Materials for Cut-And-Seam Fabrication of Soft Functional Interfaces

Interfaces between inorganic and biological systems need to be mechanically compatible with soft surfaces that grow, stretch, and distort over time. This requirement has driven the development of soft photonic and electronic circuits. However, the evolving shape of these circuits poses a problem when connecting circuits together: how can we match up distorted contacts that may be impossible to align? The problem comes up not only when repairing, modifying, or temporarily connecting to a stretchable circuit that’s already attached to a biological system, but also when connecting circuit pieces into custom 3D structures to cover one-of-a-kind biological shapes. The usual solution to the distortion problem is to create a stiffened connector region that prevents the circuit from deforming, which is good for alignment but doesn’t conform to biological surfaces as closely as a soft region does. The 3D structure problem may be solved by laying out a new electronic circuit for each individual interface, but this makes it difficult to use woven meshes and other regular structures as bases for soft functional interfaces.<br/><br/>This presentation describes our approach to connect signals across thin seams between soft materials without precise alignment at the connections. Our method relies on mm- and smaller scale electrical components inserted into thin adhesive tapes, producing materials with anisotropic electrical conductivity.<br/><br/>The alignment error tolerances we target are 2 mm in the direction perpendicular to the circuit edge, 2 cm parallel to the circuit edge, and a 70 degree angular tolerance. The reason for the large angular tolerance is that 3D cut-and-seam structures are often made from anisotropic woven or knit materials, often have curved seams, and are subject to external constraints such as packing cuts into a sheet to conserve material. These factors mean there is no guarantee that wires, optical fibers, and other anisotropic features will meet at the same angle along every seam, or even within a single seam.<br/><br/>The cut-and-seam fabrication method will also be discussed here as an interface for highly porous mesh-based biosensor circuits. Mesh circuits’ applications in tissue engineering and biosensing are promising because meshes permit vapor transport, can be perfused with nutrients, and provide paths for cells to extend through during growth. However, the task of collecting signals from individual wires in a mesh is laborious, and mesh materials are often made from fibers that can't be soldered because of a too-low operating temperature range (for example, metallized polymer threads) or because harsh chemicals are required for solder wetting (for example, stainless steel wires and threads). The seaming materials and methods in this work are designed to make contact to those conductors, forming a distribution network for power and data in the soft sensor interface.<br/><br/>In this presentation, we investigate the above alignment tolerances in circuits made from thin films and fibers, focusing on methods such as capacitance, temperature, strain, and light intensity measurements that function when encapsulated by parylene, silicone and other conformal biocompatible coatings.