MXene hydrogel for wearable electronics

The up-to-date progress of the multifunctional MXene hydrogels for flexible wearable electronics is summarized. The critical challenges and prospects are highlighted for the future developments of MXene hydrogel-based wearable electronic devices with high performance and fascinating multifunctionality for versatile applications. The up-to-date progress of the multifunctional MXene hydrogels for flexible wearable electronics is summarized. The critical challenges and prospects are highlighted for the future developments of MXene hydrogel-based wearable electronic devices with high performance and fascinating multifunctionality for versatile applications. Flexible wearable electronics—featuring with flexibility, stretchability, self-healing capability, and tactile sensing ability inspired from human skin for the next-generation electronic devices—have garnered rapid advancement in recent years because of the potential in revolutionizing the human way of life.1Chortos A. Liu J. Bao Z. Pursuing prosthetic electronic skin.Nat. Mater. 2016; 15: 937-950Crossref PubMed Scopus (1326) Google Scholar, 2Kim D.H. Lu N. Ma R. Kim Y.S. Kim R.H. Wang S. Wu J. Won S.M. Tao H. Islam A. et al.Epidermal electronics.Science. 2011; 333: 838-843Crossref PubMed Scopus (3238) Google Scholar, 3Bao Z. Chen X. Flexible and stretchable devices.Adv. Mater. 2016; 28: 4177-4179Crossref PubMed Scopus (307) Google Scholar However, conventional elastomer substrates (e.g., silicone rubbers and polyurethane) for electronic devices, exhibited much higher elastic moduli (1 MPa to 1 GPa) than that of human tissues (1–100 kPa). This apparent mechanical mismatch, coupled with the lack of biological function, gives rise to numerous problems of poor conformability, tissue trauma, and foreign-body reaction. Regarding the high-performance wearable electronics with tissue-like softness, biocompatibility, and unique electrical and chemical properties, hydrogels stand out from the existing materials by virtue of their three-dimensional bioinspired structure, high surface area, high water content, tissue resemblance, stimuli-responsiveness, and tunable conductive channels to provide a seamless interface between the electronics and the biology.4Yuk H. Lu B. Zhao X. Hydrogel bioelectronics.Chem. Soc. Rev. 2019; 48: 1642-1667Crossref PubMed Google Scholar Despite the tremendous research progress on hydrogel-based wearable electronics over the past few decades, unfavorable attributes such as the weak mechanical strength, low-sensing sensitivity, narrow detection range, easy damage, instability under extreme conditions, non-degradability, and poor adhesion significantly hindered their practical applications in flexible electronics. The design of novel and homogeneous nanocomposite through the incorporation of conductive nanomaterials into hydrogel matrix has proved to be a simple, yet effective approach to improve mechanical properties, enhance sensing performances, and facilitate fascinating functionalities for versatile applications. MXene—an emerging two-dimensional conductive transition metal carbide, nitride, or carbonitride—has attracted considerable attention in diverse research fields, owing to their negatively charged hydrophilic surfaces, high electrical conductivity, excellent mechanical strength, large specific surface area, and abundant surface functional groups (e.g., -F, -O, -OH, etc.). Recently, tremendous efforts have been devoted to developing MXene-based hydrogels aiming at solving the detrimental issues of hydrogel electronics and realizing appealing functions to broaden their potential applications (Figure 1). To tackle the poor mechanical properties and the low-sensing sensitivity of hydrogel-based electronics, H.N. Alshareef and co-authors incorporated MXene nanosheets into a commercial hydrogel composed of poly(vinyl alcohol), water, and anti-dehydration additives.5Zhang Y.-Z. Lee K.H. Anjum D.H. Sougrat R. Jiang Q. Kim H. Alshareef H.N. MXenes stretch hydrogel sensor performance to new limits.Sci. Adv. 2018; 4: eaat0098Crossref PubMed Scopus (337) Google Scholar The electrostatic repulsion interaction among MXene nanosheets and their good hydrophilicity allowed MXene nanosheets to be homogeneously dispersed in the hydrogel matrix, providing a large number of rigid physical cross-linking sites. Furthermore, polymer chains were densely entangled on these MXene nanosheets by the hydrogen bonding between the surface functional groups (e.g., -F, -O, -OH, etc.) of MXene and the -OH groups of poly(vinyl alcohol), creating a secondary cross-linked network, which further prevented the self-stacking of the MXene flakes. As a result, the prepared MXene hydrogel exhibited significantly enhanced mechanical properties, and its stretchability can reach as high as 3,400%, far exceeding the 2,200% for the pristine hydrogel. In addition, good electrical conductivity and ultrasensitive sensitivity could be obtained from the MXene hydrogel. The ultrasensitive sensitivity could be attributed to the contact or separation between the MXene nanosheets caused by the deformation of this hydrogel under external mechanical forces. Specifically, under tensile deformation, the spacing between MXene nanosheets increased, thereby reducing the probability of their contact in the hydrogel matrix, ultimately leading to the increased resistance of the MXene hydrogel. While the resistance of the MXene hydrogel decreased under the compressive deformation because of the less separation and more contact chances among the MXene nanosheets. Therefore, the MXene hydrogel-based sensor displayed unprecedented tensile strain sensitivity (gauge factor [GF] up to 25) and compressive strain sensitivity (GF up to 80). Thus, the MXene hydrogel-based sensor could be employed to sensitively detect various human motions, such as finger bending, hand gestures, voice recognition, and facial expressions. To simultaneously achieve high sensitivity and broad detection range, S.J. Jonas, X.C. Dong, V. Tung, and co-workers reported a MXene hydrogel with mixed-dimensional heterostructures through reasonable structural design.6Cai Y. Shen J. Yang C.-W. Wan Y. Tang H.-L. Aljarb A.A. Chen C. Fu J.-H. Wei X. Huang K.-W. et al.Mixed-dimensional MXene-hydrogel heterostructures for electronic skin sensors with ultrabroad working range.Sci. Adv. 2020; 6: eabb5367Crossref PubMed Scopus (58) Google Scholar Specifically, one-dimensional polypyrrole nanowires and two-dimensional MXene nanosheets were sequentially spray-coated onto a pre-stretched hydrogel substrate (1,000% strain) in the orthogonal direction. After four stretching-and-releasing cycles, the spring-like morphology of polypyrrole nanowires and the crumpled patterns of MXene nanosheets were introduced in different directions. On the one hand, the hydrophobicity of polypyrrole nanowires weakened the interactions between MXene nanosheets and water molecules in the hydrogel, preventing the MXene flakes from sliding with water molecules randomly, self-aggregating, and eventually separating from the surface of hydrogel. On the other hand, polypyrrole nanowires acted as a nano-bridging layer to combine the MXene layer with the hydrogel substrate. The large specific surface area, wrinkled morphology, and relatively weak interactions with polypyrrole nanowires enabled MXene flakes to slide evenly under large and repeated deformation in the hydrogel. Consequently, the electronic skin assembled from this MXene hydrogel possessed an ultrabroad detection range (2,800%) and super high sensitivity (GF at 40.54), affording an ideal platform for next-generation highly sensitive flexible wearable electronics. Conventional hydrogels with the dispersion medium of water, will inevitably freeze at low temperature and suffer from the evaporation of water even at room temperature, resulting in the decrease and loss in conductivity, durability, flexibility, and sensing capability, which severely hinders their potential applications in accurate and long-term stable sensing under sophisticated environmental conditions. Therefore, it is urgent to develop hydrogel-based flexible electronics with the features of anti-freezing, long-lasting moisture, and long-term stability. Inspired by the anti-freezing mechanism of biological organisms, we introduced the antifreeze ethylene glycol into the MXene hydrogel network through a facile solvent displacement method, resulting in the large amounts of hydrogen bonds between the water and ethylene glycol, which prevented the formation of ice crystal lattices at low temperature and hindered the evaporation of water at room temperature, exhibiting excellent low-temperature tolerance and remarkable moisture retention.7Liao H. Guo X. Wan P. Yu G. Conductive MXene nanocomposite organohydrogel for flexible, healable, low-temperature tolerant strain sensors.Adv. Funct. Mater. 2019; 29: 1904507Crossref Scopus (326) Google Scholar Even under extremely low temperatures (−40°C) or after being stored at 20°C for 8 days, the wearable strain sensor assembled from the MXene hydrogel with the incorporation of ethylene glycol could maintain good flexibility, excellent electrical conductivity, and reliable sensing performance and stability. It provides a fascinating strategy to fabricate MXene hydrogel featured by the anti-freezing and long-lasting moisture performances for assembling wearable electronics with long-term stability properties in extreme environmental conditions. Meanwhile, other effective strategies were also developed by introducing glycerol, dimethyl sulfoxide, inorganic salts (CaCl2, NaCl, LiCl, etc.), organic zwitterions, ionic liquids, or biomacromolecules (for example, anti-freezing proteins and ice-binding proteins) into the hydrogel to obtain the improved anti-freezing and moisturizing properties. With the booming of consumer electronic devices, electronic wastes accumulate rapidly and lead to serious environmental pollution due to their poor degradability, highlighting the urgent requirement for the development of degradable flexible electronics for next-generation wearable electronics. Inspired by the biomineralization process in nature, a MXene hydrogel was prepared by introducing MXene nanosheets network into the hydrogel network including amorphous calcium carbonate and poly(acrylic acid).8Li X. He L. Li Y. Chao M. Li M. Wan P. Zhang L. Healable, degradable, and conductive MXene nanocomposite hydrogel for multifunctional epidermal sensors.ACS Nano. 2021; 15: 7765-7773Crossref PubMed Scopus (82) Google Scholar The outstanding self-healing capability could be obtained from the supramolecular interactions among MXene, poly(acrylic acid), and amorphous calcium carbonate. The MXene hydrogel-based epidermal sensor could be largely degraded after placed in a ~pH 7.3 phosphate buffered saline solution at room temperature for 65 days. The excellent degradability in phosphate buffered saline solution could be ascribed to the collapse of the hydrogel network from the continuous swelling after absorbing excessive water, leading to the disruption of the supramolecular interactions to achieve the facile degradability. The healable and degradable MXene hydrogel-based epidermal sensor could be assembled and employed to sensitively measure human movements (finger bending, arm bending, swallowing, and pulse) and wirelessly monitor the electrophysiological signals, such as the electrocardiogram and electromyogram signals, providing key reference information for postoperative rehabilitation and disease diagnosis. It pays the way for the preparation of the degradable healable MXene hydrogel-based flexible wearables with electrophysiological signal-sensitive performance and potential applications in personalized healthcare monitoring and human-machine interactions. MXene hydrogel with good self-adhesiveness could be tightly adhered to the human tissues, thereby reducing the interface resistance to obtain high-resolution physiological sensing signals. Inspired by the mussel adhesion chemistry, a self-adhesive MXene hydrogel was fabricated by introducing dopamine-grafted polymer into the MXene hydrogel network, showing superior self-adhesive performance. The prominent self-adhesiveness mainly originated from the interactions between the catechol groups of dopamine and the material surfaces such as covalent cross-linking and hydrogen bonding. Besides dopamine, other catechol-containing compounds like tea polyphenol and tannic acid have similar effects in improving the self-adhesion of hydrogels. Great progress has been achieved in fabricating multifunctional MXene hydrogels for wearable electronics with tunable mechanical strength, high-sensing sensitivity, broad detection range, long-term stability under extreme conditions, reliable degradability, and robust self-adhesion. Nevertheless, the research on MXene hydrogel-based wearable electronics is still in its infancy. First, the trade-off among the aforementioned material performances of MXene hydrogel because of the contradictory requirements from the material molecular structures should be studied in detail for meeting the complex requirements of practical applications. The gelation of MXene hydrogel embraces both strong static covalent cross-linking and weak dynamic non-covalent cross-linking. More covalent cross-linking may lead to better mechanical strength and fatigue resistance, but the self-healing capability and degradability will be inevitably compromised. Introducing multi-bond networks and tailoring special molecular structures may be a feasible strategy to balance these properties. Second, it is still a great challenge to match different target tissues of the human body with a variety of soft tissues by MXene hydrogel with the facile tunable mechanical properties. Third, the MXene nanosheets incorporated in the reported MXene hydrogel-based electronics undergo severe oxidization in the presence of water and oxygen. Thus, further study could pay more attention to the incorporation of MXenes with the improved stability, which may bring versatile different properties to the hydrogels with the reliable stability. Fourth, most MXene hydrogels are prepared by laborious and time-consuming methods with the toxic raw materials, which prompts the urgency to develop advanced material processing technologies to realize fast, green, cost-effective, and mass preparation of MXene hydrogel. Finally, the functions of current MXene hydrogel-based wearable electronics are relatively unitary, which cannot obtain comprehensive sensing signals from real-time online human healthcare monitoring. Combining MXene hydrogel with other technologies like wireless electronic printed circuits, fluid sampling systems, and computer science will be an effective route to achieve fascinating multifunctionality. With continuous research advances, the delicate materials preparation, structure design, and device assembly of the MXene hydrogel-based wearable electronics are expected in the future for constant on-body monitoring of multiple signals (e.g., human movements, electrophysiological signals, temperature, blood pressure, glucose, sweat, etc.), ultimately enabling a comprehensive personal health diagnostics.9Lei Z. Wu P. A supramolecular biomimetic skin combining a wide spectrum of mechanical properties and multiple sensory capabilities.Nat. Commun. 2018; 9: 1134Crossref PubMed Scopus (282) Google Scholar Moreover, MXene hydrogel-based wearables could also render a potential platform to realize the integration of on-skin or implantable healthcare monitoring, personal protection, and smart medical treatment, owing to their tremendous advantages in temperature management, electromagnetic interference shielding, UV protection, anti-bacterial, hemostasis, tissue repair, and therapy (Figure 2).10Ho D.H. Choi Y.Y. Jo S.B. Myoung J.-M. Cho J.H. Sensing with MXenes: progress and prospects.Adv. Mater. 2021; (Published online May 3, 2021): e2005846Crossref PubMed Scopus (43) Google Scholar Despite daunting challenges, the MXene hydrogel-based flexible wearables are still expected to continue their rapid developments to catalyze a revolutionary change in people’s lives.