Mechanically Gradient and Electrically Conductive Structures for Soft-to-Hard Interface in E-Textiles and Other Soft Electronics

Mechanically gradient materials are a special class of advanced materials where the mechanical properties (most often, stiffness) gradually increase or decrease along one or more dimension(s). This allows for a more gradual transition between soft and hard phases as compared to an abrupt interface. Under an external deforming load, the spatial stiffness gradient enables the gradient to transfer loads between the two mechanically mismatched phases progressively. This maintains a local continuity in the materials’ response to applied stress through the system. On the contrary, in the absence of a gradient, a sharp junction is formed at the interface of soft and hard phases. This leads to an abrupt change (or discontinuity) in the materials’ response to the applied load across the soft-to-hard interface, leading to detrimental effects such as delamination, rupture, or cracking in the system. The concept of gradient is relevant in systems where mechanically mismatched materials form an interface. Some of the examples include soft robotics, electronic textiles, limb prosthetics, etc.

When it comes to soft and flexible electronic devices like wearable e-textiles and soft robotics, commercial viability requires a robust integration of soft (textile, polymer, or biological tissues) and traditional semiconductor-based hard components. Among many other issues caused by this integration, managing the interconnection between hard and soft components poses one of the most significant challenges. Here, the mismatch in the materials’ response to bending, stretching, twisting, etc. soft-to-hard interface during use may cause the system to fail. Even though several soft and stretchable electronic devices (such as textiles- and polymer-based transistors, batteries, sensors, etc.) have been shown to survive clothing-like handling and laundering, they have little commercial success. Thus, the silicon-based electronic components of electronic devices remain an indispensable part of soft electronics because of their superior electrical performance. Consequently, the significant challenges in integrating hard electronics into soft substrates and devices continue to exist. A potential solution to this issue involves the development of a structure having spatially varying mechanical properties while simultaneously possessing uniform electrical conductivity.

In the present work, stiffness gradient structures prepared from elastomer-carbon nanoparticles composite are presented. Unlike conventional composites in which the number of nanoparticles dictates the mechanical properties and vice-versa, a novel composite has been proposed in this work. The mechanical and electrical properties in these novel composites have been decoupled i.e., the changing spatial stiffness will not change the corresponding electrical conductivity, and vice-versa. This decoupling has been achieved by in-situ foaming of the composites. Foaming (or foaming agent) causes (1) re-orientation of conductive carbon particles which improves or maintains electrical conductivity and (2) arrests local polymer chain mobility to impart stiffness. By carefully controlling the spatial foaming intensity longitudinally along the composite, stiffness can be gradually increased while bearing minimum impact on the electrical conductivity. The initial prototype extruded on a lab-made setup is in a filament form and is capable of accommodating ~10X stiffness mismatch. However, this can be extended to accommodate up to 20-30X mismatch. The gradient capacity can be improved further by switching to a multi-polymer system in contrast to the single-polymer system currently. The presented work represents a potential avenue of research to strengthen soft-to-hard interface from a fundamental materials standpoint. The electro-mechanical decoupling in a conductive gradient composite holds promise to impart unprecedented durability to e-textiles and other soft electronics.