Programmable Hydrogel Ionic Circuits for Biologically Matched Electronic Interfaces

Abstract

The increased need for wearable and implantable medical devices has driven the demand for electronics that interface with living systems. Current bioelectronic systems have not fully resolved mismatches between engineered circuits and biological systems, including the resulting pain and damage to biological tissues. Here, salt/poly(ethylene glycol) (PEG) aqueous two-phase systems are utilized to generate programmable hydrogel ionic circuits. High-conductivity salt-solution patterns are stably encapsulated within PEG hydrogel matrices using salt/PEG phase separation, which route ionic current with high resolution and enable localized delivery of electrical stimulation. This strategy allows designer electronics that match biological systems, including transparency, stretchability, complete aqueous-based connective interface, distribution of ionic electrical signals between engineered and biological systems, and avoidance of tissue damage from electrical stimulation. The potential of such systems is demonstrated by generating light-emitting diode (LED)-based displays, skin-mounted electronics, and stimulators that deliver localized current to in vitro neuron cultures and muscles in vivo with reduced adverse effects. Such electronic platforms may form the basis of future biointegrated electronic systems.

Muscle-like fatigue-resistant hydrogels by mechanical training

“…Moreover, despite their high water content of around 75 wt % (3), skeletal muscles can sustain a high stress of 1 MPa over 1 million cycles per year, with a fatigue resistance over 1,000 J/m 2 (4). The combinational properties of skeletal muscles (i.e., high fatigue resistance, high strength, superior compliance, and high water content) are highly desirable for hydrogels' nascent applications in soft biological devices, such as load-bearing artificial tissues (5), hydrogel bioelectronics (6)(7)(8)(9), hydrogel optical fibers (10,11), ingestible hydrogel devices (12), robust hydrogel coatings on medical devices (13)(14)(15)(16)(17), and hydrogel soft robots (18)(19)(20).…”

Soft and Ion‐Conducting Materials in Bioelectronics: From Conducting Polymers to Hydrogels

“…Hydrogels as structural elements are a relatively mature field that has produced remarkable work summarized in these reviews . Recently, an increasing amount of research has focused on using hydrogels as the interface between electronics devices and physiological electrolytes . The soft nature of hydrogels minimizes the mechanical mismatch, and the high water content provides an ion‐rich environment at the interface .…”

“…As electrons‐ and ions‐mixed conductor, conducting polymers lower the surface impedance as well as the voltage threshold, and ion‐conducting hydrogels can even drive the stimulating voltage further due to their direct communication with biological system . An example is shown in Figure a,b where a salt/PEG hydrogel is used to trigger the twitching of muscles in chicken . In Figure 5c, the authors compare tissue damage in chicken breast from stimulation with metal, carbon, and ion‐conducting hydrogel electrodes .…”

“…An example is shown in Figure a,b where a salt/PEG hydrogel is used to trigger the twitching of muscles in chicken . In Figure 5c, the authors compare tissue damage in chicken breast from stimulation with metal, carbon, and ion‐conducting hydrogel electrodes . Carbon and metal electrodes cause significant temperature and pH change that causes lesions to tissue.…”

“…The saturated sodium dibasic phosphate solution works as a buffer in the hydrogel stimulator and minimizes pH change. Reproduced with permission . Copyright 2018, Wiley‐VCH.…”