Hydrogel bioelectronics

Bioelectronic interfacing with the human body including electrical stimulation and recording of neural activities is the basis of the rapidly growing field of neural science and engineering, diagnostics, therapy, and wearable and implantable devices. Owing to intrinsic dissimilarities between soft, wet, and living biological tissues and rigid, dry, and synthetic electronic systems, the development of more compatible, effective, and stable interfaces between these two different realms has been one of the most daunting challenges in science and technology. Recently, hydrogels have emerged as a promising material candidate for the next-generation bioelectronic interfaces, due to their similarities to biological tissues and versatility in electrical, mechanical, and biofunctional engineering. In this review, we discuss (i) the fundamental mechanisms of tissue-electrode interactions, (ii) hydrogels' unique advantages in bioelectrical interfacing with the human body, (iii) the recent progress in hydrogel developments for bioelectronics, and (iv) rational guidelines for the design of future hydrogel bioelectronics. Advances in hydrogel bioelectronics will usher unprecedented opportunities toward ever-close integration of biology and electronics, potentially blurring the boundary between humans and machines.National Science Foundation (U.S.) (CMMI-1661627)United States. Office of Naval Research (N00014-17-1-2920)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (W911NF-13-D-0001)Samsung Scholarship FoundationNational Natural Science Foundation (China) (51763010)Science Foundation for Excellent Youth Talents in Jiangxi Province (20162BCB23053)Key Research and Development Program of Jiangxi Province (20171BBH80007)Natural Science Foundation of Jiangxi Province (20171BAB216018)China Scholarship Council (201608360062

Predicting Fracture Energies and Crack-Tip Fields of Soft Tough Materials

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.Comment: 22 pages, 5 figure

Skin-inspired hydrogel–elastomer hybrids with robust interfaces and functional microstructures

nspired by mammalian skins, soft hybrids integrating the merits of elastomers and hydrogels have potential applications in diverse areas including stretchable and bio-integrated electronics, microfluidics, tissue engineering, soft robotics and biomedical devices. However, existing hydrogel–elastomer hybrids have limitations such as weak interfacial bonding, low robustness and difficulties in patterning microstructures. Here, we report a simple yet versatile method to assemble hydrogels and elastomers into hybrids with extremely robust interfaces (interfacial toughness over 1,000 Jm[superscript −2]) and functional microstructures such as microfluidic channels and electrical circuits. The proposed method is generally applicable to various types of tough hydrogels and diverse commonly used elastomers including polydimethylsiloxane Sylgard 184, polyurethane, latex, VHB and Ecoflex. We further demonstrate applications enabled by the robust and microstructured hydrogel–elastomer hybrids including anti-dehydration hydrogel–elastomer hybrids, stretchable and reactive hydrogel–elastomer microfluidics, and stretchable hydrogel circuit boards patterned on elastomer.United States. Office of Naval Research (N00014-14-1-0528)Charles Stark Draper LaboratoryMassachusetts Institute of Technology. Institute for Soldier NanotechnologiesNational Science Foundation (U.S.) (CMMI-1253495)Samsung Scholarship FoundationNational Institutes of Health (U.S.) (UH3TR000505

Fringe instability in constrained soft elastic layers

Soft elastic layers with top and bottom surfaces adhered to rigid bodies are abundant in biological organisms and engineering applications. As the rigid bodies are pulled apart, the stressed layer can exhibit various modes of mechanical instabilities. In cases where the layer's thickness is much smaller than its length and width, the dominant modes that have been studied are the cavitation, interfacial and fingering instabilities. Here we report a new mode of instability which emerges if the thickness of the constrained elastic layer is comparable to or smaller than its width. In this case, the middle portion along the layer's thickness elongates nearly uniformly while the constrained fringe portions of the layer deform nonuniformly. When the applied stretch reaches a critical value, the exposed free surfaces of the fringe portions begin to undulate periodically without debonding from the rigid bodies, giving the fringe instability. We use experiments, theory and numerical simulations to quantitatively explain the fringe instability and derive scaling laws for its critical stress, critical strain and wavelength. We show that in a force controlled setting the elastic fingering instability is associated with a snap-through buckling that does not exist for the fringe instability. The discovery of the fringe instability will not only advance the understanding of mechanical instabilities in soft materials but also have implications for biological and engineered adhesives and joints.United States. Office of Naval Research (Grant N00014-14-1-0528)Massachusetts Institute of Technology. Institute for Soldier NanotechnologiesNational Science Foundation (U.S.) (Grant CMMI- 1253495)Samsung Scholarship FoundationNational Institutes of Health (U.S.) (Grant UH3TR000505)MIT-Technion Fellowshi

Wet Adhesion and Bioadhesive Technology

Along with the rise of complex and ever more capable technologies, our body is interacting with a rapidly growing number of man-made devices and machines ranging from biomedical devices for surgical repair and disease treatment to implantable electronics for monitoring and augmentation of bodily functions. Despite the recent advances, the interfacing between man-made devices and the human body – their interactions and communications – are still dominated by relatively primitive, short-term, low-efficacy, and incompatible strategies. Owing to their close similarity in mechanical, chemical, and biological properties, hydrogels – polymer networks infiltrated with a large amount of water – have emerged as an ideal candidate to interface these two dissimilar realms. However, conventional hydrogels have suffered various limitations to serve as an effective interface. In particular, one of the central challenges in the development and practical translation of hydrogel interfaces is robust, reliable, and functional integration to wet, dynamic, and living biological tissues. This dissertation aims to provide a comprehensive set of scientific and technological advances to address the challenges in wet adhesion and bioadhesive technology. The first part of this dissertation is focused on the mechanics of wet adhesion. In particular, we will systematically discuss the mechanical design principles for achieving fast tough adhesion on wet surfaces covered by foulants. First, we propose the design principle for tough wet adhesion by a synergistic combination between the strong interfacial linkages and the mechanical dissipation in bulk tough hydrogels. Second, we propose the design principle for rapid wet adhesion by a dry-crosslinking mechanism that quickly removes the water on wet surfaces. Third, 3 we propose the design principle for foulant-resistant wet adhesion by a repel-crosslinking mechanism that cleans the foulants on wet surfaces. In the second part of this dissertation, we introduce a set of hydrogel interface technologies uniquely enabled by the wet adhesion in Part I. First, we introduce novel bioadhesive technologies in the form of a double-sided tape (DST) and a barnacle-inspired paste to achieve unprecedented rapid, robust, on-demand detachable, and blood-resistant adhesion on wet and injured tissues. Second, we introduce development and fabrication of high-performance conducting polymer hydrogels and their robust wet adhesion on a wide range of bioelectronic devices. Third, we explore a synergistic combination of bioadhesive and bioelectronic technologies for stable and functional interfacing between biological tissue and bioelectronic devices based on an electrical bioadhesive interface. In the last part of this dissertation, we will summarize and discuss the remaining challenges and opportunities in the wet adhesion and bioadhesive technology for seamless integration and communication between the human body and artificial devices and machines.Ph.D

Tough wet adhesion of hydrogel on various materials : mechanism and application

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016.Cataloged from PDF version of thesis.Includes bibliographical references (pages 82-87).In nature, robust interfacial adhesion plays crucial roles in maintaining integration and functionality of various physiological structures including tendon and cartilage to bones and epidermis to dermis in mammalian skins. For instance, the bonding of tendon and cartilage to bone is extremely tough (e.g., interfacial toughness ~800 Jm-2 ), yet such tough interfaces have not been achieved between synthetic hydrogels and various types engineering solids including rigid nonporous solids and elastomers. In this study, we report a strategy to design extremely robust interfacial bonding of synthetic hydrogeis containing 90 % water to various types of rigid engineering solids, precious metals and commonly-used elastomers. The design strategy is to anchor the long-chain polymer networks of tough hydrogels covalently to various solid surfaces, which can be achieved by diverse surface chemical treatments. We discuss the mechanism behind the proposed design strategy to further understand the tough wet adhesion of hydrogels in engineering and biological situations. We also demonstrate multiple novel applications of robust hydrogel-solid hybrids for both rigid engineering solids and elastomers. We discuss details of such new class of applications and their potential usefulness in diverse fields.by Hyunwoo Yuk.S.M

Material-stiffening suppresses elastic fingering and fringe instabilities

© 2018 When a confined elastic layer is under tension, undulations can occur at its exposed surfaces, giving the fingering or fringe instability. These instabilities are of great concern in the design of robust adhesives, since they not only initiate severe local deformations in adhesive layers but also cause non-monotonic overall stress vs. stretch relations of the layers. Here, we show that the strain stiffening of soft elastic materials can significantly delay and even suppress the fringe and fingering instabilities, and give monotonic stress vs. stretch relations. Instability development requires local large deformation, which can be inhibited by material-stiffening. We provide a quantitative phase diagram to summarize the stiffening's effects on the instabilities and stress vs. stretch relations in confined elastic layers. We further use numerical simulations and experiments to validate our findings

Strong adhesion of wet conducting polymers on diverse substrates

Conducting polymers such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), polypyrrole (PPy), and polyaniline (PAni) have attracted great attention as promising electrodes that interface with biological organisms. However, weak and unstable adhesion of conducting polymers to substrates and devices in wet physiological environment has greatly limited their utility and reliability. Here, we report a general yet simple method to achieve strong adhesion of various conducting polymers on diverse insulating and conductive substrates in wet physiological environment. The method is based on introducing a hydrophilic polymer adhesive layer with a thickness of a few nanometers, which forms strong adhesion with the substrate and an interpenetrating polymer network with the conducting polymer. The method is compatible with various fabrication approaches for conducting polymers without compromising their electrical or mechanical properties. We further demonstrate adhesion of wet conducting polymers on representative bioelectronic devices with high adhesion strength, conductivity, and mechanical and electrochemical stability. Copyright ©2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).National Science Foundation (CMMI-1661627)National Natural Science Foundation of China (51763010)National Natural Science Foundation of China (51963011)Technological Expertise & Academic Leaders Training Program of Jiangxi Province (20194BCJ22013)Research Project of State Key Laboratory of Mechanical System and Vibration (MSV202013