While human tissues are mostly soft, wet and bioactive; machines are commonly hard, dry and biologically inert. Bridging human-machine interfaces is of imminent importance in addressing grand challenges in health, security, sustainability and joy of living faced by our society in the 21^st century. However, designing human-machine interfaces is extremely challenging, due to the fundamentally contradictory properties of human and machine. At MIT SAMs Lab, we propose to use electronics and machines based on tough and bioactive hydrogels to bridge the interfaces. On one side, bioactive hydrogels with similar physiological properties as tissues can naturally integrate with human body, playing functions such as scaffolds, catheters, drug reservoirs, and wearable devices. On the other side, the hydrogels embedded with electronic and mechanical components can control and response to external devices and signals. In the talk, I will present a general framework to design bioactive and tough hydrogels as the matrices for electronics and machines bridging human-machine interfaces. I will first discuss the fundamental principles to design tough bioactive hydrogels and tough bonding between hydrogels and diverse engineering solids including metals, glass, silicon, ceramics and polymers. I will then discuss large-scale manufacturing strategies to fabricate robust and bioactive hydrogels and their microstructures, including synthetic biology for new biopolymers and 3D printing for complicated microstructures.

With the ascension of polymer electronics and the move towards soft and flexible devices, there has been a growing demand for flexible, stretchable and transparent touch and proximity interfaces. Recent research uses ionically conductive polyacrylamide based hydrogel as electrodes [1] in a mutually capacitive touch sensor [2]. Here we introduce a mutual capacitance based proximity sensor that is flexible, transparent, inexpensive and can detect objects that are 20 mm above the sensor. The sensor is based on a criss-crossing matrix of ionically conducting polyacrylamide hydrogel electrodes containing 2.7 M sodium chloride that sandwich an acrylic elastomer acting as a dielectric. The overlap between the electrodes produces fringe fields that couple with nearby objects. If an object such as a finger or metal rod is in proximity, the total capacitance between the overlapped regions is seen to drop due to the cross-coupling of the fringe fields. The fabricated device demonstrates a lateral spatial resolution of 5 mm. A change in capacitance of up to 10 % is observed when an object is hovering over the sensor within distances of 20 mm. The vertical spatial resolution is 1 mm for a finger shaped object that sits 10 mm from the surface, and drops to 4 mm at 22 mm separation. As in the work by Sun et al [2] the capacitance increases upon contact. The next steps in making practical devices for soft robotic and other applications involve distinguishing stretch and bend related changes in capacitance from touch and proximity produced signals. Encapsulation is needed for many applications, particularly for wet gels. It is demonstrated that non-volatile ionic liquid containing gel electrode capacitors also show proximity and touch sensitivity, eliminating the need for encapsulation when used in dry environments.
References
[1]
C. Keplinger, J.-Y. Sun, C. C. Foo, P. Rothemund, G. M. Whitesides and Z. Suo, "Stretchable, Transparent, Ionic Conductors," Science, vol. 341, no. 6149, pp. 984-987, 2013.
[2]
J.-Y. Sun, C. Keplinger, G. M. Whitesides and Z. Suo, "Ionic Skin," Advanced Materials, vol. 26, pp. 7608-7614, 2014.

Although hydrogels have been studied for over a century, exploring their untapped potential as ionic conductors and mechanically tough materials only began in the last decade.[i],[ii] Other tunable properties include high innate stretchability and transparency. These properties allow hydrogels to complement a wide range of applications ranging from stretchable ionic circuits, wearable soft strain sensors, to implantable medical devices and instruments. Hydrogels have also been incorporated into heterogeneous materials to great effect, such as the hydrogel and dielectric elastomer structure that forms the basis of an ionic cable capable of long distance signal transmission.[iii] However, our previous studies have highlighted a deficit in manufacturing techniques applicable to such heterogeneous materials; therefore we aim to demonstrate that additive manufacturing techniques can be exploited to fabricate heterogeneous hydrogel materials.
In this study we focused on the application of extrusion 3D printing techniques to fabricate hydrogel-elastomer structures with sub-millimeter resolution, whilst maintaining desired electrical and mechanical properties. Unlike other 3D-printing techniques available, such as digital projection based techniques, extrusion-based manufacturing is capable of multi-material printing, in addition to having already demonstrated effectiveness in hydrogel manufacturing at relatively low costs. [iv] The hydrogel under study was a PolyAcrylamide gel with high salt concentration and a rheological modifying additive to optimize print quality that was cured with exposure to a UV light source. The elastomer under study was Polydimethylsiloxane. Basic electrical characterization was performed, including impedance measurements and power/charge transfer capabilities. As a demonstration of the viability of the technique, we verified both the functionality of the printed hydrogel/elastomer structure as an ionic cable and the scaling laws as proposed in a previous study.
[i] J.-Y. Sun, C. Keplinger, G. M. Whitesides, and Z. Suo, "Ionic skin". Adv. Mater. (2014) 25, 45, 7608-7614. DOI: 10.1002/adma.201403441
[ii] J.-Y. Sun, X. Zhao, W. R. K. Illeperuma, O. Chaudhuri, K. H. Oh, D. J. Mooney, J. J. Vlassak & Z. Suo, "Highly stretchable and tough hydrogels". Nature (2012) 489, 133-136. doi:10.1038/nature11409
[iii] C.H. Yang, B. Chen, J.J. Lu, J.H. Yang, J. Zhou, Y.M. Chen, Z. Suo, “Ionic cable”. Extreme Mech. Lett. (2015), doi:10.1016/j.eml.2015.03.001
[iv] S. E. Bakarich, M. in het Panhuis, S. Beirne, G. G. Wallace & G. M. Spinks, “Extrusion printing of ionic-covalent entanglement hydrogels with high toughness”, J. Mater. Chem. B 1, 4939-4946 (2013).

Current wearable electronics provide a number of functions that span from practical (time keeping) to medically relevant (EKG). Many of the newly emerging wearable health and medical devices contain sensors, yet few make use of the interface of the device and body for biosensing applications. The challenges for such interfaces are that they should be non-irritating, non-invasive and multifunctional. We will report the progress in using hydrogels in novel biomimetic device-body interfaces. We are studying how hydrogels can enable non-invasive passive sweat sampling and microfluidic transport, as well as EKG sensing. Sweat contains many biomarkers and the ability to harvest it non-invasively is appealing. Sweating is an osmotic process in which fluid is driven out of the body based on differences in salt concentration. Hydrogels can act biomimetically as an extension of this process to extract and collect sweat in one continual osmotic pathway. We tune the ionic strength and composition of the hydrogels to create an osmotic pressure gradient to passively pump fluid through skin. Interfacing these hydrogel patches with a microfluidic network allows us to continually sample the fluid being drawn from the body through passive “capillary-osmotic” pumping principles. By visually observing the microchannels, flowrates can be calculated and correlated to ionic strength. These ionic hydrogels also have increased electrical conductivity, which allows for EKG testing. We can interface the same hydrogels with a liquid metal (eutectic gallium indium) to create a truly flexible EKG electrode. We performed potentiostatic electrochemical impedance spectroscopy (PEIS) to electrically characterize the hydrogel and hydrogel-liquid metal interface. Prototype soft electrodes were then created and demonstrated.

Current technologies which monitor the digestive system are extremely invasive and expensive, usually comprising of materials which are potentially hazardous. Edible devices offer a potential safer, simpler and cheaper alternative to these technologies as they are constructed from digestible materials which are naturally consumed by the body after performing a specific task. Hydrogels display great potential for the creation of edible devices as they are commonly used in everyday food products, such as yogurt and ice cream, and via unique chemistries can be tailored to be strong and highly conductive. Using food grade gelatin powder, alginate and sodium chloride a “one-pot” synthesis method was developed to 3D print hydrogel electrodes which were not only safe to consume but achieved extremely high conductivities (150 mS/cm), compared to the literature, with water a content greater than 90% w/w. In addition to this, I will demonstrate the potential of our materials to construct edible devices, such as strain gauges, through additive manufacturing (3D printing) and details of the mechanical and electrical behaviour of these devices.

Next generation wearable robots will use soft materials such as textiles and elastomers to provide a more conformal, unobtrusive and compliant means to interface to the human body. These robots will augment the capabilities of healthy individuals (e.g. improved walking efficiency, increased grip strength) in addition to assisting patients who suffer from physical or neurological disorders. This talk will focus on two different projects that demonstrate the design and fabrication principles required to realize these systems. The first is a soft exosuit that that can apply assistive joint torques to synergistically propel the wearer forward and provide support to minimize loading on the musculoskeletal system. Advantages of the suit over traditional exoskeletons are that the wearer's joints are unconstrained by external rigid structures, and the worn part of the suit is extremely light, which minimizes the suit's unintentional interference with the body's natural biomechanics. The second part of the talk will focus on the development of a soft robotic glove for hand rehabilitation that consists of a wearable textile with attached elastomeric fluid-powered actuators specially designed to match the natural movements of the fingers and thumb. A component of the research is to develop the knowledge and techniques required to design soft multi-material fluid-powered actuators. These actuators are of particular interest to the robotics community because they are lightweight, inexpensive, easily fabricated with emerging digital fabrication techniques and capable of producing complex three-dimensional outputs with simple control inputs. This is accomplished via a multi-step molding process where some combination of fillers (e.g. cloth, paper, particles and fibers) is integrated into a soft elastomeric matrix to create anisotropy in the bulk material properties. Upon pressurization, embedded channels or chambers in the soft actuator then expand in directions with the lowest stiffness and give rise to linear, bending, and twisting motions.

The properties of selected biopolymers are such that they can effectively chaperone living cells through printing processes.
Here we will review the required properties of a bioink and reveal examples of biopolymers that induce appropriate behaviour in cell containing media. The ability to create gels from these biopolymers is a crucial feature. These gels must support cell growth and provide sufficient structure to enable us to build in 3 dimensions.
Gels based on gellan gum or alginate have proven particularly useful as have other polysaccharides extracted from seaweed. Gelatin also provides a low cost and reliable material for printing. We have investigated the use of ionic cross linking during printing to enable gel formation and to stabilise 3D structures based on such materials. For some biopolymers methacrylation and subsequent crosslinking induced by UV or visible light provides an attractive alternative.
Ongoing advances in printer design are also critical in expanding our cell printing capabilities. Here we will discuss the advantages that can be gained from the use of a coaxial printing approach.
Our ultimate goal is to print stem cells within 3D structures wherein the environment can be manipulated to control subsequent cellular development. Our progress towards this goal will be discussed.

Selection of a suitable scaffold material for bioprinting is critical in tissue engineering. By programming the precise recognition of DNA hybridization, here we designed an in-situ assembled polypeptide-DNA hydrogel and for the first time applied for 3D bioprinting. The two-bio-ink mixing deposition and rapid in-situ formation under physiological conditions make our hydrogel an excellent material for 3D ink-jet printing. Based on the dynamically crosslinked supramolecular hydrogel network, the printed structures via layer-by-layer assembly can merge and heal together, resulting in geometrically uniform and structurally precise constructs without obvious boundaries and defects. The no shrinking or swelling property and strong mechanical strength can support the printed desired shapes without collapse or deformation. Cellular biocompatibility and permeability of nutrients make it possible for cell printing to produce living-cell-containing structures with high viability and normal cellular functions. Designable biodegradability by dual-enzymatic responsiveness to protease and nuclease provides the possibility of complete removal of hydrogel networks on-demand from systems. These results demonstrate our hydrogel is a promising printing material for fabrication of desired complex 3D tissue-like constructs with controllably precise scale and highly cellular viability.
References
[1] C. Li, A. Faulkner-Jones, A. R. Dun, J. Jin, P. Chen, Y. Xing, Z. Yang, Z. Li, W. Shu, D. Liu, R. R. Duncan, Angew. Chem. Int. Ed. 2015, 54, 3957-3961.
[2] C. Li, P. Chen, Y. Shao, X. Zhou, Y. Wu, Z. Yang, Z. Li, T. Weil, D. Liu, Small2015, 11, 1138-1143.
[3] C. Li, M. J. Rowland, Y. Shao, T. Cao, C. Chen, H. Jia, X. Zhou, Z. Yang, O. A. Scherman, D. Liu, Adv. Mater.2015, DOI: 10.1002/adma.201501102.
[4] Y. Wu, C. Li, F. Boldt, Y. Wang, S. L. Kuan, T. T. Tran, V. Mikhalevich, C. Fortsch, H. Barth, Z. Yang, D. Liu, T. Weil, Chem. Commun. 2014, 50, 14620-14622.
[5] P. Chen, C. Li, D. Liu, Z. Li, Macromolecules2012, 45, 9579-9584.
[6] A. Faulkner-Jones, S. Greenhough, J. A. King, J. Gardner, A. Courtney, W. Shu, Biofabrication2013, 5, 015013.

Hydrogels are smart and multifunctional materials with a real potential for use in a variety of novel applications including soft robotics, sensors and bionic implants. Consisting of a highly swollen polymer network, hydrogels are typically soft and brittle which means that they are not compatible with many traditional techniques used to process raw materials into useful structures. This limitation makes handling hydrogels alongside other materials for the fabrication of functional devices difficult. 3D printing is an additive manufacturing technique that is becoming an increasingly popular method for processing hydrogels and is now commonly used for the purpose of constructing scaffolds for tissue engineering purposes. A feature of some 3D printers is the capacity to print multiple material inks within a single build or component. This feature gives an operator digital control of an object&’s material properties throughout 3D space by the positioning and blending of inks. Manipulation of this capacity provides a viable means to fabricate structures or devices that can harness the functionality of hydrogel materials.
We have developed a variety of extrusion printing techniques for processing hydrogel inks alongside other inks of structural polymers to create a range of composite architectures including fibre and particulate reinforcement. [1] The printing techniques use UV irradiation to simultaneously polymerise layers of the patterned inks to provide strong adhesion between the different phases. The material properties of these hydrogel composites are dependent on the volume fraction of hydrogel present and can be programed into a printed object through digital modelling software. The elastic modulus of the printed composite materials has been shown to range over several orders of magnitude and can be modelled with appropriate composite theory. With these printing techniques functional gradients have been built into 3D structures, as demonstrated by a prototype artificial meniscus cartilage.
Our printing techniques have been further developed to incorporate an ink for printing a stimuli responsive hydrogel that actuates in response to changes in its environment. This added functionality was achieved by incorporating the components for the formation a thermally responsive poly(N-isopropylacrylamide) network into a hydrogel ink. Combined this temperature sensitive hydrogel ink with the composite hydrogel printing techniques facilitate 4D printing and has been used to fabricate a smart valve device that can autonomously control the flow of water.[2]
[1] S. E. Bakarich, R. Gorkin III, M. in het Panhuis, G. M. Spinks, ACS Appl. Mater. Interfaces 2014, 6, 15998.
[2] S. E. Bakarich, R. Gorkin III, M. in het Panhuis, G. M. Spinks, Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201500079.

Drawing inspiration from nastic plant composition, structure, and movements, we create phytomimetic active matter composed of a hydrogel matrix and cellulose nanofibrils. Hydrogel-based architectures are printed and simultaneously encoded with localized, anisotropic swelling behavior through the controlled alignment of cellulose fibrils along prescribed pathways defined by theoretical modeling. These programmable architectures exhibit complex and dynamic shape transformations akin to those observed in flowers. Several motifs will be presented, including but not limited to those that exhibit bending, folding, twisting, ruffling and combinations thereof.

While high resilience of a materials requires low mechanical dissipation of the material under deformation, high toughness requires significant mechanical dissipation during crack propagation. Here we reconcile this pair of seemingly contradictory properties to design extremely tough and resilient hydrogels. We propose a resilient domain for hydrogels&’ deformation, below which hydrogels are deformed with low mechanical dissipation, but above which the deformation is highly dissipative. Therefore, hydrogels will appear resilient under moderate deformation within the resilient domain, but materials around crack tips will be deformed beyond the resilient domain and thus dissipate significantly to toughen the hydrogels. We implement the resilient domain by pre-stretching an interpenetrating-network hydrogel to damage the short-chain network to a controlled degree. The resultant hydrogel is highly resilient and compliant if deformed within the pre-stretched range (i.e., resilient domain), but highly dissipative if deformed beyond the resilient domain because of further damage of the short-chain network - achieving both high resilience of 95%, low shear modulus of 1 kPa and high toughness of 1900 J/m2. The hydrogels can be formed and printed into various shapes with different dimensions. Owning to their high resilience, low rigidity and high robustness, the gel can be conformably attached to different regions of human body. One particular application of the new gel system is as ultrasound transmission couplant in a number of applications such as ultrasound imaging, ultrasound therapy and continuous blood pressure measurement.

What can we do if water is a tough solid? A hydrogel aggregates water and a polymer network. The polymer network makes the hydrogel a stretchable solid, but water retains its exceptional physical and chemical properties. Several recent findings show that hydrogels can achieve properties and applications well beyond previously imagined. Most existing hydrogels, like Jell-O and tofu, are fragile and dry out in open air. We make hydrogels as tough as rubber, and retain water in low-humidity environment. We use hydrogels ionic conductors to transmit signals at high speed and over long distance. We show that hydrogels outperform existing fire-retarding materials. We also demonstrate fiber-reinforced hydrogels. This talk describes the mechanics and chemistry of these materials and applications.

Hydrogels are a popular choice for cartilage tissue engineering (TE) scaffolds due to their hydrophilicity. Closely mimicking the highly hydrated extracellular matrix of biological tissues increases the scaffold&’s biocompatibility. However, use of hydrogels within TE is limited by poor mechanical properties, preventing their use as a long term solution for cartilage damage. The properties of natural cartilage could be reproduced by successfully imitating the nanostructure of cartilage, creating a scaffold that models the extracellular matrix as a fibre reinforced hydrogel. The fibres mimic the nanosized collagen fibres within cartilage, and the hydrogel mimics proteoglycans and allows for the high water content of natural cartilage.
This work utilises electrospinning to create a continuous mesh of gelatin nanofibres that are crosslinked and stable in water. These fibres are then successfully infiltrated with a hydrogel to create a fibre reinforced composite. The tensile mechanical properties are recorded for the nanofibres alone, the hydrogel alone, and for the composite. The addition of aligned fibres to a hydrogel can increase the tensile strength by a factor of over 17 and the elastic modulus by a factor greater than six. The fibres not only improve the tensile properties in comparison to the hydrogel, but also increase the toughness. Mode I and III toughness tests are conducted in order to assess this. There are good models that predict the effects of fibre reinforcement within a composite. However these are difficult to apply to electrospun fibre reinforced hydrogels due to the uncertainty in parameters caused by the small length scale and variability in the method of manufacture. Mechanical properties of single electrospun fibres are used to calculate the upper and lower bounds of the composite properties and compared with the experimental results.

Network dynamics is a crucial factor that determines the macroscopic self-healing rate and efficiency in polymeric hydrogel materials, yet its controllability is seldom studied in most reported self-healing hydrogel systems. Inspired by mussel&’s adhesion chemistry, we developed a novel approach to assemble inorganic nanoparticles and catechol-decorated PEG polymer into a hydrogel network. When utilized as reversible polymer-particle crosslinks catechol-metal coordination bonds yield a unique gel network with mechanics controlled directly by interfacial crosslink structural dynamics. Taking advantage of this polymer-particle interfacial structure-property relationship, we next designed a hierarchically structured gel with two distinct relaxation timescales. By tuning the relative contribution of the two hierarchical relaxation modes, we are able to finely control the gel&’s dynamic mechanical behavior from a viscoelastic fluid to a solid yet fast self-healing hydrogel.

The stiffness of hydrogels is crucial for their applications. Nature&’s hydrogels become stiffer as they are strained. This stiffening is used, for instance, by cells that actively strain their environment to modulate their function. When optimised, such strain-stiffening materials become extremely sensitive and very responsive to stress. Strain-stiffening, however, is unexplored in synthetic gels since the structural design parameters are unknown. We recently reported one of the first synthetic hydrogels that is truly mimicking biogels in morphology and mechanical properties,¹ and uncovered how readily tuneable parameters such as concentration, temperature, polymer length and salt addition impact the stiffening behaviour.^2,3 Our work also reveals the marginal point, a well-described, but never observed, critical point in the gelation process.⁴ At this point, we observe a transition from a low-viscous liquid to an elastic gel upon applying minute stresses. Our experimental work in combination with network theory yields universal design principles for future strain-stiffening materials. Preliminary cell studies show the potential for application of these ‘smart&’ hydrogels in wound healing and tissue engineering.
References:
1. P. H. J. Kouwer, M. Koepf, V. A. A. Le Sage, M. Jaspers, A. M. Van Buul, Z. H. Eksteen-Akeroyd, T. Woltinge, E. Schwartz, H. J. Kitto, R. Hoogenboom, S. J. Picken, R. J. M. Nolte, E. Mendes, A. E. Rowan, Nature2013, 493, 651.
2. M. Jaspers, M. Dennison, M. F. J. Mabesoone, F. C. MacKintosh, A. E. Rowan, P. H. J. Kouwer, Nat. Commun.2014, 5, 5808.
3. M. Jaspers, A. E. Rowan, P. H. J. Kouwer, submitted.
4. M. Dennison, M. Jaspers, P. H. J. Kouwer, C. Storm, A. E. Rowan, F. C. MacKintosh, arXiv2014, 1407.0543.

Solid electrolytes in supercapacitors have been studied to solve the leakage issues present when utilizing liquid electrolytes and also to reduce the number of required components. However, with gel electrolytes the highly porous nature of the electrode has led to problematic air bubble formation with the addition of the high surface tension electrolyte. Through the use of vacuum and suction, the air bubbles can be removed after the deposition of the gel. Although, this results in a non-uniform surface topography and a film thickness that cannot be carefully controlled. Since the film thickness affects the setting time of the gel, the reliability of the supercapacitor fabrication process becomes problematic. Fortunately, enhanced control in the deposition of the electrolyte hydrogel film can be accomplished through spin coating. Even though spin coating is traditionally used for manufacturing thin coats of solution in submicrometer scale, the method can be adapted for processing high viscosity gels with adjustable layer properties.
In this work, thick yet even electrolyte films are prepared from a gelatin based hydrogel. The substrate used in this study is a flexible polymer film, upon which a current collector and electrode of graphite and activated carbon were previously blade coated. The gelatin based electrolytic gel was heated to 323 K and was deposited over the highly porous active area of the electrode. Furthermore, the substrate and glass carrier plate were equally warmed to 323 K to prevent rapid setting of the gel. Through the spin coating process, electrolyte film thicknesses up to 100 µm were reached with low angular velocities. Also, by utilizing a mask on the substrate uniform layers of gel with distinct edges were produced.

Hydrogels have been widely recognized as soft and wet materials. Novel strategies have been developed to create hydrogels with improved strength and toughness, and responsiveness to external stimuli with great potentials for applications to soft machines, actuators, artificial muscles, and medical devices, etc. Non-covalent interactions, including hydrogen bonding, hydrophobic association, metal ion coordination, electrostatic interaction, and supramolecular recognition, etc, have been used to completely or partly replace covalent bonds to create three-dimension networks. Therein, the non-covalent interactions not only impart reversible energy dissipation mechanism, but also entitle multiresponsiveness to the hydrogels.
This talk will overview our recent progresses in the design and synthesis of hydrogels with very high strength and toughness, and actuators based on these hydrogels. Inorganic nanospheres, nanorods, and nanosheets are exploited as multi-functional crosslinkers to adsorb or bond with hydrophilic chains, leading to hydrogels with very high strength, toughness, fatigue resistance, and/or self-healing. Such new concept hydrogels are introduced with functional groups including ionic monomers, amino groups, and/or host/guest units to create hydrogels reponsive to pH, ionic strength, electric field, and competitive molecules, etc. For example, vinyl-modified triblock copolymer micelles have been used as multi-functional crosslinkers for in situ copolymerization with neutral and ionic monomers. The resulted hydrogels show very high mechanical strength and toughness and reversibly response to pH and ionic strength changes. Such polyelectrolyte hydrogels are actuated by electric field in salt solutions. Besides, as amino groups are introduced into hydrogels, local coordination of amino groups with multi-valent metal ions results in changes in local crosslink density, leading to deformation and inhomogeneous swelling/deswelling of the hydrogels. As a result, the hydrogels are actuated. These features have been exploited to design and fabricate actuators with rapid response to scratch and move small objects.

Hydrogel materials present innovative solutions for problems presented by hard components in fields such as soft robotics, bionics and electroactive materials science. The preparation of an ionic-covalent entanglement (ICE) hydrogel of PNIPAm (poly(N-isopropylacrylamide)) and alginate in the presence of large quantities of carbon nanofibers is described. The resultant material is a robust, composite, high water-content hydrogel with thermal actuation properties due to the inclusion of PNIPAm. The carbon nanofiber network serves to introduce a percolative pathway for electrical and thermal conduction along with a four-fold strengthening of the hydrogel.
These complex hydrogels can be prepared through casting or additive manufacturing techniques such as extrusion printing and, due to their thermal actuation, may be described as 4D printed hydrogels. Potential applications of this tough hydrogel range from soft strain gauges, valves to artificial muscles and haptic sensors for soft robotics.

Ligaments are unique, wet biological tissues with high tensile modulus and fracture stress, combined with high bending flexibility. Developing synthetic materials with the same balance of properties is a significant challenge. Hydrogel composites made from high stiffness fabrics is a strategy to develop such unique materials; however, the ability to produce these materials has proven difficult, since common hydrogels swell in water and interact poorly with solid components, limiting the transfer of force from the fabric to the hydrogel matrix. We have overcome this problem and successfully produced extraordinarily tough hydrogel composites by strategically selecting a recently developed tough hydrogel that de-swells in water. These new composites, consisting of polyampholyte hydrogels and glass fiber woven fabrics, exhibit extremely high effective toughness (250,000 J/m²), high tear strength (~65 N/mm), high tensile modulus (606 MPa), and low bending modulus (4.7 MPa). Even though these composites are primarily composed of water-containing, biocompatible materials, their mechanical properties are comparable to high toughness Kevlar/polyurethane blends and fiber-reinforced polymers. Importantly, the mechanical properties of these composites greatly outperform the properties of either individual component. A toughening mechanism is proposed based on established fabric tearing theory, due to three events: 1) fiber-fiber friction, 2) fiber-hydrogel friction, and 3) energy dissipation within the hydrogel. Utilizing these energy dissipation mechanisms will enable the development of a new generation of mechanically robust soft composites materials. These results will be important towards developing soft biological prosthetics, and more generally for commercial applications such as tear-resistant gloves and bullet-proof vests.

Dynamic, reversible crosslinks have been shown to specifically control the mechanical properties of a wide variety of mechanically tough and resilient biomaterials. We have shown that reversible histidine-metal ion interactions, known to contribute to the strong mechanical properties and self-healing nature of mussel byssal threads, can be used to control and engineer the hierarchical mechanical properties of model polyethylene glycol hydrogels orthogonally from the spatial structure of the material.
Here, we explore the scaling relationships in our model networks to further expand our abilities to control the relative elasticity and energy dissipation on hierarchical timescales. We show that the elasticity is dominated by long-range entanglements, while the dissipation is controlled by the exchange kinetics of the transient crosslinks. Understanding the interplay between elasticity and dissipation allows us to rationally design high-strength hydrogels for specific states of dynamic loading.

It&’s widely known that mechanical property of hydrogels is crucial for their functional performance. However, due to the structural complexity, it is rather difficult to characterize mechanical property of hydrogels. In addition to nonlinear elasticity, it is further complicated by the time dependency due to viscoelasticity and poroelasticity. Furthermore, nanocomposite (NC) hydrogels, as one of the tougher hydrogels, are reinforced by fillers and/or extra bonds, which further increase the complexity of the structure. This, therefore, exacerbates the difficulty to characterize their mechanical properties.
This talk presents the investigation on mechanical behavior of NC hydrogels by taking hyperelasticity, viscoelasticity and poroelasticity into consideration. A unique experimental-analytical framework was established to characterize and model time-dependent mechanical property of a poly(ethylene glycol) diacrylate (PEGDA)/nano-silica NC hydrogel. Normal compression experiments were performed to obtain time-dependent mechanical responses. Hereditary integral assisted Prony series is used to characterize time-dependent mechanical behavior obtained from experiments. Mechanical parameters thus obtained from the model also provide intuitive physical meanings. In addition, the influence of nanoparticle upon viscoelasticity of the NC hydrogel was quantified and analyzed.
This work not only endeavored to explain comprehensive deformation mechanism of NC hydrogels, but also provided guidance to the design of time-dependent hydrogel applications such as cartilage, heart valve leaflets, artificial skin, etc.

Hydrogels are highly versatile materials that achieve a variety of unique properties. In particular, there are growing demands for applications that require toughness against repetitive deformation, such as molecular switches, tissue adhesives, artificial skins, artificial muscles, and flexible screens. However, the conventional design concept of hydrogels does not include ‘reliability&’, although robust hydrogels must maintain constant mechanical properties for an extended period of time in the working environment. Here, we present a new class of hydrogel with great reliability using a shorthand method. We designed and fabricated homogeneous amphiphilic co-network where hydrophilic segments (poly(ethylene glycol)) and hydrophobic segments (poly(dimethylsiloxane)) were alternatingly aligned, simply by mixing two polymer solutions. In an aqueous environment, our gel spontaneously formed a unique well-ordered structure, dissipating infinitesimal stress concentrations. Our results demonstrate that our new polymer gel can endure 3-fold stretching for 100 times without any mechanical hysteresis. To the best of our knowledge, such a polymer gel with a wide range of flexible deformation region and great toughness to endure repetitive deformation has not been reported yet. We anticipate that our new method will give new perspective to conventional design concepts of polymer gels, and that our indefatigable hydrogel will expand the scope of hydrogel applications in more unforgiving environment.

It has been recognized for some time that polymeric hydrogels are a class of “intelligent” materials, which respond to external stimuli, like temperature and/or pH. Moreover, biodegradable hydrogels are finding applications in medical applications like tissue engineering and drug delivery. That is, these ‘smart&’ biomaterials change properties in response to selective biological recognition events. Hydrogels are inherently weak materials, thus presenting hurdles to expand its applications. Therefore, we have synthesized hybrid PEG-based hydrogels reinforced at the nanoscale utilizing organic-inorganic polyhedral oligosilsesquioxane (POSS) nanocages. A series of hydrogels were synthesized utilizing thiolene chemistry and a photopolymerization process. Dyamic mechanical analysis (DMA), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and rheology, were utilized to determine structure, transition temperatures, microstructure and viscoelasticity of the resulting molecular networks. PEG is a semicrystalline polymer thus exhibiting a melting endotherm, and therefore defining an activation temperature for shape memory behavior. Moreover, the results showed that swelling and mechanical modulus of the hydrogels could be tuned by either controlling the molecular weight of PEG or the concentration of POSS. The fine tuning of hydrogel physical properties and shape memory behavior opens up the opportunity for smart shape changing scaffolds for application in tissue engineering.

3D printing is an increasingly important aspect of manufacturing in the modern world. Polymeric 3D printing is commonly used for rapid prototyping, but the inferior mechanical properties of 3D printed components compared to traditionally manufactured ones limits their use in many other applications. Built in thermal stress and poor interlayer bonding are common problems in 3D printing resulting from deposition of heated material onto cooled material and uneven cooling. The temperature profiles within a 3D printed component during printing were modeled to obtain insight into where these issues were occurring within components and how to mitigate them. Thermal diffusivity, ambient temperature, glass transition temperature of the printed polymer, nozzle temperature, and layer height were also examined as factors affecting printing and resultant mechanical properties. The observed relationships indicate built in stress and poor interlaying bonding can be minimized with the choice of appropriate printing parameters and use of polymers with properties amenable to printing with such parameters.

Immobilizing proteins into 3D matrices contrary to flat planar surfaces stabilizes them better and might facilitate the study of the interaction between proteins, antibodies and other biomolecules as well as the analysis of cellular responses to extracellular environment in a more physiologically relevant manner. Photocrosslinkable hydrogels have received significant attention in recent years as they provide not only highly hydrophilic 3D environment to promote protein stabilization and its interactions with analyte molecules, but also optically addressable to enable spatially controlled protein localization. Protein immobilization can be achieved mainly through covalent bonds, physical interactions, and physical entrapment. Even though physical entrapment without a need of any chemical modifications would offer a universal approach for protein immobilization, leaching of protein through the pores of hydrogel is the greatest challenge and the biggest limitation of this approach. Thus, here an alternative method is developed to provide better control on protein localization and immobilization using naturally existing reactive groups of proteins. To this end, a genetically encoded FRET based glutamate indicator protein (FLIPE) is modified with diacrylated poly (ethylene glycol) (PEGDA) by Michael-type addition between acrylate groups and the thiol side chains of the cysteine residues. SDS-PAGE and free thiol assay is used to check the conjugation efficiency. We optimize the molecular weight of PEGDA (300, 740, and 3400 Da) as well as concentrations of the photoinitiator (0.1, 0.5 and 1 % (w/w)) and monomer (10, 20, and 30 % (w/w)) in the precursor solution. Later, the precursor solution is grown at the distal end of an optical fiber to test the spectroscopic properties and characteristic bioactivities of proteins in the hydrogel network. Further optimizations on the applied irradiation parameters of light intensity, and exposure time are performed to improve the spatial resolution of 3D hydrogel tips. This study examines the capability of fabricating 3D hydrogel sensors covalently modified with a member of recently growing genetically encoded fluorescent biosensors, which can later be extended to all conformation-dependent protein biosensors and be used intracellularly for physiological and biological sensing purposes.

In the past few years, superabsorbent materials such as hydrogels have attracted considerable scientific attention owing to their excellent water-retention capability and wide potential of application in drug delivery, wastewater treatment, and agriculture. Recently, renewable natural resources have been proposed as alternatives to partially substitute conventional petroleum-based polymers in the synthesis of nanocomposite (NC) hydrogels. Particular interest has been put on cellulose due to its large abundance in nature, biodegradability, and low cost. Furthermore, the inclusion of layered double hydroxides (LDHs) into hydrogels can result in new nanocomposites with promising properties. This work was aiming at synthesizing a novel hydrogel composed of polyacrylamide (PAAm) and methylcellulose (MC), and investigating the inclusion effect of hydrotalcite (HT) (Mg₆Al₁₂(CO₃)(OH)₁₆middot;4H₂O) on their morphological, structural, and swelling properties. The NC hydrogels were obtained by in situ polymerization of acrylamide (AAm) at 25 °C in aqueous solutions of MC containing reconstructed HT. The N-N&’-methylene-bisacrylamide and the N,N,N&’,N&’ tetramethylenediamine were used as cross-linking agent and as catalyst, respectively. A series of NC hydrogels were prepared by varying the HT content between 10 - 50 wt.%. HT was previously calcined at different temperatures in order to analyze the polymer interleaved extent during the hydrogel synthesis. All materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and single pulse ²⁷Al MAS NMR. The swelling-deswelling behavior of the NC hydrogels in water was also investigated. It was found a remarkable increase in the swelling degree (SD) when the HT mass fraction in the hydrogels was 20 wt.%. The calcination temperature of HT also exerted an effect on SD. SEM observations revealed an organic-inorganic hybrid structure for all PAAm-MC/HT hydrogels. XRD analyses indicated the HT nanolayers within the hydrogel matrix and certain displacement and broadening of the basal spacing (d₀₀₃) for some hydrogel samples. ²⁷Al NMR results showed that different states of coordination of Al ions in the reconstructed HT are formed depending on the calcination temperature. The obtained hydrotalcite-loaded NC hydrogels presented super-absorption properties, thereby being promising hybrid materials for the development of smart materials for drug delivery and slow release applications.

Elastomers and gels which are synthesized by employing interpenetrating polymer networks on a molecular scale, possess attractive mechanical properties which include: extremely large deformations, very high fracture toughness, dramatic Mullins-effect with very large dissipation, and desirable rate-dependent response. This paper formulates a thermodynamically-consistent theory to model the large deformation response of interpenetrating polymer networks. A particular double-network hydrogel is synthesized and used to verify the applicability of our theory. We have conducted systematic experiments and analyzed the mechanical response of our double network hydrogel under three different large deformation modes: plane-strain tension, simple compression, and simple shear. A suitable set of material parameters for our constitutive equations is shown to quantitativley reproduce the experimentally-measured mechanical response under the three different modes of testing. We have numerically implemented our theory in a finite element program and checked the predictive capability of the numerical simulation methodology.

Gelatin based hydrogels present the advantages of a bio-compatible and physiologically degradable natural bio-polymer as they mimic the extra cellular matrix (ECM) mechanically and chemically. In order to create a stable scaffold at body temperature the gelatin matrix can be chemically cross-linked by catalytic agents such as Microbial Transglutaminase (MTG). The effects on the hydrogel properties of an enzymatic cross-linking by MTG are quantifiable, however the physical nature of the resulting matrix and its protein conformation remains unclear [1]. This study focuses on the network structure and topology of the cross-linked hydrogels. Rheology data first showed improvement of thermal stability of the gel function of crosslinking density. Elastic moduli of the gels were increased from 6110Pa for a softly cross-linked hydrogel to 9940Pa for a fully cross-linked hydrogel. The rubber elasticity theory (RET) as well as the Flory-Huggins Theory (FHT) were used to characterize fully cross-linked hydrogels exhibiting a homogeneous and percolated network. These theories were found to not properly describe the system&’s network in the case of partially cross-linked hydrogels. We developed a physical model based on the consideration of the persistence length of the biopolymer chains as well as statistical energetics studies to describe non-percolated hydrogel networks. Cryo-SEM imaging of the samples was performed which allowed a visualization and comparison between the network topography of the hydrogels in their natural state with the theoretical models developed to describe them. The theoretical models have been found to provide great agreement with experimental results. Images have also shown that MTG cross-linking of gelatin leads to strengthening of the network through reduction of the mesh size as well as conformational changes in the protein matrix. Natural gelatin configuration was shown to be transformed from a coiled, fibril like, structure to a mono stranded conformation upon cross-linking action. Even though chemically similar to the ECM these artificial scaffolds then become structurally novel to cellular life. Future work needs to be performed to evaluate cells behavior through interaction with these unique ECM-like scaffolds as well as assessing the feasibility of their design with technologies such as 3D printing.
This work was supported by the NSF inspire program.
[1] Yung, C.W., Wu, L.Q., Tullman, J.A., Payne, G.F., Bentley, W.E. and Barbari, T.A. (2007), Transglutaminase crosslinked gelatin as a tissue engineering scaffold. J. Biomed. Mater. Res., 83A: 1039-1046. doi: 10.1002/jbm.a.31431

Despite actuation being commonly reported as a potential application for pH-responsive hydrogels, there are few examples of refined macro-scale hydrogel actuators. pH-responsive hydrogels have many desirable actuation properties, such as high free strains, significant blocking stresses and the ability to directly convert chemical energy into mechanical work, but there are also many challenges facing their development. These challenges span from the fundamentals of characterising the swelling response to the engineering aspects of device integration and control. However, the potential for these compliant muscle-like actuator materials to be applied in the rapidly expanding field of soft robotics makes addressing these challenges a worthwhile endeavour. Here we present an overview of recent work addressing some of these challenges, including improved characterisation techniques, simplified modelling approaches, advances in balancing material toughness with actuation potential, optimised micro-structures for both load transmission and fast response, as well as a method for the integration of hydrogels into compact composite actuator devices. Finally and most significantly, novel control strategies based on photo-responsive and far-from-equilibrium chemical systems are introduced. Although further developments are still required for pH-responsive hydrogels to be considered as competitive actuators, this work shows that good progress is being made in their development.

To investigate the fracture process of double network (DN) hydrogels, we synthesized DN poly(acrylamide)/poly(acrylamide) (PAAm/PAAm) hydrogels, in which the first and the second networks are identical units. A linear polyelectrolyte was introduced via a molecular stent method to show a significant effect of reinforcement. The order of introducing linear polyelectrolyte was changed to obtain PAAm/poly(2-acrylamide-2-methylpropane-sulfonic acid) as molecular stent/PAAm (PAAm/St-PAMPS/PAAm) and PAAm/PAAm/St-PAMPS hydrogels. The swelling degrees of PAAm/St-PAMPS/PAAm and PAAm/PAAm/St-PAMPS hydrogels were higher than that of the PAAm/PAAm hydrogel. During the cyclic tensile test, the deformation of the DN in PAAm/PAAm/St-PAMPS and PAAm/PAAm hydrogels was homogeneous until the sample was completely destroyed. However, the first network of PAAm/St-PAMPS/PAAm hydrogel was initially broken, and energy was dissipated. The second network was then gradually broken with increasing strain. Meanwhile, PAAm/St-PAMPS/PAAm hydrogel had high failure strain and failure stress. The friction behaviors of PAAm/St-PAMPS/PAAm and PAAm/PAAm hydrogels were also investigated using a rotational rheometer.

Hydrogels are under intense development for biomedical applications such as scaffolds in tissue engineering and carriers for drug delivery. Most existing hydrogels are weak, brittle, and not very stretchable; furthermore they dry out as water evaporates. These issues have severely limited their scope of applications. Here we present our efforts to create hydrogels with enhanced mechanical properties and environmental stability with a goal of opening up new applications, ranging from stretchable transparent conductors to fire-retarding blankets.
We demonstrate how the use of permanent and sacrificial crosslinks results in hydrogels with vastly enhanced toughness, and present data for several types of sacrificial crosslinks. Depending on the nature of sacrificial crosslinks, hydrogels can also be made to self-heal. Tough hydrogels can be reinforced with fibers, thus providing a completely different avenue to engineer their mechanical response. A fiber-reinforced tough hydrogel does not fail by fibers cutting through the hydrogel matrix; instead, it fails by fibers pulling out of the matrix, dissipating energy in the process. Both stiffness and strength can be increased significantly using this technique. A fiber-reinforced gel can dissipate a significant amount of energy at a tunable level of stress. We will also discuss how the stability against drying out can be enhanced by controlling the chemical potential in water in the hydrogel through additives.

Hybrid combinations of hydrogels and solid materials including metals, ceramics, glass, silicon and polymers are used in areas as diverse as biomedicine, adaptive and responsive materials, antifouling, actuators for optics and fluidics, and soft electronics and machines. Although hydrogels with extraordinary physical properties have been recently developed, the weak and brittle bonding between hydrogels and solid materials often severely hampers their integrations and functions in devices and systems. Whereas intense efforts have been devoted to the development of tough hydrogel-solid interfaces, previous works are generally limited to special cases with porous solid substrates. The need for general strategies and practical methods for the design and fabrication of tough hydrogel bonding to diverse solid materials has remained a central challenge for the field. Here, we report a general design strategy and a simple fabrication method to achieve extremely tough and functional bonding between hydrogels and diverse solids including glass, silicon, ceramics, titanium and aluminum with interfacial toughness over 1000 Jm^-2. The new design strategy relies on a synergistic integration of moderate intrinsic work of adhesion on the interfaces and significant mechanical dissipation in the hydrogels during detachment. The fabrication method does not require porous or topographically patterned surfaces of the solids and allows the hydrogels to contain over 90 wt. % of water. The resultant tough bonding is optically transparent and electrically conductive. In addition, we demonstrate novel functions of hydrogel-solid hybrids uniquely enabled by the tough bonding including tough hydrogel superglues, hydrogel coatings that are mechanically protective, hydrogel joints for robotic structures, and robust hydrogel-metal conductors. The general strategy and simple yet versatile method opens new avenues not only to addressing fundamental questions on hydrogel-solid interfaces in biology, physics, chemistry and material science but also to practical applications of robust hydrogel-solid hybrids in diverse areas.

Hydrogels are continuous networks of polymeric chains and water where the solid fraction is small compared to the fluid, making them generally compliant and often brittle. One possible route to robust hydrogels from conventional polymers is the assembly of a bulk material from smaller components: micro- or nanostructured gels are composed of a large number of gel particles with a diameter much smaller than the bulk gels. When assembled into lattices, it is expected that the inter-particle bonding, rather than the strength of single polymer chains, will determine the toughness of the bulk gel.
In this work, the resistance to fracture of nanostructured gels was investigated as a result of different inter-particle crosslinking methods. Polyvinyl alcohol nanogels were fabricated by solution precipitation and assembled into bulk gels through a range of polymer-specific chemical and physical crosslinking methods. The mechanical properties, measured by indentation-based stiffness and strength testing, were compared to the ones for monolithic gels of the same polymer.
The mechanical response of the structured hydrogels was found to be superior to that of conventional gels: their toughness can be enhanced by the choice of inter-particle bonding. Their modulus of elasticity was found to be affected by the structured nature of the gels, increasing as the deformation of single particles is constrained. The permeability to fluid flow within the gels, which determines the time-dependent deformation of the materials, was also observed to be affected by the geometry of the lattice. The ability to tailor the properties of the nanostructured hydrogels, through the choice of inter-particle crosslinking method and particle size, makes this method a viable option for the fabrication of robust soft materials.

Chronic wound care is a major cost to national health systems worldwide [1]. Improved exudate management via the use of wound dressings is a recognized route to decreased healing times [2]. Despite largely applied in the clinic, however, wound dressing products still leave much to be desired in terms of hydrated mechanical properties and material customization [3]. To address this challenge, collagen was selected as a biomimetic building block for the formation of highly-swollen hydrogels with tunable water uptake, remarkable mechanical properties and bespoke material format [4]. Type I rat tail collagen was covalently functionalized with monomers of varied molecular stiffness, e.g. 4-vinylbenzyl chloride. The type and degree of collagen functionalization governed the structure-property relationships in photo-activated collagen networks, so that swelling ratio (SR: 707±51-1996±182 wt.-%), bulk compressive modulus (E_c: 30±7-168±40 kPa) and atomic force microscopy elastic modulus (E_AFM: 16±2-387±66 kPa) were readily adjusted and enhanced compared to a leading dressing product. Non-toxic hydrogels and mechanically#8209;stable water-swollen fibres could also be prepared from a GMP collagen source. In light of the remarkable hydrated mechanical properties, GMP collagen materials are currently being investigated in an in vivo wound healing study with diabetic mice.
[1] C.J. Phillips, I. Humphreys, J. Fletcher, K. Harding, G. Chamberlain, S. Macey (2015). “Estimating the costs associated with the management of patients with chronic wounds using linked routine data”, Int. Wound J., DOI: 10.1111/iwj.12443
[2] C. Dowsett (2011). “Moisture in wound healing: exudate management”, Br. J. Community Nurs. (16) S6-S12
[3] J.J. Schmidt, J.H. Jeong, V. Chan, C. Cha, K. Baek, M.-H. Lai, R. Bashir, H. Kong (2013). “Tailoring the dependency between rigidity and water uptake of a microfabricated hydrogel with the conformational rigidity of a polymer cross-linker”, Biomacromol. (14) 1361-1369
[4] G. Tronci, C.A. Grant, N.H. Thomson, S.J. Russell, D.J. Wood (2015). “Multi-scale mechanical characterization of highly swollen photo-activated collagen hydrogels”, J. R. Soc. Interface (12) 20141079

3D biomaterial scaffolds are promising materials for mimicking the natural environment of many cell types, which are normally integrated into well-organized and dense material environments in vivo. In particular, materials with high structural flexibility offer interesting possibilities, e.g. for mimicking the extracellular matrix and bone structures. Using 3D networks of ZnO tetrapods, we have developed a method to generate 3D biomaterials that contain interconnected maze-like channels in the micrometer and nanometer range. These materials mimic in vivo conditions in the extracellular matrix, where cells are embedded into a dense network of fibers and only occupy a tiny fraction of the space available. Importantly, we can independently define the overall shape and size of the scaffold, channel size, porosity, scaffold stiffness and surface functionalization. The interconnectivity of our hollow multi-channel structure is guaranteed by the fabrication procedure, and is independent of channel and pore density. Our novel method is based on hydrogels so that enough nutrients reach the cells in spite of the small channel diameters. In addition, the cells can be guided through these channels with the aid of chemotactic reagents. Cells can grow deep into such scaffold materials, and cell migration is controlled by channel diameter. Most importantly, cells reside for long periods in larger cavities inside the material and are thus captured by the system.

Four dimensional (4D) printing combines smart actuating and sensing materials with additive manufacturing techniques to offer an innovative, versatile and convenient method for crafting custom-designed sensors, robotics and self-assembling structures. Stimuli-responsive volume-change materials incorporated into multi-material structures can be harnessed to create movement in the same way that biological muscles achieve motion in animals and nastic movements are generated in plants. We have shown relatively fast and reversible, skeletal muscle-like linear actuation in 3D printed tough hydrogel materials and their incorporation into a smart valve that can control the flow of water[1].
As much as demonstrating the potential of 4D printing, this work has also highlighted some remaining challenges. The hydrogel of choice was an hybrid blend of ionically-crosslinked alginate and covalently crosslinked poly(N-isopropylacrylamide). These hydrogels are known to be tough [2], which is an important characteristic for load-bearing artificial muscles, and are compatible with extrusion-based 3D printing. However, these hydrogels also suffer rapid stress relaxation under load [3], which severely limits the stress generation capabilities of hybrid gel artificial muscles.
Modelling the stress-strain properties of hybrid hydrogels and also double network hydrogels provides insights into the molecular mechanisms responsible for their high toughness. Based on prior work [4], we show that the damage process occurring during loading involves a progressive scission of the shortest strands in the tight network [5]. The strand density of the intact gel network is progressively reduced and the contour length of the remaining next-shortest load-bearing chains became longer with increasing strain. The parameters of the Wang-Hong model, which applies a log-normal distribution to describe the probability distribution for the strand fracture of a double network hydrogel [4], were considered to describe the strand length distribution of the tight network. Using these insights we can speculate on network topologies that can provide high strain elasticity and toughness without excessive stress relaxation.
1. S.E. Bakarich, R. Gorkin III, M. in het Panhuis, G.M. Spinks, “4D printing with mechanically robust, thermally actuating hydrogels” Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201500079
2. J. Y. Sun, X. Zhao, W. R. K. Illeperuma, O. Chaudhuri, K. H. Oh, D. J. Mooney, J. J. Vlassak, Z. Suo, “Highly stretchable and tough hydrogels” Nature 2012, 489, 133.
3. H. Xin, H.R. Brown, S. Naficy and G.M. Spinks, “Time-dependent mechanical properties of tough ionic-covalent hybrid hydrogels” Polymer 2015, 65, 253-261.
4. X. Wang, W. Hong, “Pseudo-elasticity of a double network gel” Soft Matter 2011, 7, 8576.
5. H. Xin, H.R. Brown, and G.M. Spinks, “Molecular weight distribution of network strands in double network hydrogels estimated by mechanical testing”, Polymer.2014, 55, 3037-3044.

It has been shown that a muscle-like actuator can be created by constraining a cylindrical pH-responsive hydrogel within a stiffer fibrous braid. Modelling of this composite hydrogel actuator has shown that it is able to increase a hydrogel&’s useful axial work output by an order of magnitude. This improvement in hydrogel actuation performance, achieved without compromising the compliant nature of the hydrogel, suggests this concept may have great potential in the field of soft robotics. However, the slow diffusion-driven rate of response remains a challenge facing the development of macro-scale hydrogel actuators. To improve the rate of response a variety of techniques have previously been developed to create micro- and nano-dimensioned hydrogel structures, typically simple films, fibres or beads. Although these simple, easy-to-manufacture structures may be fast to swell/shrink in isolation, their actuation speed, elongation and force can be hindered by unfavourable deformations when grouped together in larger units. The work presented here employs finite element analysis (FEA) to show that for a composite hydrogel actuator, a complex structure composed of helical coiled vascules can provide superior actuator performance compared with a structure composed of more simple geometry. However, the work also reveals the requirement for such complex structure to be accompanied by material toughness. The relationship between structure, material toughness and actuator performance is investigated through the use of a tough interpenetrating network hydrogel. Findings outline required geometric and material properties for the creation of optimised hydrogel actuators, ultimately revealing the need for 3D printing techniques with the ability to create complex structures from tough stimuli-responsive hydrogels.

Stimuli-responsive hydrogels can drastically and reversibly change their volume in response to environmental changes. Poly(N-isopropylacrylamide) (PNIPAAm) is a widely used temperature-sensitive hydrogel that exhibits a large volume change depending on temperature. Taking advantage of this unique attribute, PNIPAAm hydrogels have been used in a broad range of applications such as microfluidic device, smart membrane and tissue scaffold. However, brittleness of the PNIPAAm hydrogel has been a major issue for further application, especially when the deformation causes large mechanical stress which often leads to material failure.
Much effort has been made to improve toughness of hydrogels. It has been reported that introducing ionic crosslinking to covalently cross-linked polymer network to form hybrid double-network significantly enhances toughness of the resulting hydrogel. Recently, this principle was applied to photo-curable polymers to create mechanically robust hydrogel, which was then three-dimensionally fabricated using nozzle-based 3D printers. However, strictly limited by the size of the extrusion nozzles, smallest feature size of the 3D printed hydrogel structures still remains ~100 µm or above. This manufacturing limitation impedes exploration of this material in micro-scale where mechanically, physically, and biologically more interesting phenomena can be found and studied.
Here, we present 3D micro-fabrication of temperature sensitive and tough PNIPAAm hydrogel using projection micro-stereolithography (PµSL). PµSL is a lithography-based additive manufacturing technique using a digital dynamic photo-mask. Using photopolymerization, highly complex 3D micro structures can be rapidly fabricated with resolution of ~5 µm. Photo-curable PNIPAAm solution including alginate is first 3D printed using Pmu;SL to form a desired three-dimensional micro structure. Then ionic crosslinking for alginate is subsequently induced by allowing ionic crosslinker to permeate through the covalently crosslinked polymer network. In this way, both micro-scale 3D printing and toughness enhancement are achieved for PNIPAAm hydrogel. Material-process-property relationship is established by conducting Pmu;SL process parameter study as well as mechanical characterization of the resulting hydrogels. Based on this result, large, reversible, and temperature responsive deformation without material failure is demonstrated in a highly three-dimensional PNIPAAm hydrogel micro structure.
With versatile manufacturing capabilities of PµSL, various reconfigurable and mechanically robust 3D micro devices and structures can be developed into novel applications. The applicability of designed 3D hydrogel bolsters its potential for a variety fields, such as soft robots, biomedical micro-devices, and tissue engineering.

Rapid prototyping of new hydrogel-based structures, including tissue scaffolds, organs-on-chip, and hybrid bioelectronics materials, requires flexible fabrication techniques capable of positioning multiple materials in 2D and 3D with micro-scale resolution and accuracy. Despite their impressive accomplishments, current methods for micro-scale additive manufacturing are limited to a single material (e.g. projection micro stereolithography) or require sequential or parallel use of multiple nozzles to deposit different materials. An alternative approach would be to digitally place discrete building blocks, e.g. microbeads or cells within a 3D matrix such as a hydrogel. This strategy would enable control over both the global and local properties of the fabricated part.
We have invented and demonstrated a new micro-scale additive manufacturing process, enabling local and regional control over the placement of microscale objects in a 3D matrix. This method uses feedback from a high-speed machine vision system to guide the placement of microscale objects using a digital light processing device (DLP). The 3D part is designed using standard software (e.g. Solidworks, etc.) and sliced into layers and then voxelized into print locations using custom software. As microbeads are delivered in a fluid flow over the part surface, the software uses the image feedback and forward trajectory prediction to project a pattern to photoanchor the building block into a predefined XYZ location in space. We demonstrate the use of this system to pattern arrays of polystyrene microbeads and hepatocyte aggregates within a PEG-DA hydrogel matrix, and assess the viability of the resultant solids under culture conditions. We derive the relationship between printing resolution and throughput, and assess the influence of the thermal and optical limits of the system, and the biocompatibility of the hydrogel on this performance. Moreover, we optimize the system to accurately track and anchor particles with an accuracy of ~10 mu;m. Challenges to this method include clogging and clumping of building blocks, printing high packing densities of objects, and false positive/negative tracking events. Nevertheless, our results show promise to enable on-demand additive manufacture of soft material structures with digital three-dimensional placement of discrete building blocks.

Polyrotaxane is a supramolecule consists of a larger number of macrocycles where they can slide and rotate through the long polymer axle and presence of bulky end groups in the polymer axle prevent the dethreading of these macro cycles. Here, we focused on the exploitation the fascinating properties of polyrotaxanes in syntheses of polymer networks, more specifically, hydrogels and elastomers. The hydroxyl groups of α-Cyclodextrins or its derivatives of polyrotaxanes were modified by isocyanate monomer to obtain the polyrotaxane-based hydrophobic and hydrophilic type cross-linkers, MPRs and MHPRs, respectively.
Stimuli-sensitive hydrogels changing their volumes and shapes in response to various stimulations which have potential applications in multiple fields. However, these hydrogels have not yet been commercialised due to their poor mechanical properties. The fixed cross-links in the polymer networks of the conventional hydrogels cannot avoid the localization of the stress to a short polymer chains and soon the gel looses its mechanical integrity. We can prepare extremely stretchable thermosensitive N-isopropylacrylamide hydrogels with good toughness by using polyrotaxane derivatives composed of α-cyclodextrin and polyethylene glycol as cross-linkers and introducing ionic groups into the polymer network. The ionic groups help the polyrotaxane cross-linkers to become well extended in the polymer network. The resulting hydrogels are surprisingly stretchable and tough because the cross-linked α-cyclodextrin molecules can move along the polyethylene glycol chains. In addition, the polyrotaxane cross-linkers can be used with a variety of vinyl monomers; the mechanical properties of the wide variety of polymer gels can be improved by using these cross-linkers. However, polymer gel mostly contains solvent, leakage and evaporation of the solvent is common under open condition and it gradually dries. We also focused on the fabrication of solvent free polymeric networks using Oligo (ethylene glycol) methyl ether methacrylate (MEOxMA) molecules. MEOxMA has a ethylene oxide chain in the side chain and behave like a liquid. Therefore, a highly stretchable three-dimensional poly(MEOxMA) elastomers can be formed using a MHPR . The length of the side chain of MEOxMA and MHPR concentration have great impact on the thermal and mechanical properties of the MEOxMA slide-ring elastomer.
Our results indicate that using a small amount of a supramolecular building block as a cross-linker in a polymer network results in dramatic changes in its mechanical properties. Using the preparation methods described here, which are simple, the mechanical properties of various polymer gels and elastomers can be improved. Thus, hydrogels or elastomers with good mechanical performance and controllable functionality can be produced for drug delivery systems, tissue engineering, artificial muscles, and sensors.

Material science has been a leading influence in the rapidly developing field of Soft Robotics. This field has advanced so much in the last five years that it is now appropriate to develop subclasses of Soft Robots. In this talk, I define a sub class that is soft regardless of architecture - they are Intrinsically Soft. The materials used to make the sensors and actuators comprising these systems are organic elastomers and are ideally suited for use in many additive manufacturing systems. In this talk, I will describe extrusion and stereolithography of sensors and actuators using elastomeric material, including hydrogels, and set the results in context of the field of Soft Robotics.

Three-dimensional (3D) cultuere cell scaffolds, which can mimic the natural extracellular matrices, have enormous potential in biomedical engineering applications because they can provide a promising tool for the controlled release of drugs and the study of cell interactions. The supramolecular hydrogel can be used potential 3D cell scaffolds due to their flexible adjusting of physical and chemical properties.
In these studies, we have synthesized two kind smart supramolecular hydrogels scaffold including photo-sensitive and pH-sensitive respectively based on poly(methyl vinyl ether -alt-maleicacid) (P(MVE-alt-MA)) formed by host-guest interaction and applications for three-dimensional (3D) culture of ovarian cancer cells SKOV3. The photo-sensitive hydrogel were prepared by inclusion complexes between trans-azobenzene (Azo) groups and cyclodextrin cavities from azobenzene grafted P(MVE-alt-MA) and β-cyclodextrin (β-CD) grafted P(MVE-alt-MA). The hydrogels are sensitive to light because of azobenzene unique characteristic and can provide a convenient way to separate cells and materials so that study cell behavior in the hydrogel during different stages (shown as Figure1 ). The other pH-sensitive hydrogel prepared by inclusion complexes between adamantane groups and cyclodextrin cavities from adamantane grafted P(MVE-alt-MA) and β-cyclodextrin derivative were also bound to P(MVE-alt-MA). The hydrogels are sensitive to pH value because of the carboxylic acid groups of polymer backbone. In addition, The pH-sensitive hydrogel have self-healing performance as shown by rheological properties. In vitro 3D cell culture experiments proved that the two hydrogels are biocompatible and support cell proliferation.
Our results suggest that the two smart supramolecular hydrogels scaffold based on P(MVE-alt-MA) could potentially provide a good platform for the study behavior of ovarian cancer cells after further improvement.

We present a new class of injectable hydrogels formed from the flow-induced gelation of microfiber suspensions. Monodisperse, concentrated, aqueous suspensions of high aspect ratio, flexible poly(ethylene glycol) microfibers were generated using microfluidic methods. When subjected to certain flow conditions, such as extrusion from a needle, the suspension of fibers entangles irreversibly and forms a network. To study the flow conditions that lead to the formation of entanglements in the fiber suspensions, we conducted shear rheology experiments, and observed that the onset of shear thickening behavior and the development of entanglements depend primarily on the concentration of the suspension and the shear rate. Furthermore, the entangled microfiber network exhibits mechanical properties typical of a hydrogel, and it absorbs and swells in water, another distinctive feature of hydrogels. This flow-induced process is a simple mechanical approach to hydrogel formation that does not depend on chemical reactions. Using microfluidics, we are able to control the physical properties of the fibers, such as fiber dimensions and modulus. In addition, with microfluidics, different cargos, such as drugs and cells, can be readily encapsulated in the fiber structure, and so we propose that these microfiber suspensions are potentially useful material candidates for in situ scaffold fabrication in bioengineering applications.

We have recently developed a new technique for creating hybrid polymer hydrogels wherein two or more gels with distinct chemical formulations are combined into the same material. In our approach, the unique character of each individual gel is preserved in its specific zone, and the different zones are covalently linked to each other, thereby ensuring robust interfaces. The utility of this approach is that it can give rise to materials with unique properties. A target material from nature in this context is the “squid beak”, which has different portions with very different mechanical properties: specifically, one end of the squid beak is hard and strong while the opposite end is soft and pliable. To realize a gel with such diverse properties, we choose different monomers for different zones in our hybrid gel design. In one zone, we combine a conventional acrylate monomer with a silica precursor, and the resulting gel is structured with nanoscale silica particles. Thereby its elastic modulus is about 1000 times that of an adjacent zone composed of the acrylate monomer alone. We use dynamic rheology and scanning electron microscopy (SEM) to characterize these hybrid biomimetic gels.

Cartilage is a tough load bearing tissue that has many biological functions. It absorbs shocks during loading and distributes the load applied to the articular ends of bones. The increase in water content affects the compressive stiffness of cartilage by reducing its ability to bear load. Increased hydration and loss of stiffness are early indicators of irreversible tissue degeneration. Knowledge of the swelling behavior of cartilage is essential to understand its biological function. Cartilage swelling is governed by the thermodynamic interactions between its constituents. The main macromolecular components of cartilage extracellular matrix are collagen (10-30% of the wet weight of healthy tissue), proteoglycans (4-10%), and water containing dissolved electrolytes (60-85%). Proteoglycans (PGs) consist of a protein core to which linear polysaccharide chains (glycosaminoglycans) are attached through covalent bonds. The negatively charged PGs generate an osmotic swelling pressure within the tissue, which is balanced by the collagen network. In cartilage the major PG is aggrecan, which binds to hyaluronic acid and forms large aggregates. In the bottlebrush-shaped aggrecan molecule chondroitin sulfate and keratan sulfate chains are tethered to a core protein. Relatively little is known how the molecular and supramolecular organization of PG assemblies influences the macroscopic properties of the tissue, such as its compressive resistance and load bearing capacity. To better understand the function of cartilage at the tissue level, we studied its osmotic and mechanical properties using complementary macroscopic and microscopic techniques. We developed a multiscale experimental approach to determine the physical properties of the main macromolecular components of cartilage extracellular matrix by combining macroscopic techniques (osmotic pressure measurements and mechanical tests) with scattering methods (light scattering, small-angle neutron and X-ray scattering) that probe the static structure at higher resolution. The dynamic response of proteoglycan assemblies is investigated by rheological measurements and dynamic light scattering.

Using intelligent materials to perform accurate strain sensing has been a plight for materials chemists for a very long time. Strain gauges have many different uses, including remote actuation, rehabilitation of damaged joints, and sensors on mechanical moving parts. One predominant type of strain gauge that is widely used is to 3D print conducting ink onto a flexible hydrogel substrate (polydimethylsiloxane, PDMS), and to then monitor the resistance of the tracks change with respect to how strained the sample has been. I shall be presenting a hydrogel-based strain gauge that has been fully fabricated using 3D printing, with the conducting ink consisting of a co-dispersion of hollow vapour-grown carbon nanofibres and carbon black, and the polysaccharide gellan gum. The design shall be discussed in detail (including the reason for requiring two conducting carbon fillers and their synergistic effects), the dispersion method and optimisation, as well as other unique factors within the inks&’ structure. I will present a comparison between the performance of this hydrogel strain gauge with other developed strain gauges, and its potential application in soft robotics.

3D printing has emerged as a promising technology for complex tissue engineering due to flexible design capabilities. However, a severe limitation to the growth of bioprinting is the lack of inks and the ability to tune mechanical, biological, chemical, and physical properties of the material without compromising “printability”. In this work, we present a single ink method capable of producing extrudable, soft hydrogels of various formulations in which polymer concentration, material type, degree of cross-linking, and type of cross-linker can all be manipulated. In this method, polymer solutions are lightly cross-linked with a polyethylene glycol (PEG) functionalized with reactive groups (PEGX). PEG is an ideal cross-linker since it can be easily synthesized into many physical and chemical variants to produce a variety of ink properties and versatile cross-linking chemistries that will be necessary for expanding the number of 3D printable hydrogels for biofabrication as well as other applications. Phase plots of varying polymer concentrations against varying PEGX:polymer mass to mass (m:m) ratios were performed to screen formulations and identify candidate hydrogels with 3D printable consistencies (i.e. soft, lightly cross-linked). Material “phase“ (i.e. solution, soft or robust gel) was determined by tube inversion, and gels were qualitatively assessed for consistency with a spatula. Soft gels found from phase plots were able to be extruded through fine diameter nozzles (200 µm) and were shape-maintaining and self-supporting. Soft gels of both synhethic and natural polymers were 3D printed into well-defined, multi-layered structures. To probe mechanical characterisitics associated with printability, rheological studies of printable and unprintable formulations uncovered differences in yield strains and stresses. From these differences, we hypothesize that 3D printing of these hydrogels is possible by local yielding/rupture at the nozzle wall. Printable gels having low yield stresses and high yield strains are able to break at the nozzle and extrude, whereas unprintable gels having high yield stresses and low yield strains are unable to break and therefore, unable to extrude. This indicates that yield stress and strain are measurable parameters that can be correlated to printability. To investigate the utility of this method for biofabrication, multi-material and cell printing were demonstrated. Multiple types of materials were successfully co-printed, illustrating the ability to print single 3D constructs with heterogeneous properties. Cells can also be mixed within polymer solutions prior to PEGX cross-linking to produce printable cell-laden hydrogels with good cell viability. We believe this method will contribute to advancing hydrogel additive manufacturing, specifically for biofabrication of compositionally and structurally complex structures and functional tissues.

In tumor treatment, it is highly desirable that drugs can be focusedly delivered to the tumor core, and that nutrients and oxygen can be selectively proofed from the tumor region to hunger the tumor. To prove the key concept, we design 3D biomimetic hydrogel structures that can freely control the diffusion of small molecules within the structures; for example, nanoparticles can be rapidly transported to a small region, or can be cloaked from a small region. The hydrogel structures are fabricated with projection micro-stereolithography that constructs the structures with multiple distinct hydrogel components. The diffusion focus and cloak are achieved by the interaction of the hydrogel components with different diffusion properties. We also construct theoretical and computational models that agree well with the experimental results. The proposed biomimetic hydrogel structures are expected to provide implications for better tumor therapies and artificial organs or tissues.

Soft materials including elastomers and gels are pervasive in biological systems and technological applications. Whereas it is known that intrinsic fracture energies of soft materials are relatively low, how the intrinsic fracture energy cooperates with mechanical dissipation in process zone to give high fracture toughness of soft materials is not well understood. In addition, it is still challenging to predict fracture energies and crack-tip strain fields of soft tough materials. Here, we report a scaling theory that accounts for synergistic effects of intrinsic fracture energies and dissipation on the toughening of soft materials. We then develop a coupled cohesive-zone and Mullins-effect model capable of quantitatively predicting fracture energies of soft tough materials and strain fields around crack tips in soft materials under large deformation. The theory and model are quantitatively validated by experiments on fracture of soft tough materials under large deformations. We further provide a general toughening diagram that can guide the design of new soft tough materials.

The live tissue is a complex hydrogel-like material, whose functions, such as muscle actuation and neural transduction are accomplished by ionic currents and metabolite transport through vascular “microfluidic” networks. We will discuss how the principles of ionic transport and osmotic actuation can be transcribed into hydrogel-based biomimetic devices. Earlier, we reported a new class of gel diodes with rectifying junction formed by interfacing water-based gels doped with polyelectrolytes of opposite charge and showed how water-based gels doped with polyelectrolytes can be used as the core of novel photovoltaic cells. We will discuss how these concepts could be applied in the making of bioinspired “artificial leaves” with microvascular channel networks embedded in permeable hydrogel, and will illustrate them with experimental and modelling results of self-regenerating dye sensitized solar cells and leaf-like photocatalytic hydrogel reactors. We will then discuss how ionic transport, electrostatic crosslinking, and osmotic pressure gradients can also be used in novel hydrogel actuators and soft robotic components. The first example is the "ionoprinting" technique that allows directed and reversible gel bending by directional ion injection in the hydrogel. A second type of hydrogel actuator can be built on electroosmotic ion redistribution through a bi-gel system. Finally we will present the ongoing work on hydrogel devices combined with colloidal assemblies. These biomimetic hydrogel actuators may have future applications in new types of "grabbers", "walkers" and other soft robotic components.

The fabrication, characterization and operation of untethered, environmentally responsive microgrippers will be discussed. In these soft-robotic microdevices, a stimuli responsive hydrogel such as poly(N-isopropylacrylamide) is paired with a rigid non-swelling polymer to create reversible self-folding devices. Environmentally responsive swelling and de-swelling of the hydrogel is used to power opening and closing of multi-fingered microgrippers. Using simulations and experiments, the self-folding characteristics of the devices are investigated. Further, by incorporating magnetic nanoparticles, the devices can be precisely manipulated from afar using magnetic fields to perform tasks such as pick-and-place and micro-assembly. We anticipate widespread use of these devices in minimally invasive surgery, drug delivery and soft-robotics.

Hydrogels are useful for various industrial and biomedical applications such as tissue engineering, drug delivery and immunotherapy. In tissue engineering, hydrogel was used as ECM (extracellular matrix) for stem cell differentiation. The mechanical properties of hydrogels are an important aspect of their ability to fulfill the requirements of various applications. To overcome these limitations, the fabrication of hydrogels synthesized by many methods has been studied. For example, gelatin/bacterial cellulose or alginate/polyacrylamide hydrogels exhibited much stronger than their single-network counterpart hydrogels^1-3. Although there have been several successful approaches to improving the mechanical properties of hydrogels, it is still challenging to achieve a very high stiffness above a certain limit. Here, we propose a facile process, a remodeling of polymer networks followed by a secondary cross-linking (RsC process), to significantly enhance the mechanical strength of alginate/polyacrylamide hybrid hydrogels. By utilizing high stretchability of alginate/polyacrylamide hydrogels, the polymer network was first linearly remodeled by stretching and fixed by the subsequent ionic cross-linking of alginate chains. The stretched polymer network fixed by RsC process resulted in anisotropic property of the hydrogel. to the resulting hydrogel showed unprecedented, extremely high stiffness up to 5.3 MPa of elastic modulus that is about 300 times higher than that of the original hybrid gels with same polymer composition. This simple approach can make it possible to increase the upper limit of hydrogel stiffness under a defined polymer composition, which may be beneficial in many applications including bone regeneration, artificial tissue construction, or other mechanical studies of hydrogels.
References
1. J. Sun, X. Zhao, W. R. Illeperuma, O. Chaudhuri, K. H. Oh, D. J. Mooney, J. J. Vlassak, Z. Suo, Nature 2012, 489, 133-136.
2. C. H. Yang, M. X. Wang, H. Haider, J. H. Yang, J. Sun, Y. M. Chen, J. Zhou, Z. Suo, ACS Appl. Mater. Interfaces 2013, 5, 10418-10422.
3. Nakayama A., Kakugo A., Gong J. P., Osada Y., Takai M., Erata T., Kawano S., Advanced Functional Materials, 2004, 14, 1124-1128.

Gelatin based hydrogels provide great biocompatibility and biodegradability due to their Extra Cellular Matrix (ECM) like nature. The properties of these hydrogels can be controlled by addition of cross-linker and photo-initiator to stabilize, the gelatin network upon light exposure, both mechanically and thermally. However, due to gelatin&’s viscosity, chemistry and structural specificity it is challenging to design these materials through automated 3 Dimensional scaffolding techniques such as 3D printing. Kolesky et al[1] have shown the ability of using methacrylate cross-linked gelatin (GelMA) to produce 3D printable ink through UV irradiation, but the procedure applied is lengthy and costly. This work focuses on the development of a novel technique to produce these hydrogels in a faster, safer and easier way. We showed that the lyophilization process, used today in these systems, has shown to be necessary for the bio-compability of the ink due to the toxicity of the unreacted methacrylate group still present in the ink if it is not filtered. Human dermal fibroblast cells were platted on samples and did not prosper healthily on the GelMA samples. We developed a novel ink, unique both in the chemistry used as well as the design process, that provides the user with a faster production time thanks to the suppression of the lyophilization step. The use of a different cross-linker provides an advantage on the biocompatibility of the ink and the safety of the operator while showing similar mechanical strength as the current formulation through rheology. 3D printing of this novel ink has shown to be possible, but needs further implementation to provide the user great control and precision in the process.
[1] Kolesky, D. B., Truby, R. L., Gladman, A. S., Busbee, T. A., Homan, K. A. and Lewis, J. A. (2014), 3D Bioprinting of Vascularized, Heterogeneous Cell-Laden Tissue Constructs. Adv. Mater., 26: 3124-3130. doi: 10.1002/adma.201305506

We demonstrate a method to increase the Young&’s Modulus of a hydrogel using multiple in-diffusion cycles of precursor solution and subsequent exposure cycles (DE cycles). Employing a single biocompatible initial precursor solution of polyethylene glycol dithiol (PEG-SH) and polyethylene glycol norbornene (PEG-ene), we show an order of magnitude modulus increase for gels with an initial monomer weight percent of 5, 10, and 20. In addition to increased modulus, the soluble fraction within the gels decreases by a factor of two using the multiple DE cycle technique, demonstrating the presence of a higher crosslink density within the hydrogel. Using this technique, the modified hydrogels also demonstrate increased toughness by fracturing at compressive forces five times greater than hydrogels with the same Young&’s Modulus that did not undergo multiple DE cycles. We attribute the increase in toughness to the formation of double and interpenetrating networks within the gels due to the in-diffusion of monomer, which subsequently polymerizes to and around the existing polymer matrix. This modified hydrogel fabrication technique is designed for use in the replication of the bone-cartilage interface region, which is located in every joint structure within the body. To replicate this region, three-dimensional control of hydrogel mechanical properties is critical because both bone and cartilage reside at the interface. Using a projection stereolithography system, we demonstrate three-dimensional control of our biocompatible hydrogel cross-link density and Young&’s Modulus using a single material, which enables the use of this technique to fabricate interface-like structures.