermal expansion, the coefficient of thermal expansion of CNTs is nearly zero. This mismatch results in a large deflection of the film with small changes in temperature. Furthermore, films exhibit excellent cycle stability, owing to the covalent interface and reversible nature of the conformational change. Such low-energy stimulus induced systems could enable development of ultrasensitive sensors, soft bio-robotics and wearable devices, as well as low-grade energy harvesting.

The authors acknowledge funding from NSF CHE-1900647, NSF DMR-1309673, National Aeronautics and Space Administration (NASA) grant numbers NNX15AQ01A and NNH18ZHA008CMIROG6R.

In this talk, recent research advances in next-generation 3D microdevices for interfacing cells, organoids, and humans will be described. These devices feature three dimensionality, shape-change, appropriate mechanical compliance, and integrated sensing and actuation of widespread relevance to biomedical engineering and human health. They leverage ultrathin and biocompatible materials, heterogenous and hybrid materials integration, transfer patterning and self-folding paradigms. Applications include shape change theragrippers, shell microelectrode arrays, bio interfacing tattoos, bioprogrammable robots and single cell manipulation tools.

The urgent demand for remote health monitoring has attracted massive attention to develop wearable bioelectronic devices. Smart contact lenses are not only applicable for AR/VR but also useful for vision correction and drug delivery. However, the lack of a reliable energy supply to the micro-electronic elements mounted on the lens impedes its widespread adoption. Photovoltaics, triboelectric, and wireless power transfer are the most well-known strategies to supply electrical power to smart contact lenses. However, all of these methods suffer from several drawbacks. For instance, due to the limited available area on the contact lens, the radio scavenging's output power (~ 0.1 µW.cm^-2) is much lower than the required energy.
Accordingly, we have developed a motion-activated metal-air battery driven by natural eye-blinking motion. Our innovative micro-battery is independent of environmental conditions. The natural eye-blinking motion, which happens approximately 10⁴ times a day, is the main driving force of this novel energy generator. This battery comprises an anode, a cathode, and an electrolytic solution, which is the eye tear. When the eyelid is completely open, the device is in off-mode. During the eye-blinking motion, eye tear first touches the anode, and the spontaneous oxidation reactions take place, resulting in the generation of electric charges. Finally, when the eyelid is completely closed, the eye tear goes into contact with both the anode and the cathode, resulting in the energy generation.
In the present study, to fabricate the metal-air battery we have utilized Zn (80 µm foil) and Pt as the anode and the air-electrode, respectively. A 200 nm Pt was sputtered using Denton discovery 18 and photolithographically patterned. Next, we have spin-coated and patterned a 1 µm Cytop in between the electrodes to serve as the hydrophobic layer, which effectively prevents the internal short-circuit issue. Furthermore, in order to investigate the effect of the air-electrode’s surface area, 20 µm long silicon pillar arrays were photolithographically patterned, dry etched, and coated with Pt. Siglent oscilloscope with the input impedance of 1 MΩ was used to measure the voltage output of the device. The current output was detected using the SR570 low noise current pre-amplifier from Stanford Research Systems.
Electrical results demonstrate the maximum voltage and current of 1.25 V and 300 µA, respectively. Using this innovative 3×8 mm² micro-battery, we have fully charged up a 100 µF capacitor in 8 s, corresponding to the power density of 40.1 µW.cm^-2. In addition, the integration of the current-time plot over one pulse shows the energy output of 260.4 µJ per blinking cycle. Having use of pillar arrays with different filling ratios of 20 to 80 % exhibits the importance of air-electrode’s surface area for oxygen reduction rate. The current output of the motion-activated metal-air battery enhances to 780 µA while using pillars with the filling ratio of 60 %. It should be noted that over increasing the pillars’ density brings about a semi-permanent wetting of the air-electrode, causing the oxygen inhaling deficiency. To further investigate the superior performance of the motion-activated battery compared to the conventional encapsulated and static metal-air battery, we have analyzed their discharge behavior under the maximum power-consuming load (740 Ω). The encapsulated Zn-air battery delivers 0.8 mJ and fully discharges in less than 30 min. However, the motion-activated battery exhibits about 100 mJ energy (more than 2 orders of magnitude higher than the static version). Given the average blinking frequency of 12 times per min, this device is functional for 111 hr at the discharge current density of 40 µA.cm^-2. The superior performance of the motion-activated battery over the encapsulated version is mainly due to the infinite amount of ambient oxygen and fresh electrolyte (eye tear with uniform resistance) during the blinking cycles.

Muscles help us move, enable us to interact with objects and the environment, and regulate critical internal functions. Unfortunately, they are susceptible to damage due to disease, ageing and trauma and are a central factor in diverse serious healthcare conditions including sarcopenia (age-related loss of muscle mass and function, where decline in muscle mass between 40 and 80 years ranges from 30% to 50% [1]), stroke, muscular dystrophy, multiple sclerosis, soft-tissue cancers, venous ulceration, diabetes, degenerative myopathy and incontinence. There are over 10.8 million people living with disability in the UK alone today. Nearly 6.5 million have mobility impairments, with the largest causes being age-related frailty and stroke (there are over 1.2 million stroke survivors in the UK). There is, therefore, significant demand for new treatments of muscle dysfunction, which will have significant global impact of the quality of life of many millions, with implantable medical devices steadily increasing for orthopaedic and diagnostic applications.
New functional, active and addressable materials are therefore urgently required to address these significant global challenges, and is the current focus of a larger EPSRC-funded collaboration (emPOWER, £6M). emPOWER comprises of a large multidisciplinary team of chemists, engineers and clinicians (>20). The aim of the emPOWER project is to develop fully implantable and directly bio-interfacing artificial muscles that seamlessly integrate with the body to restore muscle function, health and independence. To deliver this ambitious research and to lay the foundations for major healthcare impact throughout the project and onwards to 2050, the chemistries of pneumatic, thermal and electrical actuators have been developed and explored[2-3]. In addition the bio-compatibilities and adhesiveness of the formed actuators (with respect to muscles) are under exploration. The project aims to not only develop actuators but forms proof of concept devices as well as tackle the social, ethical and economic consequences.
Here we present our initial investigations into the production of addressable actuating polymer-based materials. Keeping the final application in mind (i.e., in-body implant), it is of crucial importance to show that our materials will be able to actuate temperatures relevant to the body (i.e., 37 °C). We are therefore developing a range of epoxy-based materials that show significant changes in physical and mechanical properties at these temperatures by simple tuning of type and density of cross-linking agents, composition and stoichiometries. Simple actuation tests show the validity of our approach for in-body actuation. However, several challenges (including biocompatibility) will have to be addressed in the future.
References
1. J. Rudge, D. Kim, Age and Ageing, 2014, 43, 821–826.
2. R. Liang, H. Yu, L. Wang, B. Ul Amin, N. Wang, J. Fu, Y. Xing, D. Shen, Z. Ni, Chemistry of Materials 2021, 33, 1190-1200.
3. J. M. Rossiter, Artificial Life and Robotics 2021, 26, 1-6.

Precise nanomanipulation techniques endow functional nanoparticles with the ability to perform complex tasks as active machines for various applications, including nanosurgery, drug delivery, and enhanced biosensing. In this work, we present an anti-Brownian-motion manipulation scheme with electric fields to control both the position and orientation of an anisotropic particle in suspension with precision under 20 nm and a fraction of a degree, respectively. The manipulation is also versatile that a nanoparticle can orient in arbitrary direction in three-dimensions (3D) and trace designed patterns; they also transport during agile rotation. With this technique, we successfully assembled a nanoarray and further applied a plasmonic-active nanorod as a nanopen for location-pinpointed analysis of bacterial cell membrane. The versatile and precision manipulation technique can have wide applications in position deterministic biochemical delivery, sensing, nano-assembly, and dynamic nanodevices.

Animals have long served as an inspiration for robotics. However, many of the mechanical properties, physical capabilities, and the behavioral flexibility seen in animals have yet to be achieved in robotic platforms. Standard materials for robotic fabrication do not exhibit self-healing or have the ability to autonomously generate energy, as is seen in biological systems. Additionally, traditional robotic actuators lack the compliance, energy efficiency, and power-to-weight ratio combinations observed in musculoskeletal systems. Finally, existing robotic materials typically rely on the use of synthetic, and often hazardous materials that can not biodegrade in natural environments. However, with recent developments in tissue engineering, it is now possible to create biohybrid and even completely organic robots using organic components for robotic structures, actuators, sensors, and even controllers. In this talk, I will present recent work from my lab on the creation and characterization of biodegradable structures for use in biohybrid actuators. I will focus on three key research questions: 1) How can biodegradable protein-based materials be used to guide the organization of living actuators, 2) How can we control the mechanical and geometric properties of these biomaterials towards engineering design, and 3) How can we scale up fabrication of living actuators for sustainable robots. To address these questions, we have developed robotic actuators using electrocompacted and aligned collagen as a guiding substrate. Subsequently, to control the mechanical and geometric properties of these materials we undertook a multi-factor study on the impact of fabrication parameters on material property outcomes. Finally, I will present a novel 3D printing approach developed in our lab to embed these materials in 3D printed hydrogels towards the scalable fabrication of biohybrid robots using biodegradable and biocompatible materials. I will conclude by presenting future challenges in biohybrid robotics. Such robotic systems have future applications in medicine, search and rescue, and environmental monitoring of sensitive environments (e.g., coral reefs).

Adaptability is one of the hallmarks of living systems that provide resilience to survive and flourish in dynamically changing environments. I will present our ongoing efforts for multifunctional materials that can autonomously change their mechanical properties and mitigate damages depending on loading conditions. Nature produces outstanding materials for structural applications such as bones and woods that adapt to their surrounding environment. For instance, bone regulates mineral quantity proportional to the amount of stress. It becomes stronger in locations subjected to higher mechanical loads. This capability leads to the formation of mechanically efficient structures and damage mitigation. However, it has been a challenge for synthetic materials to change and adapt their systems and properties to address the changes in loading conditions. To address the challenge, we are inspired by the findings that bones are formed by the mineralization of ions from blood onto collagen scaffolds. I will present a material system that triggers mineral deposition from ionic solutions on scaffolds upon mechanical loadings so that it can self-adapt to mechanical loadings. For example, the mineralization rate could be modulated by controlling the loading condition. A 30-180% increase in the modulus of the material was observed upon cyclic loadings whose range and rate of the property change could be modulated by varying the loading condition. Moreover, our preliminary results showed that the material system showed improved fatigue resistance from its damage mitigation mechanism. We envision our findings open new strategies for making autonomous material systems with improved durability from their self-sensing and self-adapting behaviors.

In recent years, the advent of highly deformable and healable electronics is exciting and promising for next-generation electronic devices. In particular, self-healable triboelectric nanogenerators serve as promising candidates based on the combination of the triboelectric effect, electrostatic induction, and self-healing action. Moreover, the output performance of triboelectric devices critically depends on their surface charge density that is greatly influenced by the dielectric constant of material. However, the self-healable polymer dielectric films have poor output performance because of their poor dielectric properties. Herein, we demonstrate that interfacial polarization in self-healable layered polymer films significantly enhances their dielectric constants, increasing the surface charge density and triboelectric performance. This is the first report of the introduction of interfacial polarization of dielectric materials for the enhancement of triboelectric output performance. The heterostructured elastic films with dynamic urea bonding as negative triboelectric materials are utilized in triboelectric devices, which shows an increase in the dielectric constants through interfacial polarization, resulting in significant enhancement of triboelectric performance. Our device displays the best output performance (169.9 V/cm²) compared to the other self-healable triboelectric ones. Based on the concept of self-healable dielectric material with dynamic urea bonding, we designed self-powered and self-healable triboelectric sensors consisting of self-healable polymers and aligned silver nanowires (AgNWs), leading to five times higher triboelectric performance (403.9±46.2 V and 12.1±1.5 μA) than those without the NWs. Moreover, owing to the anisotropic dielectric property through the aligned AgNWs, the proposed triboelectric sensor could detect eight-directional movements of the human eye, suggesting its potential applicability as a self-powered and healable EOG sensor. The described approach involving the use of the self-healable polymer and aligned AgNW composite gel for triboelectric devices provides a robust platform for the design of flexible sensors with controlled tactile sensitivity and self-healability.

Systems such as traffic and security are constantly operated in order to maintain their operation and this operation induces the waste of tremendous energy despite the necessity of these systems. By supplying the energy generated from the triboelectric nanogenerator (TENG) to these systems, the issue related to waste of the energy as the standby power can be handled because the TENG generates the electricity from the wasted energy circumstance of these systems. Herein, the spray coating-based TENG (STENG) is developed to harvest this wasted mechanical energy. The electrical outputs of the STENG are investigated and the applicability of the STENG is demonstrated by adopting the various materials for fabrication and the STENG shows great stability. Finally, invasion alarming system (IAS) and hidden keypad (HKP) are implemented as applications, respectively. The violation in the restricted areas is successfully detected with the developed IAS. The HKP successfully plays a role as the keypad of the conventional keyboard. Considering these results, the STENG can be expected to apply in the smart traffic system as well as advanced security systems in near future.

Piezoelectric fibers are promising structures for smart textile and composite applications. Such materials can be used to harvest energy from ambient vibrations in wearable systems or to sense deformations in composites. Currently available inorganic piezoelectric fibers display remarkable piezoelectric coefficients. However, they suffer from brittleness and cannot be easily implemented in wearable textiles. On the other hand, organic fibers made of piezoelectric polymers can be processed by conventional melt spinning technologies. These fibers are flexible and are well appropriate for wearable systems. However, they are less efficient than inorganic materials in terms of energy conversion. We discuss in this presentation an alternative approach, which consists in developing nanocomposite piezoelectric fibers. These fibers are made of inorganic piezoelectric nanoparticles embedded in a polymer matrix. We will show how these fibers can be processed by a scalable wet spinning technology. First results show that the fibers display suitable mechanical properties for textile applications. However, their piezoelectric performances are not yet at the level of the most optimistic theoretical expectations. We will discuss the physical mechanisms of electrical poling and of mechanical stress involved in such fibers to explain the observed behaviors. Routes and work under progress for future improvements will be presented. It is believed that the concept of nanocomposite fibers cand lead to fibers that combine high piezoelectric efficiency and flexibility as needed for demanding smart textile applications.

As the demand for scattered energy sources has increased with the development of the Internet of Things (IoT) and mobile/portable devices, diverse energy harvesting mechanisms such as electrochemistry, thermoelectricity and their hybridizations have attracted much attention. They can meet stringent requirements for scattered devices in environmental, military, and industrial applications because of their integrability and simple structures, which do not contain mechanical moving parts. Their common characteristic is their suitability for integrated energy harvesting in various platforms involving waste energy sources. However, their low voltage level and limited operation depending on ambient environments should be resolved to bring them to feasible energy harvesting devices. The hybridization of multiple stimulus sources is efficient strategy for the energy harvesters. For instance, humidity is a basic element that exists everywhere, and its relative humidity (RH) is coupled with temperature. If utilizing the ambient humidity and temperature gradient as dual stimulus sources is realized in one integrated energy harvesting device, it has a high potential to complete the sustainable energy harvesting with the amplified potential level.
Here, we report a humidity-thermoelectric-hybrid energy harvesting device (HT-HED) to amplify the potential level of typical thermoelectricity by means of combing humidity-driven energy harvesting through PN junctions. First, the palladium-based oxide composites (Pd/Pd_xO_y) deposited on the carbon cloth were fabricated by an electrothermal process via Joule-heating, and they were combined with a poly (4-styrenesulfonic acid) (PSSh) as a hydron supplier to form a humidity-driven energy harvester (H-dH). A Pd/Pd_xO_y generating a reduction potential depending on the RH environment was utilized as an active material, while a poly (4-styrenesulfonic acid) (PSSh) served as a hydron supplier by the desulfonation reaction in the RH environment. Second, attach the n-type Bi₂Te₃ pellet on the bare carbon to form a thermoelectrically-driven energy harvester (T-dH). A Bi₂Te₃ generate potential by using temperature gradient. HT-HED were fabricated by put H-dH and T-dH on the same carbon cloth. The underlying mechanism for hybridizing dual stimulus was elucidated by correlating the humidity-temperature reactivity and electrical impedance.
To optimize the OCV of HT-HED, the electrothermal processing conditions in terms of power and duration were explored to manipulate the Pd/Pd_xO_y@C composites within carbon cloth substrates. The unit cells for T-dH-only, H-dH-only, and their combined device (HT-HED) were prepared to compare the OCV performances depending on the hybridized stimulus. A HT-HED, which was developed to compensate the low potential of the thermoelectricity could show the enhanced OCV compared to the T-dH only unit cell. The output voltage was enhanced from 647 μV to 0.133 V at RH of 28.5 % and temperature of 308.5 K (delta T=10.35 K). Based on the investigated design from the unit cells, a 5 by 5 array of HT-HEDs employing the optimized cell was fabricated and its potential generation was investigated at the general ranges of RH (~30-60 %) and ambient temperature (~298.15-323.15 K) to derive the contour map depending on the ambient environments. An optimal OCV of the HTHD array was obtained from 0.505V to 0.4868V in the range of 30-50% RH at the temperature of 308.5 K. As a demonstration of feasible applications, the HT-HEDs was attached to a real human skin and the average OCV of 0.63V was measured for 2.8 h in ambient environments. This humidity-thermoelectric dual-mode energy harvesting strategy in the ambient environments enables more amplified and sustainable power generation compared with the single-mode energy harvesting, thereby developing potential energy harvesting devices for wearable or IoT platforms.

3D printing (additive manufacturing) where materials are deposited in a layer-by-layer manner to form a 3D solid has seen significant advances in recent decades. 3D printing has the advantage of creating a part with complex geometry from a digit file, making them an ideal candidate for making architected materials. Multimaterial 3D printing is an emerging field in recent years in additive manufacturing. It offers the advantage of the placement of materials with different properties in the 3D space with high resolution, or controllable heterogeneity. In this talk, we present our efforts in developing multimaterial 3D/4D printing for functional composites. We first present our efforts of using digital light processing (DLP) 3D printing to create multimaterial printing. Conventional DLP method uses a single vat resin, making them not suitable to multimaterial printing. Here, we use light grayscale (or light intensity) to control local properties of printed parts to achieve multimaterial-like printed parts. The advantage of this grayscale DLP (g-DLP) method is that it still uses a single resin vat and thus retains the advantages of conventional DLP printing. Second, we present a new method of integrating DLP with direct-ink-write (DIW) method to print multifunctional composites. Here, the DLP can be used to print bulky parts and the DIW can be used to add functional materials, such as liquid crystal elastomers, conductive materials, etc. Third, the capability of spatially control mechanical properties in multimaterial 3D printing offers a vast design space for 4D printing to create shape-changing devices. We present our recent efforts of using evolutionary algorithm and machine learning to rapidly explore the design space and to obtain optimized design. Finally, we will briefly discuss the future work in the multimaterial 3D/4D printing.

Freeform liquid 3D printing (FL-3DP) is a promising new additive manufacturing process that uses a yield stress gel as a temporary support, enabling the processing of a broader class of inks into complex geometries (i.e., overhanging structures, shapes with high-aspect ratio, etc.), including those with low viscosities or long solidification kinetics that were previously not processable. However, the full exploitation of these advantages for the fabrication of complex multi-materials structures has been restricted by chemical compatibilities and difficulties in controlling the interfaces between inks and supports. For example, the new class of silicone/epoxy hybrids recently reported by our group could not be processed by FL-3DP because of the lack of compatibility of stiffer hybrids containing a high amount of epoxy with commonly used aqueous-based supports, due to the presence of acid-anhydride hardener. These hybrids exhibit the highest tunable range of elasticity (five orders of magnitude, from ~20 kPa to ~2 MPa), providing an excellent material family for the manufacture of multi-material devices with strong interfaces ^[1]. While direct-ink-writing could be used for the manufacture of "2.5D" multi-material structures ^[1], processing these hybrids through FL-3DP is highly desirable to reach higher complexities (fully 3D structures with overhanging parts) and functionalities. In this work, two enhancements of FL-3DP are reported. First, a novel support bath based on vegetable oil was developed, enabling the process of the whole range of hybrids, from extremely soft to stiff materials. However, the use of vegetable oil induced minor alterations in the mechanical properties of the hybrids, a second approach combining different baths was developed. Vegetable-based and aqueous-based supports were combined to enable selective printing of inks based on support affinity. These key developments enabled the manufacture of more complex structures and were illustrated through three examples applied to healthcare and soft robotics. First, a knee articulation could be replicated by printing hybrids in vegetable-based support. Second, a tooth-like structure with gum could be printed by combining vegetable-based and aqueous-based supports. Third, a functional soft robot could be manufactured.

[1] V. S. Joseph, T. Calais, T. Stalin, S. Jain, N. K. Thanigaivel, N. D. Sanandiya, P. Valdivia y Alvarado, Applied Materials Today 2021, 22, 100979.

Active 3D printed structures capable of shape morphing when subject to an external stimulus are highly useful for a vast amount of applications, ranging from soft robotics to biomedical devices. To actuate such structures, pneumatics offer a readily available and reliable solution and enjoy increasing popularity. However, when used to actuate shape morphing structures, pneumatic actuation usually takes place via monolithic linear actuators. Therefore, a mechanism is inherently needed to translate the linear actuation to a shape change of the overall structure. Even though this allows for simple and standardizable actuators, the involved mechanisms increase the manufacturing and assembly cost and complex target shapes are difficult to achieve. Other pneumatically actuated shape-morphing structures employ more complex designs, but are often manually designed for a certain use case, leaving the design process tedious and the full potential of additive manufacturing remains unused.
The concept proposed in this study takes the conversion of using pneumatic pressure for complex shape morphing to the structural level and avoids any translating mechanisms. To do so, we employ spinodal metamaterials, which have a microstructure consisting of two phases with a complex labyrinthine geometry. Our approach is to 3D print these metamaterials, using an elastomer for one phase while leaving the other phase void and to actuate the metamaterial by applying pneumatic pressure to the void phase. In this study, we investigate the relations between the design of the spinodal microstructure and the material deformation when pneumatically actuated. The focus lies on the possibility of introducing anisotropy into the microstructure, leading to a highly anisotropic deformation response of the material. This anisotropy can be varied continuously within the metamaterial, allowing for designing for a desired deformation when actuated.
Our characterization of these metamaterials starts by generating a set of microstructures with varying degrees, types and directions of anisotropy and different volume fractions of the material phase. The pneumatic actuation is then simulated using nonlinear Finite Element Analysis, considering the nonlinear (hyperelastic) behavior of the elastomer, as well as the option of applying a negative pneumatic pressure (i.e. pull vacuum). The active metamaterial concept is also verified experimentally on a selected set of designs and the experimental and numerical results are compared. Using the gathered knowledge, a more complex structure is designed in which the anisotropy is varied, such that when actuated, a complex shape change is obtained.
The results of this study demonstrate the validity and potential of the concept by its ability to translate pneumatic actuation into a significant deformation. Considering tunability, varying the microstructure, especially its anisotropy, provides a powerful tool to control the material response to pneumatic actuation. Varying the volume fraction of the material phase can increase the resulting deformation while lowering the stiffness or vice versa. Overall, the presented concept for an active metamaterial represents a promising approach for shape morphing, as it allows for complex 3D-to-3D shape changes.

4D printing is a novel approach that enables dynamic functionality in printed objects. The objects can change their shape and dimensions in response to time or external triggers such as heat, electric and magnetic fields. We will describe various materials-driven approaches which enable fabrication of printed responsive objects and devices, mainly based on inks composed of polymers and monomers with dispersed nanomaterials, such as carbon nanotubes and magnetic particles. The polymers are obtained by localized photopolymerization reactions, thus enabling printing by stereolithography processes, such as in Digital Light Processing printers. Various responsive printed objects will be presented, in which the polymer itself is responsive, such as shape memory polymers, or the response results from embedded particles such as carbon nanotubes or magnetic particles. Such examples are methacrylated oligomers having a shape memory behavior, or stretchable urethan acrylate monomers that change shape under heat, a magnetic field or upon exposure to light at a specific wavelength due the presence of magnetic or plasmonic nanoparticles, respectively. The various inks will be demonstrated mainly as actuators for soft robotics.

There is high interest in new materials and assembly methods from the nanoscale up to the macroscale. A combination of surface and interfacial chemistry, thermo-mechanical properties, self- or directed- assembly enable unique designs and structure-property relationships that are challenging when carried at various hierarchies. The use of lithographic and non-lithographic methods is key towards demonstrating high throughput fabrication in thin-film materials. Hybrid and multilayer structures that have anisotropic thermo-mechanical properties that can drive actuation and the appropriate stimuli-response. This talk will highlight our work on nanostructured multilayer films that can be structured using various deposition methods ranging from polyelectrolyte layer-by-layer approaches to polymer multilayer melt co-extrusion. In another demonstration, we have utilized the CLiPS process of polymer melt co-extrusion of layers to produce microparticles with unique geometries and high multilayer internal structure useful for applications such as photonic materials and controlled release technologies. Lastly, we demonstrate scalability with 4D printing using stimuli-response and shape-memory properties in thermoset polymer materials.

4D printing is commonly defined as the targeted evolution of a 3D printed structure to change its shape, properties, and functionalities. This targeted evolution is achieved by utilizing different 3D printing techniques, stimuli, materials, interactions, and modeling approaches. 4D printing greatly enhances the design space of 3D printed structures giving engineers the possibility to create highly integrated and reconfigurable structures. Especially in complex structures such as valves, 3D and 4D printing have the potential to offer significant improvements. Valves are essential parts of almost any pneumatic, hydraulic or other pressurized system. They fulfill a variety of tasks from pressure regulation to flow control and emergency backup systems. Currently, valves are often made from a variety of materials such as plastics and metals, often consisting of multiple parts that require assembly and tuning before system integration. Here we show a fully integrated 4D printed tunable pneumatic pressure release valve, which can be fabricated in one piece requiring no further assembly. The valve is modeled in a top-down approach, the build room for the valve is determined based on commercially available pneumatic pressure release valves. All parts of the valve are fabricated out of related photopolymers using Stratasys PolyJet printing technology. The valve can be manually released through bistable actuation and the release pressure can be tuned prior to fabrication using a numerical optimization scheme. Tunability is achieved by manipulating the geometry and material distribution in parametrized wave and bar springs. Relevant spring parameters and properties are determined through numerical simulations and mechanical testing of 3D-printed, physical prototypes. This concrete example illustrates several possible approaches to how 4D printing can be utilized to retain functionality and performance while minimizing weight and eliminating assembly time in highly integrated additively manufactured systems.

The multi-material structures found in soft robot bodies, from stretchable elastomers to hard connectors, can display highly distinct physicochemical properties, challenging the engineering of robust interfaces required for long-term performance in real-world scenarios. A novel family of hybrid resins combining platinum-catalyzed silicones with epoxy resins cured by acid anhydrides was recently reported ^[1]. These hybrids enabled the additive manufacturing of functional multi-material bodies exhibiting five orders of magnitude of elasticity (from 20 kPa to ~2 GPa) with strong interfaces (from 1 to 3 kJ m^-2), by adjusting the composition of the hybrid resins. The excellent control of ink rheological behavior via nanoclay addition allowed for direct-ink-writing, enabling seamless control of topology and mechanical properties with sub-millimeter resolution in three dimensions.
In this work, the significance of this new class of hybrids is further demonstrated via the fabrication of membranes for flying devices. Two types of devices were manufactured: (i) a bird-inspired folding wing made of an elastic membrane with anisotropic deformation bonded on a 3D-printed bone structure made in PLA; (ii) an insect-inspired flapping wing for micro-air vehicles. For the first device, the formulation of the hybrids was modified through the addition of cellulose-based fillers to increase the interfacial toughness between the hybrids and PLA. In the second case, adjustments in the fabrication strategy were made to successfully manufacture light-weight thin wings (<50 μm).

[1] V. S. Joseph, T. Calais, T. Stalin, S. Jain, N. K. Thanigaivel, N. D. Sanandiya, P. Valdivia y Alvarado, Applied Materials Today 2021, 22, 100979.

We present a model cactus plant with developmental strategies, structures and means of colonizing its natural habitat that have been inspirational for the development of hydrogel-elastomer actuator systems. Climbing plants represent a rich source for ideas and concepts for the creation of new technologies and materials.
Many climbing plants develop a mechanical architecture where apical branches (so called "searchers") have a stiffer structure compared to the basal climbing stem. This allows the apical branches to navigate and find a substrate to anchor themselves. The climbing cactus stands out from the usual organization present in most lianas. Selenicereus setaceus (Cactaceae) occurs in dry coastal forests of southeastern Brazil. Like most cacti, it has succulent stems adapted to arid and semi-arid conditions that undertake photosynthesis. Its leaves are extremely reduced and transformed into spines. Unlike close related species, individual plants are climbers. Initially they creep along the ground, then attach to tree trunks using specialized roots and then develop apical searchers that spread out from the tree canopy into the open light. S. setaceus can grow and develop in rough unstructured environments and is able to anchor on many kinds of substrates, keeping the stem firmly attached even in harsh climatic conditions.
This climbing plant adapts its stem geometry to optimize rigidity. Creeping basal stems have circular cross-sections, giving rise to root-climbing stems (triangular cross sections), which produce searcher stems (star-shaped cross-sections). These maximize second moment of area and rigidity whilst retaining a physiologically “cheap” hydrogel-like internal structure. The searcher architecture is capable of crossing voids up to 1 m.
The searcher stem is occupied by a hydrogel-like material: thin-walled water storage cells, surrounded by mucilage and enclosed by a stiff "skin". The skin is a composite material in three layers: an outer waterproofing layer, an inner layer of flattened cells (epidermis) and inside that several layers of spindle-shaped cells with hygroscopic cell walls. This arrangement ensures that the skin accommodates increases in volume due to water loss or gain, a condition that varies greatly in these plants. Swelling and de-swelling experiments showed significant increases and decreases of cross-sectional area in demineralized water or 0.5 M sucrose respectively, leading to profound changes in their geometry.
Based on this cactus, two actuating systems have been developed, one based on the searcher geometry, in which movement is generated from external light or humidity stimuli, as in natural conditions. A second one was based in the developmental changes observed along the cactus stem, from cylindrical to triangular to star-shaped, using the materials swelling/deswelling capacity. In both systems, an artificial hydrogel mimicking the cactus soft cortex is employed as a self-actuating component, generating movement (Devo system 1) or changes in geometry (Devo system 2).
This biological system provided insight for how variable developmental pathways —“devos”— in a biological model might be used to develop different kinds of technological actuator. These “variations on a developmental theme” are based on phases of development seen naturally in the cactus that are related to different “stages” of its climbing-attaching and searching behavior. Ongoing studies are exploring these traits further by investigating the autonomy, survival and growth of these skin-core, hydrogel-based plant systems, especially following catastrophic damage in harsh environmental settings. It is expected that these will give further bio-inspired clues for autonomy and survival of actuating technologies in challenging environments.

During the last century, humankind has enjoyed the convenience brought by the invention of petroleum-based plastics, while at the same time dealing with the negative consequences of nondegradable plastic wastes after their service life. After over 70 years of industrial development, both humankind and mother earth need rethinking of this path, leading to an urgency to develop products from nature-derived materials. Nature has provided humankind abundant biodegradable resources that can be processed into high-performance and multifunctional materials, including cellulose, hemicellulose, lignin, chitin, alginate, and silk, among many others. If these biopolymers can be carefully processed, modified and designed, they can be turned into advanced materials with potential applications as structural and engineering products, with utility in environmental remediation, optical devices, electronic substrates, energy harvesting systems and storage materials, as well as biomedical devices. Nature provides humankind with rich biodegradable resources, while also offering tremendous inspiration in structural and functional designs in concert with sustainable technology.
In this talk, for the first time, we will report an ion selective membrane derived from natural loofah sponge, which has structural features of high hydrophilicity, superior ion conductivity, and 3D interconnected long fibers. The permselectivity and ion conductivity of loofah-based anion selective membranes (ASMs) and cation selective membranes (CSMs) are designed through chemical modification of the surface functional groups of loofah fibers and followed with compression, resin filling and polishing. Meanwhile, the unique isotropic structure endows excellent dimensional stability under the NaCl solution for months. When these loofah-based ISMs were used for salinity gradient power generation from the gradient of artificial seawater (0.6 M NaCl) and river water (0.01 M NaCl), the maximum power density was 18.3 mW m−2. When ten units of loofah-based ISMs were stacked in series, a voltage as high as 1.55 V was achieved. The results highlighted the great potential of natural fibers for fabricating affordable, durable, and high-performance ISMs, paving a sustainable pathway for developing high-performance, durable, and low-cost salinity gradient power generators. Meanwhile, the resulting device can also be used as an electrically controlled sieve to remove salt from seawater to produce fresh water.

Robotics has undergone a major expansion in scope, dimensions and multi-disciplinarity, thanks to the maturity and the advances of the related technologies. From predominantly industrial applications, robots are rapidly challenging other scenarios, being safer and more dependable at homes, workplaces, and natural environments. The application in such unstructured settings requires increased abilities of perception-action delivered by a control architecture which relies on the information on the state of the robot and its surrounding environment, and exploits its learning and body adaptation skills to respond to a continuously changing situation. In a constantly changing world, natural organisms’ life and evolution strategies can provide engineers with the rules to design and develop functional embodiments and energy-efficient adaptive behaviours, that are the keys for artificial machines to better deal with unstructured and challenging environments.
Natural systems’ secret lies in the smart characteristics of how their body is structured, in how they are able to use or preserve energy resources, in how their intelligence is embodied and distributed, and in the synergies that they create each other to effectively adapt, grow and survive.
With this vision, our approach is to take inspiration from plants and soft animals in order to design robots with high morphological adaptability, distributed sensory systems, as well as energy-saving mechanisms.
Specifically, this talk will show recent results in the field of bioinspired soft robotics based on the investigation of plants and soft animals' features, with the double goal to identify and extract the key principles underlying these biological functions and to translate them in a technological solution, and to improve scientific knowledge on these biological systems that we take as models.
Examples will include artificial self-growing roots for soil monitoring, growing robots inspired by climbing plants for infrastructures’ explorations, anchoring systems inspired by plants hooks, soft manipulators inspired by the octopus’ arms for adaptive grasping, biohybrid energy harvesting systems, and self-deployable, biodegradable seed-like soft robots for environmental monitoring.
A new wave of environmentally-responsible robots is envisioned, by merging bioinspired soft robotics, material science, nanocomposite technologies, and environmental science.
These “green robots” will be developed to better integrate into the natural ecosystems and react accordingly to changes by varying their structural, morphological or behavioural features, to be sustainable and operate in natural scenarios in support to the challenge of biodiversity and climate protection.

Stereolithography (SLA) 3D printing offers a capability to shape photopolymers into a variety of periodic structures (lattices) with feature sizes ranging from submicron to millimeters. Such lattices of photopolymers can serve as a scaffold for carbon nano- and microlattices, microarchitected pyrolytic carbon materials which replicate the original topology at 60-80% shrunk dimensions after pyrolysis in an inert atmosphere. So far carbon nano- and microlattices are three-dimensional, rigid and strong, offering favorable properties especially for structural applications. Here, we employed the same SLA 3D printing and pyrolysis to fabricate two-dimensional carbon microlattices that exploit an untapped mechanical properties of microarchitected carbon: elasticity. Prepared in the same diamond patterns with three different thicknesses (70, 100, and 150µm), 2D carbon microlattices endured cyclic bending without disintegrating the structure and degrading mechanical properties, visually indicating forces at a magnitude of few tens of millinewton. The combination of sufficient structural robustness, elastic deformability and the degree of freedom in architectures is unprecedented for bulk, monolithic pyrolytic carbon, unlike the graphene assemblies that are made of nanocarbon chemically bonded or the carbon fibers whose shape is constrained to one-dimensionality. Further provided with an electrical conductivity and biocompatibility as was made of pyrolytic carbon, this emerging class of carbon material can find applications in microrobotics, micro electromechanical systems (MEMS), medical devices and so on.

A series of dynamic covalent network films containing the catalyst-free, reversible thia-Michael bond were used to understand structure/property/processing relationships in materials that exhibit dynamic reaction-induced phase separation (DRIPS). By controlling both crosslinking density and thermal processing conditions of the networks, a range of morphological, rheological, and mechanical properties were realized. Critically, this system uses thermal processing as the sole handle to access a wide range of mechanical properties with a single variant of Michael acceptor, instead of tuning the electron-donating/withdrawing nature of the Michael acceptor. Furthermore, it was demonstrated that different shape-memory behavior can be programmed into a material through straightforward thermal processing.

The triboelectric nanogenerator (TENG) is attractive as a power source of small electronics because the TENG generates electricity when friction occurs between two different materials. This simple working principle allows the TENG to select the various materials in order to fabricate the TENG. Considering this advantage, the thin films based TENGs are proposed to be utilized in flexible or wearable electronics. However, these flexible TENGs are easily broken when the strong impact is applied to the TENGs. Hence, the TENGs possessing robustness and flexibility are highly desired. Shear thickening fluid (STF) is a suitable material to fabricate the robust and flexible TNEG because it can change its phase from liquid to solid by applying the external force and returning its original state when the force is removed. In this work, the STF based TENG is proposed composed of rubber container, cornstarch-based STF, and Al electrode. The water-assisted oxidation is conducted on the surface of the electrode and the electrical outputs of 30 V and 350 nA are generated from the STF based TENG. Additionally, by adding 10 wt% of PVDF powder into the STF, the electrical output generated from the STF based TENG is dramatically increased. Furthermore, the STF based TENG can endure a strong impact even more than 115 kg of weight and the STF based TENG shows the highly shock-absorbing ability. Considering these results, the proposed STF based TENG can be utilized as a robust and flexible power source as well as a sensor for speed and weight in near future.

Self-powered user-interactive displays that facilitate the visualization of human information acquired by sensors are of great interest in emerging human–machine interface technology with efficient energy consumption. Herein, a self-powered motion-sensing display capable of simultaneously detecting and visualizing finger motions is presented. Our device is based on a one-dimensional photonic crystal of an interpenetrated hydrogel network block copolymer (IHN-BCP) consisting of alternating water-absorbable and non-absorbable lamellae. Triboelectrification is achieved as a function of relative humidity from 30% to 80%. The direct visualization of the humidity is also achieved through the humidity-dependent structural color of the photonic crystal in the full visible range.

The functionality of hydrogel actuators can be enhanced by speeding up response times and converting their isotropic swelling and contraction into more complex deformations. We present a composite microactuator which combines poly(N-isopropylacrylamide) (PNIPAM), a widely used temperature-responsive polymer, with carbon nanotubes (CNTs) to achieve these goals. The anisotropy of vertically aligned CNT forests transforms the isotropic (de)swelling of PNIPAM into anisotropic motion. The high optical absorption and anisotropic heat conductivity of carbon nanotubes allows us to use low power light illumination for both local and global PNIPAM actuation, with sub-second response times. We demonstrate a wide variety of CNT skeleton microstructures to show a range of actuation behaviours, including fast reversible movement, swimming, active switching from low to high light absorption states, auxetic lattice shape changes, and good cycling stability.

In a thermal energy storage system, the ability to harvest heat is vital. Hawaii’s plan to transition to clean energy is by the year of 2045 [1]. This introduces to optimizing a solar water heater system towards Hawaii’s climate. As Hawaii is in the tropical region where abundant solar energy is available yearly, we can achieve decarbonization and help transition to renewable energy before 2045. Some of the prior works toward enhancing a solar water heater system focuses on enhancing the solar absorber’s material. However, impact of the working fluids on the indirect solar water heater system has gained less attention. Conventional working fluids lack in their thermophysical properties that lead into poor heat transfer towards energy conversion systems. Here, I introduce to use nanofluids to overcome this issue and enhance the heat transfer of the solar water heater system. Nanofluids are engineered colloidal suspended solutions that enhances the overall heat transfer of the base liquid. Nanofluids are well known for their enhanced thermal conductivity in which they’re reliant on the nanoparticle’s material, size, and shape. I have also developed a transient hot wire apparatus to measure nanofluid’s thermophysical properties to help determine the best selected enhanced working fluid from the innumerable samples. The nanomaterials used are multiwalled carbon nanotubes, alumina, copper oxide, and silver nanoparticles. I will present the obtained thermophysical properties of the water based nanofluids and compare their results with water to demonstrate the thermal conductivity enhancement. I will also present the overall efficiencies of the solar water heater system with the selected nanofluids to illustrate how the heat transfer has been enhanced. We expect these results to be valuable in understanding the energy conversion enhancement of a solar water heater system to achieve decarbonization and enable sustainable energy for Hawaii before 2045.

[1] Borland, J., & Tanaka, T. (2018, June). Overcoming barriers to 100% clean energy for Hawaii starts at the bottom of the energy food Chain with residential Island Nano-grid and everyday lifestyle behavioral changes. In 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC)(A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC) (pp. 3829-3834). IEEE.

The blue phases are observed in highly chiral liquid crystalline compositions that nascently organize into a three-dimensional, crystalline nanostructure. The periodicity of the unit cell lattice spacing is on the order of the wavelength of visible light and accordingly, the blue phases exhibit a selective reflection as a photonic crystal. Here, we detail the synthesis of liquid crystalline elastomers that retain blue phase I, blue phase II, and blue phase III. The mechanical properties and optical reconfiguration via deformation of retained blue phases are contrasted to the cholesteric phase in fully solid elastomers with glass transition temperatures below room temperature. Mechanical deformation and chemical swelling of the lightly crosslinked polymer networks induces lattice asymmetry in the blue phase evident in the tuning of the selective reflection. The lattice periodicity of the blue phase elastomer is minimally affected by temperature. The oblique lattice planes of the blue phase tilt and redshift in response to mechanical deformation. The retention of the blue phases in fully solid, elastomeric films could enable functional implementations in photonics, sensing, and energy applications.

Thermal energy storage (TES) based on phase-change materials (PCMs) has many current and potential applications, such as climate control in buildings and thermal management for batteries and electronics. In all of these applications, the PCM works near the ambient temperature and is typically designed to operate at a fixed temperature. One fundamental challenge in the adoption of PCM-based TES is that there is limited tunability in the usage temperature, especially for the near-ambient applications. For example, the use temperature in buildings during the summer and winter months can vary significantly (there can be also significant diurnal variations). In reality, that translates into reduced use of the PCM, because either the PCM melts partially or does not melt at all. There has been significant interest in recent years in making thermal materials and devices tunable, such that their properties and performance can be dynamically changed. However, most of the focus has been on changing thermal transport properties, such as thermal conductivity, rather than thermodynamic properties, such as the melting/transition temperature (Tm) of a material. Changing the Tm of a material using an external stimulus such as pressure, an electric field, or a magnetic field, is a non-trivial task, as the required magnitude of the stimulus to achieve a sizable change in Tm is typically large, and the enthalpy change at Tm is only moderate for thermal storage applications. Besides, dealing with the liquid phase of PCMs during the phase change transition also prevents TES from practical application. To circumvent the aforementioned problems, here, we report on the solid-state tunable thermal energy storage of a shape-stabilized PCM. We achieve a transition temperature tunability up to 10 degrees C and shape stability up to 1 month, which may enable simpler and safer TES devices/system designs.

An introduction in the field of soft actuators and soft robotics followed by various application areas of soft actuators will be covered here. A detailed survey on materials in use and fabrication methodologies for soft actuators will be given. Furthermore, a range of stimuli for actuation, deformation capabilities, motion complexities and varied multifunctionality in the soft actuators will be discussed.

Bio-inspired structures and materials

The most recent research on polymer-based bio-inspired structures and materials will be presented. The focus will be hybrid and multimaterial systems inspired by plants, for which the arrangement of different materials and their structure-function relations will be discussed. Finally, plant inspired movements in such artificial systems and their applications will be covered in this topic.

Stimuli-sensitive material for actuators, e.g., shape-memory polymers, liquid crystalline elastomers

Fundamentals of stimuli-sensitive materials and how the shape-memory effect or actuation can be utilized to fabricate bioinspired actuators will be covered in this topic. Network architectures of shape-memory polymers and liquid crystalline elastomers, understanding of various types of shape-memory effects and actuation, parameters or factors influencing the performance of actuators and their characterization methods will be discussed.

Polymer processing technologies

The topic will cover the introduction into various conventional polymer processing such as extrusion, injection or compression molding. An introduction into 3D printing will also be the part of this topic and later the use of various technologies in the fabrication of actuators will be elaborated. The usage and various product ranges from different processing techniques will also be discussed.

Atomic force microscopy, as a platform for implementing and characterizing functional materials on the micro/nano-scale, was developed and will be introduced here from the basics, such as scanning modes, influence of cantilever tip, force measurements and other advanced applications. Particularly, in-situ characterization of different morphologies, monitoring shape reconfiguration of structured surfaces, particles and fibers during active movement will be discussed.
Micro/nano-technologies of soft actuators
The design/fabrication, functionalization, actuation, and localization of soft actuators on micro/nanoscale will be introduced. The principles of different actuation methods and their limitations will be analyzed. Examples of biomedical applications, micro-/nanorobots or sensors will be presented. The key challenges and future development directions will be discussed.

It will be described how the materials occurring in leaves could enable to convert mechanical energy such as from wind and rain drops into electricity using solid-solid or liquid-solid contact electrification of the leaf cuticle.
It will be shown how the resulting electrical signals differ from active electrophysiological signals like action potentials that can be analyzed with soft organic electrodes on the plant surfaces.

Smart materials are widely used in sensors, actuators and robotics, and their multi-functional capabilities enrich engineering applications. In this presentation, material designs, form factors and scale effects of Ni-Ti SMA are discussed. As an example of material designs, soft smart composite (SSC) based on SMA and anisotropic polymer fibers embedded in soft elastomer matrix is introduced. Actuation characteristics of the SSC and its applications to various mechanical systems are presented. To extend conventional form factors, shape-memory knit actuators are developed in order to produce high actuation force and stretchable features. Scaling down to meso/micro level, 25~40 micrometer thick SMA wires in SSC show 35 Hz, and light-driven actuation of 500 nm-thin SMA frames enhance the actuation frequency up to 1,600 Hz. The actuator is applied to make a standalone microrobot actuated by remote laser beam, and several micro robots are controlled individually to form a swam activity as a group. Extending nano fabrication to SMA, meta surfaces are introduced to the micro actuator by focused ion beam to show wavelength selectivity of the SMA actuators.

Dielectric Elastomer Actuators (DEAs) exhibit a collection of excellent performances to be the next-generation artificial muscles, yet they suffer from short lifespan and premature breakdown. Self-clearing of defects from thin, compliant electrodes can potentially solve these problems, yet this process is currently neither observable nor controllable. In this work, we propose a dimensionless indicator named capacitor retention to indicate the remaining capacity to generate force/displacement of a DEA during self-clearing. Through self-clearing preprocess using appropriate combinations of factors derived from the above investigations, we achieve high-performance DEAs with unidirectional strain of 9%, power density of 300W, >1 million life cycle, and the ability to recover from multiple external damages. This work can potentially produce long-lifespan and highly robust artificial muscles for future mass applications of DEAs.

Ferroelectric materials with large linear strain are suitable candidates to be used in actuator applications. Ferroelectric materials exhibit an electrocaloric effect to be used in the solid-state cooling device. In the present work, we have synthesized a multifunctional ferroelectric material Ba_0.95Ca_0.05Sn_0.09Ti_0.91O₃ (BCST) by a conventional solid-state reaction route. This BCST composition is close to polymorphic phase boundary composition. Structural analysis using X-ray diffraction and Raman spectroscopy confirm the coexistence of orthorhombic (O) (Amm2) and tetragonal (T) (P4mm) phases at room temperature. Using Rietveld refinement technique, a reasonable match between observed and simulated XRD patterns with the minimal value of agreement factors (R_p~5.52, R_wp~7.39, χ²~2.1) confirms a good fit for the chosen P4mm (65%) + Amm2 (35%) coexisting phases. In the Raman spectrum, the bands present at [E(TO+LO), B₁] ~300, A₁(LO) ~470 and [A₁(LO), E(LO)] ~719 cm^-1 represents the T phase of the material. A small peak at 190 cm^-1 and a wider peak at 185 cm^-1 indicates the O phase. Scanning electron microscope (SEM) micrographs show grains of random orientation and polygonal morphology with an average grain size of 14 μm. The formation of stoichiometric solid solution is ensured by the Energy-Dispersive Spectroscopy (EDS) spectrum. The dielectric permittivity (ε') as a function of temperature is measured in the cooling cycle at different frequencies from 20 Hz to 1 MHz. In comparison to BTO (T_c =393 K), T_c (C→T) has shifted to 330 K due to the simultaneous substitution of 5 at% Ca²⁺ and 9 at% Sn⁴⁺ ions at Ba²⁺ and Ti⁴⁺ sites, respectively. The highest value of ε_r = 12000 at Tc is observed at 1 kHz frequency. Dielectric loss (tan δ) is seen to be <6% over a broad temperature range of 100 K to 450 K. P-E hysteresis loops and S-E strain curves too were recorded at 30 kV/cm field and 10 Hz frequency in the temperature range 303 K to 380 K. The room temperature P-E curve with an enhanced value of P_r = 4.73 μC/cm², P_m = 16.76 μC/cm², reveals a typical FE behavior with complete domain switching. A maximum electro-strain of 0.12% is obtained at room temperature in a well-saturated “butterfly-like” S-E loop. The converse piezoelectric constant (d^*₃₃) calculated from the slope of S-E curve ~1113 pm/V is quite large in comparison with Pb-based FE materials. We inferred that large electromechanical properties are due to enhanced polarizability arising from the coupling between two equivalent energy states, i.e., the O and T phases, allowing favorable domain reorientation during the application of electric field. With the increase in temperature, P-E, as well as S-E loops, becomes hysteresis-free and the nonlinearity decreases across the phase transition temperature. Thus, P_r, E_c, S_max(%) and d^*₃₃ decreases with an increase of temperature, which is maybe ascribed to the reduction in the ferroelectric domain size. We have extracted the electrocaloric effect (ECE) from the polarization Vs temperature (P-T) curve by employing an indirect method for this composition. We have calculated the adiabatic temperature change ΔT_EC as a function of temperature. The broad span in ΔT_EC is associated with a quenched chemical disorder and relaxor behavior introduced by Ca/Sn-substitution in BTO ceramics. The maximum value of ΔT_EC is ~0.6 K (E = 30 kV/cm) at 360 K and ~0.5 K at room temperature. The corresponding value of the EC coefficient (ξ = ΔT_EC/E) ~0.17 at 300 K and ~0.2 K.mm/kV at 360 K are quite high in comparison to many other lead-free ceramics. The synthesized BCST composition is a good relaxor ferroelectric with high ECE (ΔT_EC = 0.5 K) and large linear strain (0.12%) with low E_c (0.70 kV/cm) in comparison with other linear dielectrics and normal ferroelectrics. These representative parameters make the synthesized BCST a useful Pb-free material suitable for displacement actuator, electrocaloric, and ac-field applications at room temperature.

Recently, a novel soft electrohydraulic actuator called “electrohydraulic actuator powered by induced interfacial charge (EPIC)” has been developed. [1] This actuator provides a high output force (16 times its body weight) at a relatively low operating voltage compared to existing electrohydraulic actuators such as the hydraulically amplified self-healing electrostatic (HASEL) actuator. [2] Based on the high performance of this actuator, various potential applications are expected, such as a soft varifocal lens, artificial muscles, and soft vibrotactile actuators. Indeed, a soft vibrotactile actuator based on EPIC technology has been developed and has achieved high acceleration performance (>3 G) over a broad frequency range (90–700 Hz). [3] However, despite the high potential of this actuator as a powerful soft transducer, it is plagued by durability issues, which are commonly encountered in the case of soft actuators.
In this work, we propose a novel approach to improve the durability of EPIC-based actuators. We have identified the main source of the durability problem as the viscoelastic characteristics of the polyvinyl chloride (PVC)-gel polymer film used in the EPIC actuator. Owing to the nature of its ionic intermolecular bonds, PVC-gel dissipates strain energy by breaking these bonds. This phenomenon can be described well by using the Maxwell model, in which an elastic component and a viscous component are connected serially. A material that follows the Maxwell model cannot maintain its internal stress when subjected to a step stretch owing to stress relaxation. The durability problem is probably caused by this relaxation phenomenon.
Therefore, we have developed a skin-inspired hybrid composite film to alter the mechanical properties of PVC-gel. The hybrid composite film is composed of a PVC-gel layer, an Elastosil P7670 layer, and a polyacrylamide (PAAM)-gel layer as an adhesive layer between the PVC-gel and Elastosil P7670 layers. Unlike PVC-gel, Elastosil P7670 is elastic owing to its covalent intermolecular bonds, meaning that this polymer can be described using the Voigt model, in which an elastic component and a viscous component are connected in parallel. Therefore, the Elastosil P7670 layer serves as an elastic layer in the hybrid film to compensate for the dissipative properties of the PVC-gel layer. These different materials were hybridized by means of benzophenone-assisted grafting polymerization of acrylamide (AAM). [4] We confirmed that this method can be utilized not only for fabricating hydrophilic–hydrophobic hybrids but also for fabricating mechanically different materials.
Finally, the fabricated PVC-gel and Elastosil P7670 hybrid composite film was compared with the PVC-gel film in terms of durability when used in an EPIC actuator. Each actuator was operated five times consecutively with frequency variation (1–400 Hz) at 2 kV. We compared the blocking force variation of these actuators at their resonant frequencies over five consecutive frequency-sweep runs. The results indicated that in the case of the hybrid film, the blocking force decreased by only 5.1%, whereas in the case of the PVC-gel film, it decreased by 18.4%. This result proves that the durability issues associated with the existing EPIC actuators are mainly related to the viscoelastic properties of the polymer film, and this durability problem can be solved by introducing an elastic component covering the entire strained area.
[1] H. Kim, J. Ma, M. Kim, J. Nam, K. Kyung, Advanced Intelligent Systems 2021, 3, 2100006.
[2] E. Acome, S. Mitchell, T. Morrissey, M. Emmett, C. Benjamin, M. King, M. Radakovitz, C. Keplinger, Science 2018, 359, 61.
[3] H. Kim, J. Nam, M. Kim, K. Kyung, IEEE Robotics and Automation Letters 2021, 6, 8245.
[4] H. Yuk, T. Zhang, G. Parada, X. Liu, X. Zhao, Nature Communications 2016, 7, 12028.

Is it possible to realize an assembler that can assemble material including cells into a defined shape of a 3D object? Here, I show our latest results using holograms to form defined pressure distributions in space with which it is possible to assemble material from a suspension. In order to realize a 3D assembler, one needs to solve a formidable inverse problem: how to encode an entire 3D object in the form of planar (2D) holograms that in turn can be used to recreate the object in space. This is a phenomenally complex optimization problem, because of the astronomically large number of computational degrees of freedom. Nevertheless, we have found practical solutions, and moreover, we have managed to use the 3D pressure distributions to assemble objects, including from living cells. Our most recent results offer the possibility to directly assemble cell aggregates and possibly entire organoid structures in ‘one shot’ from a suspension of cells. Our method is much faster than alternative schemes including 3D printing. Further, the assembled objects are “alive”. Our scheme even offers the chance to design and build more complex cell systems with a view towards obtaining robotic living matter.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) tin monosulfide (SnS), and tin disulfide (SnS₂) have gained much attention in various scientific applications due to their inherent piezoelectric characteristic, high electron mobility, excellent chemical stability, and wide accessibility to optoelectronic devices. In particular, inherent piezoelectric characteristics suggest the possibility that SnS and SnS₂ can be used in piezoelectric nanogenerators (PENGs) for scavenging resources. However, despite the theoretical and experimental investigation of these materials to possess excellent piezoelectricity, the piezoelectric output performance of the SnS- or SnS₂-based 2D thin-film piezoelectric nanogenerator is still relatively low, and the fabrication process is not suitable for practical applications.
In this report, we adopted the formation of the SnS₂/SnS heterostructure thin film for the enhanced output performance of piezoelectric nanogenerator (PENG) using atomic layer deposition (ALD). The piezoelectric response of the heterostructure thin film was increased by ∼40% compared with that of the SnS₂ thin film, attributed to the large band offset induced by the heterojunction formation. In addition, thickness-controllable large-area uniform thin-film deposition via ALD ensures that the reproducible output performance is achieved. As a result, the output voltage and current density of the heterostructure PENG were 60 mV and 11.4 nA/cm² at 0.6% tensile strain, respectively. The approach introduced here provides a facile strategy to effectively enhance the output performance of 2D thin-film PENGs.

Soft matter architectures for robots and electronics are having transformative impact on applications that require direct physical contact between machines and soft, delicate objects. These range from applications in industrial automation, such as high throughput food handling and packaging, to emerging wearable devices for physiological monitoring and therapy. Progress in the field has been enabled by the development of novel material technologies for actuation, sensing, and electronics that are mechanically compliant, elastic, and compatible with soft natural tissue. However, continued progress also depends on advancements in new material systems for providing power to these emerging classes of soft machines. This is especially true for wearable technologies used in computing, healthcare, and human-machine interaction. Such systems require on-board sources of energy to power sensors for tactile or physiological monitoring, actuators for haptic feedback, and electronics for signal processing and communication. For many applications, miniature battery technologies (e.g. coin cells, LiPo) are inadequate. Operation over sustained durations instead depends on larger batteries or methods of harvesting energy from the body or environment. Moreover, these energy systems must be soft and stretchable so that they can be worn on the body without constraining natural motion.

I will present efforts by my group to create soft material systems that can be used to generate and store energy in wearable applications. For the most part, these devices utilize eutectic gallium indium (EGaIn) as the functional material. EGaIn is a “liquid metal” alloy that has high electrical conductivity and is liquid at room temperature. Combining EGaIn and elastomers allows for the creation of soft and highly elastic transducers that can be used for electrochemical energy storage, voltage-controlled actuation, and energy generation using a wide variety of transduction mechanisms. This includes energy harvesting through electromechanical, thermoelectric, and triboelectric coupling. I will review our progress on each of these topics and present demos that highlight the potential role of these systems in future soft electronics and soft robotic applications. As time permits, I will also cover related work on other novel material architectures that exhibit unique properties for sensing, actuation, and circuitry through the use of EGaIn and other forms of soft matter.

Polymeric microdevices with multifunctional features like defined geometrical actuation or spatially segregated surface properties are inspiring materials with attractive potential applications in sensors, microfluidic systems, on demand carriers or biomedical devices. Here we aim at micro-objects allowing a sequential actuation at different geometrical level on demand. In this study, temperature-memory polymer based micro-cuboids were designed and fabricated as model system [1, 2]. The actuation should occur on the microscopic cuboid level as well as on nanoscale changes of cavities implemented on the cube's top side. The programming of the microcuboid was achieved by compression between glass slides with external force or indentation using atomic force microscopy (AFM) on the surface at certain programming temperatures T_prog. Micro-bowls were realized by indentation using a spherical tip on the surface of the microcuboid to achieve temporary nanocavities at a different T_prog. The geometry and surface structure of the microcuboids was analyzed from AFM height images. By varying T_prog and the sequence in the procedure, the step wise actuation on nanocavities and whole micro-bowl can be realized with reversible actuation of 2-5%. Besides, multiple nanocavities can be generated on the same micro-bowl to achieve sequential recovery and actuations by designing step wise programming at different T_prog. Finally, a demonstration of micro-bowl trapping and sequential elevating submicron particles was performed to proof the concept as on demand carriers. The technology presented in this work can inspire further design ofmultifunctional micro-objects in order to fulfill complex tasks with actuation functions on micro/nano-level.
1. Y. Liu, M. Y. Razzaq, T. Rudolph, L. Fang, K. Kratz and A. Lendlein, Macromolecules 50 (6), 2518-2527 (2017).
2. Y. Liu, O. E. Gould, T. Rudolph, L. Fang, K. Kratz and A. Lendlein, Macromolecular Materials and Engineering 305 (10), 2000333 (2020).

Magnetoelectric thin films can be used for various flexible electronics and biomedical applications but are limited by achieving strong magnetoelectric coupling at room temperature in a simple device. We report on the formation of a hybrid magnetoelectric system based on a new three-step grow-etch-fill process. First, a vertically aligned nanocomposite thin film containing a magnetostrictive CoFe₂O₄-based material and a sacrificial passive component (MgO) is grown. Then the MgO is etched out, to leave a mesoporous structure having 10’s nm dimensionality, and then a ferroelectric polymer PVDF-TrFE is coated onto the mesoporous structure.

Our study shows a novel approach to high-performance magnetoelectric composites. The films have advantages over previous composites of the same composition of having much higher density of interfaces for more effective strain coupling between the ferroelectric and magnetostrictive phases at directional, high-quality interfaces between the two materials. Also, the ability to more carefully tune and change the magnetic anisotropy in the CoFe₂O₄ films to design strong magnetoelectric coupling is demonstrated. The strain coupling and strain profile in these films were modelled using finite element analysis as a guideline to optimise the magnetoelectric effect in these films.

Active fibers can serve as artificial muscles in robotics or components of smart textiles. [1] Here, we present an origami hand robot, in which single fibers control the reversible movement of the fingers. The fiber actuators consist of extruded poly[ethylene-co-(vinyl acetate)] (PEVA) with 18 and 28 wt% vinyl acetate (VA) content which were afterwards covalently crosslinked by gamma-irradiation from 99, 132 to 165 kGy. [2] Broad melting transitions were obtained from 50 to 90 °C for cPEVA18 and from 40 to 80 °C for cPEVA28 while the thermal properties were not influenced by change in radiation dose. An increase in elastic modulus with decrease in elongation at break was observed when the radiation dose was increased. In cyclic, thermomechanical uniaxial tensile tests a high reversible actuation of ε_rev = 10±2% was observed for cPEVA28 fibers, while a lower ε_rev = 2±1% was obtained for cPEVA18 fibers. Here, the actuation was solely attributed to the crystallite orientation implemented in extrusion process. The feasibility of the fabricated thermally-controlled fiber actuators as muscles in robotics was demonstrated by deploying these fibers as actuators to trigger the movement of fingers in an origami style robot’s hand. A recovery/contracting force of 0.2 N with a work capacity of 0.175 KJ/kg was observed in cPEVA28 fibers, which could enable the bending movement of the fingers by contraction upon heating. The reversible opening of the fingers was attributed to a combination of elastic recovery force of the origami structure and crystallization induced elongation of the fibers upon cooling. The actuating fibers based on commodity thermoplastic materials may lead to readily deployable actuating materials for future robotics and prosthetic devices.
References
Maziz, A., et al., Knitting and weaving artificial muscles. Science Advances, 3, e1600327 (2017).
Farhan, M., et al., Origami hand for soft robotics driven by thermally controlled polymeric fiber actuators. MRS Communications, 11, 476–482 (2021).

Examples of soft functional composite structures with arrangements of active and passive materials of dissimilar properties are widely observed in nature. Biological organisms exploit the swelling and growth mismatch inherent in these structures to enable complex shape change and motion. Our work has been focused on developing computational models of stimuli-responsive hydrogel composite structures to investigate how soft active and stiff passive material segments can be arranged in two-dimensional and three dimensional structures to achieve targeted shape changes. More recently, we have also investigated how the sequence of shape changes in a multi-segmented, hydrogel composite structure can be coordinated to achieve locomotion. In this presentation, I will describe the experimental and computational modeling investigation of an untethered dual-segmented thermo-responsive bilayer hydrogel crawling robot. Direct ink write was used to fabricate a robot composed of 2 pNIPAM-pAAM bilayer segments with different thicknesses ratios joined by a thin pNIPAM segment. Cooling and heating the robot through the LCST produced a crawling motion with a consistent stroke for each temperature cycle. We developed a finite element model of the hydrogel robot to investigate the crawling mechanism. The chemo-thermo-mechanical of Chester et al. (2015) was applied to represent the coupled thermo-responsive swelling and mechanical deformation response of pNIPAM and a simple Coulomb friction model was used to describe the surface interactions of a crawling robot. The elastic properties of the hydrogel materials were measured using dynamic mechanical analysis, and imaging techniques were used to measure the transient and equilibrium swelling properties of the hydrogel to calibrate the parameters of the constitutive model. We found that the rapid deswelling of the pNIPAM gel and difference in deswelling kinetics between the bilayers of different thickness ratios produce crawling movement of the robot during heating above LCST. Parametric studies were performed to identify the effect of geometric and materials parameters on controlling the net displacement and temporal scaling of the robot. The net displacement of the robot over a thermal cycle increased with increased swelling and deswelling diffusivity, and elastic moduli ratio of leading bilayer. Simulation based analysis provided us a geometric and material design space for thermo-responsive soft robots for biomedical application in future.
References:
Chester, S.A., Di Leo, C.V., Anand, L., 2015. A finite element implementation of a coupled diffusion-deformation theory for elastomeric gels. Int. J. Solids Struct. 52, 1–18. https://doi.org/10.1016/j.ijsolstr.2014.08.015