• Chen G, Wu J, Wang Z*, Zhu H, Zhu S, Zhang Q*. Armored Polymer-fluid Gels with Integrated Damping and Impact Protection Across Broad Temperatures. Science Advances. 2025, eadv5292. doi: 10.1126/sciadv.adv5292. PDF

Abstract: Unpreferable vibrations and impacts pose substantial risks to sensitive devices, structures, and the human body, demanding materials capable of providing both high energy dissipation and impact protection across a broad temperature range. Traditional damping materials often fail to meet these demands due to a trade-off between damping and mechanical strength. We introduce an innovative strategy to fabricate armored polymer-fluid gels (APFGs) that combine high damping and high modulus for effective damping and impact protection under extreme conditions. By employing a controlled surface crosslinking process through diffusion, we greatly enhance the mechanical strength of polymer-fluid gels without sacrificing their damping capabilities. This asymmetric design results in an unprecedented loss factor (  > 0.5 from -45 °C to 135 °C, peaking at  = 2.2), while achieving a tensile modulus of 20 MPa. This method resolves the long-standing damping-modulus trade-off, positioning APFGs as promising candidates for robust damping and impact protection in electronics and human motion applications.

• Wang Z, Lin Z, Mei X, Cai L, Lin K-C, Flores Rodríguez J, Ye Z, Salazar Parraguez X, Mireles Guajardo E, García Luna PC, Zhang JYJ, Zhang YS*. Engineered Living Systems based on Gelatin: Design, Manufacturing, and Applications. Advanced Materials, 2025, 2416260. doi:  10.1002/adma.202416260. PDF.

Abstract: Engineered living systems (ELSs) represent purpose-driven assemblies of living components, encompassing cells, biomaterials, and active agents, intricately designed to fulfill diverse biomedical applications. Gelatin and its derivatives have been used extensively in ELSs owing to their mature translational pathways, favorable biological properties, and adjustable physicochemical characteristics. This review explores the intersection of gelatin and its derivatives with fabrication techniques, offering a comprehensive examination of their synergistic potential in creating ELSs for various applications in biomedicine. It offers a deep dive into gelatin, including its structures and production, sources, processing, and properties. Additionally, the review emphatically explores various fabrication techniques employing gelatin and its derivatives, including generic fabrication techniques, microfluidics, and various three-dimensional printing methods. Furthermore, it discusses the applications of ELSs based on gelatin in regenerative engineering as well as in cell therapies, bioadhesives, biorobots, and biosensors. Future directions and challenges in gelatin fabrication are also examined, highlighting emerging trends and potential areas for improvements and innovations. This comprehensive review underscores the significance of gelatin-based ELSs in advancing biomedical engineering and lays the groundwork for guiding future research and developments within the field.

• Wang Z, Qiu W, Zhang Q*. Constructing phase separation in polymer gels: Strategies, functions and applications. Progress in Polymer Science. 2024, 154, 101847. doi: 10.1016/j.progpolymsci.2024.101847. PDF

Abstract: Over the past decades, there has been a flourishing of phase-separated polymer gels. Unlike traditional design methods that rely on chemical structure and polymer network construction, phase separation enables polymers to tune morphologies across the microscopic, mesoscopic, and macroscopic levels, thereby creating a new path for regulating and innovating the performance of polymer gels. This comprehensive review offers a deep dive into the mechanisms underlying phase separation formation in polymer gels and makes a particular focus on the methods used to induce phase separation in polymer gels. Additionally, the review highlights the potential performance improvements and innovations of polymer gels based on phase separation and explores the promising applications of phase separation polymers in various fields. Finally, this review emphasizes the potential benefits yet significant challenges associated with phase-separated polymer gels. The versatility and multi-scale applicability of this approach make it a promising pathway for developing cutting-edge materials with tailored properties and functionalities.

• Wang Z*, Cai Q, Lu L, Levkin PA*. High‐Performance Pressure Sensors Based on Shaped Gel Droplet Arrays. Small. 2024, 20, 2305214. doi: 10.1002/smll.202305214. PDF

Abstract: Polymer gel-based pressure sensors offer numerous advantages over traditional sensing technologies, including excellent conformability and integration into wearable devices. However, challenges persist in terms of their performance and manufacturing technology. In this study, a method for fabricating gel pressure sensors using a hydrophobic/hydrophilic patterned surface is introduced. By shaping and fine-tuning the droplets of the polymer gel prepolymerization solution on the patterned surface, remarkable sensitivity improvements compared to unshaped hydrogels have been achieved. This also showcased the potential for tailoring gel pressure sensors to different applications. By optimizing the configuration of the sensor array, an uneven conductive gel array is fabricated, which exhibited a high sensitivity of 0.29 kPa−1 in the pressure range of 0–30 kPa, while maintaining a sensitivity of 0.13 kPa−1 from 30 kPa up to 100 kPa. Furthermore, the feasibility of using these sensors for human motion monitoring is explored and a conductive gel array for 2D force detection is successfully developed. This efficient and scalable fabrication method holds promise for advancing pressure sensor technology and offers exciting prospects for various industries and research fields. 

• Wang Z, Cui H, Liu M, Grage SL, Hoffmann M, Sedghamiz E, Wenzel W, Levkin PA*. Tough, Transparent, 3D-Printable, and Self-Healing Poly(ethylene glycol)-Gel (PEGgel). Advanced Materials. 2022, 34, 11, 2107791. doi: 10.1002/adma.202107791. PDF

Abstract: Polymer gels, such as hydrogels, have been widely used in biomedical applications, flexible electronics, and soft machines. Polymer network design and its contribution to the performance of gels has been extensively studied. In this study, the critical influence of the solvent nature on the mechanical properties and performance of soft polymer gels is demonstrated. A polymer gel platform based on poly(ethylene glycol) (PEG) as solvent is reported (PEGgel). Compared to the corresponding hydrogel or ethylene glycol gel, the PEGgel with physically cross-linked poly(hydroxyethyl methacrylate-co-acrylic acid) demonstrates high stretchability and toughness, rapid self-healing, and long-term stability. Depending on the molecular weight and fraction of PEG, the tensile strength of the PEGgels varies from 0.22 to 41.3 MPa, fracture strain from 12% to 4336%, modulus from 0.08 to 352 MPa, and toughness from 2.89 to 56.23 MJ m–3. Finally, rapid self-healing of the PEGgel is demonstrated and a self-healing pneumatic actuator is fabricated by 3D-printing. The enhanced mechanical properties of the PEGgel system may be extended to other polymer networks (both chemically and physically cross-linked). Such a simple 3D-printable, self-healing, and tough soft material holds promise for broad applications in wearable electronics, soft actuators and robotics.

1. Wang Z, Wei Z, Huang Y, Wei Y*, Chen J*. Naturally Sourced Hydrogels: Emerging Fundamental Materials for Next-generation Healthcare Sensing. Chemical Society Reviews. 2023, 52, 2992-3034. doi:10.1039/D2CS00813K. PDF 

Abstract: The flourishing development of flexible healthcare sensing systems is inseparable from the fundamental materials with application-oriented mechanical and electrical properties. Thanks to continuous inspiration from our Mother Nature, flexible hydrogels originating from natural biomass are attracting growing attention for their structural and functional designs owing to their unique chemical, physical and biological properties. These highly efficient architectural and functional designs enable them to be the most promising candidates for flexible electronic sensing devices. This comprehensive review focuses on the recent advances in naturally sourced hydrogels for constructing multi-functional flexible sensors and healthcare applications thereof. We first briefly introduce representative natural polymers, including polysaccharides, proteins, and polypeptides, and summarize their unique physicochemical properties. The design principles and fabrication strategies for hydrogel sensors based on these representative natural polymers are outlined after the fundamental material properties required in healthcare sensing applications are presented. We then highlight the various fabrication techniques of natural hydrogels for sensing devices, and illustrate the representative examples of wearable or implantable bioelectronics for pressure, strain, temperature, or biomarker sensing in the field of healthcare systems. Finally, concluding remarks on challenges and prospects in the development of natural hydrogel-based flexible sensors are provided. We hope that this review will provide valuable information for the development of next-generation bioelectronics and build a bridge between the natural hydrogels as fundamental matter and multi-functional healthcare sensing as an applied target to accelerate new material design in the near future. 

• Wang Z, Heck M, Yang W, Wilhelm M, Levkin PA*.Tough PEGgels by In-situ Phase Separation for 4D Printing. Advanced Functional Materials. 2023, 2300947. doi: 10.1002/adfm.202300947. PDF

Abstract: Polymer gels, consisting of cross-linked polymer network systems swollen by a solvent, show great potential in biomedicine, flexible electronics, and artificial muscles, due to their tissue-like mechanical properties. Due to the presence of a large amount of solvent, the improvement of the mechanical properties of the polymer gel is a challenge. Moreover, combining high toughness with useful properties, such as 3D printability or shape-memory, in one polymer gel system is even more challenging. In this study, a simple and efficient method is developed for the fabrication of tough polymer gels by polymerizing 2-hydroxyethyl methacrylate (HEMA) in a mixture of poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG). The polymerized elastic networkpresents distinct compatibility with PEG (compatible) and PPG (poorly compatible), resulting in in-situ phase separation at the microscale. The resulting phase-separated gel demonstrates high strength (8.0 MPa), favorable fracture strain (430%), and large toughness (17.0 MJ m−3). The separated hard phasewith a high glass transition temperature (75 °C) endows the whole soft polymer gel with the property of shape memory at room temperature. Finally, the fabrication of tunable tough PEGgels is combined with 3D printing as well as with shape memory properties, demonstrating the use of PEGgels for 4D printing. 

• Wang Z, Chen J, Cong Y, Zhang H, Xu T, Nie L, Fu J*. Ultrastretchable Strain Sensors and Arrays with High Sensitivity and Linearity Based on Super Tough Conductive Hydrogels. Chemistry of Materials. 2018, 30, 21, 8062-9. doi: 10.1021/acs.chemmater.8b03999. PDF

Abstract: Biocompatible conductive hydrogels with intrinsic flexibility, high sensitivity, linearity and outstanding reliability are highly demanded for wearable devices or implantable sensors. Here we report novel tough conductive hydrogels composed of interpenetrating polyaniline (PANI) and poly(acrylamide-co-hydroxyethyl methyl acrylate) (P(AAm-co-HEMA)) networks. Intrinsic interactions between the conductive PANI network and the flexible P(AAm-co-HEMA) endowed hydrogels with outstanding strength and toughness to cyclic loadings. The conductive hydrogels show very high sensitivity (gauge factor 11) and outstanding linear dependence of sensitivity on strain. Strain sensors based on the conductive hydrogels demonstrate reliable detection of repeated large strains and subtle vibrations, including the movements of various human joints, pulses and voiceprints. Moreover, a prototype 2D sensor array is fabricated to sense strains or pressures in the two dimensions, which is promising for electronic skin, touchpads, biosensors, human-machine interfaces, biomedical implants, wearable electronic devices and so on. 

• Wang Z, Lai Y-c, Chiang Y-t, Scheiger JM, Li S, Dong Z, Cai Q, Liu S, Hsu S-h, Chou C-c, Levkin PA*. Tough, Self-Healing, and Conductive Elastomer - Ionic PEGgel. ACS Applied Materials & Interfaces. 2022, 14, 44, 50152-62. doi: 10.1021/acsami.2c14394. PDF

Abstract: Ionically conductive elastomers are necessary for realizing human–machine interfaces, bioelectronic applications, or durable wearable sensors. Current design strategies, however, often suffer from solvent leakage and evaporation, or from poor mechanical properties. Here, we report a strategy to fabricate ionic elastomers (IHPs) demonstrating high conductivity (0.04 S m–1), excellent electrochemical stability (>60,000 cycles), ultra-stretchability (up to 1400%), high toughness (7.16 MJ m–3), and fast self-healing properties, enabling the restoration of ionic conductivity within seconds, as well as no solvent leakage. The ionic elastomer is composed of in situ formed physically cross-linked poly(2-hydroxyethyl methacrylate) networks and poly(ethylene glycol) (PEG). The long molecular chains of PEG serve as a solvent for dissolving electrolytes, improve its long-term stability, reduce solvent leakage, and ensure the outstanding mechanical properties of the IHP. Surprisingly, the incorporation of ions into PEG simultaneously enhances the strength and toughness of the elastomer. The strengthening and toughening mechanisms were further revealed by molecular simulation. We demonstrate an application of the IHPs as (a) flexible sensors for strain or temperature sensing, (b) skin electrodes for recording electrocardiograms, and (c) a tough and sensing material for pneumatic artificial muscles. The proposed strategy is simple and easily scalable and can further inspire the design of novel ionic elastomers for ionotronics applications.

• Wang Z, Cui H, Li S, Feng X, Aghassi-Hagmann J, Azizian S, Levkin PA*. Facile Approach to Conductive Polymer Microelectrodes for Flexible Electronics. ACS Applied Materials & Interfaces. 2021, 13, 18, 21661-8. doi: 10.1021/acsami.0c22519. PDF

Abstract: Conductive polymers have been intensively investigated as materials for electrodes in flexible electronics due to their favorable biocompatibility and reliable electrochemical stability. Nevertheless, patterning of conductive polymers for the fabrication of devices and in various electronics applications confronts multifarious limitations and challenges. Here, we present a simple but efficient strategy to obtain conductive polymer microelectrodes via utilization of surface-tension-confined liquid patterns. This method shows universality for various oxidizers and conductive polymers, high resolution, stability, and favorable compatibility with different surfaces and materials. The developed method has been demonstrated for creating conductive polymer microelectrodes with a customized reaction process, defined geometry, and flexible substrates. The obtained microelectrodes were assembled into flexible capacitive sensors. Thus, the method realizes a facile approach to conductive polymer microelectrodes for flexible electronics, biomedical applications, human activity monitors, and electronic skin. 

• Wang Z, Chen J, Wang L, Gao G, Zhou Y, Wang R, Xu T, Yin J, Fu J*. Flexible and Wearable Strain Sensors based on Tough and Self-adhesive Ion Conducting Hydrogels. Journal of Materials Chemistry B. 2019, 7, 1, 24-9. doi: 10.1039/C8TB02629G. PDF

Abstract: Inspired by biosystem, ionic hydrogels have been extensively studied as promising materials for wearable or implantable devices. Herein, we report novel ionic hydrogels that comprise dynamically crosslinked polyzwitterion and physically crosslinked polyvinyl alcohol, which demonstrate excellent mechanical properties, repeatable self-adhesion, and high and linear strain sensitivity. The obtained hydrogels can be directly attached to human skin as sensors to detect or monitor physiological signals.

• Wang Z, Cong Y, Fu J*. Stretchable and tough conductive hydrogels for flexible pressure and strain sensors. Journal of Materials Chemistry B. 2020, 8, 16, 3437-59. doi: 10.1039/C9TB02570G. PDF 

Abstract: Flexible pressure and strain sensors have great potential for applications in wearable and implantable devices, soft robotics and artificial skin. Compared to flexible sensors based on filler/elastomer composites, conductive hydrogels are advantageous due to their biomimetic structures and properties, as well as biocompatibility. Numerous chemical and structural designs provide unlimited opportunities to tune the properties and performance of conductive hydrogels to match various demands for practical applications. Many electronically and ionically conductive hydrogels have been developed to fabricate pressure and strain sensors with different configurations, including resistance type and capacitance type. The sensitivity, reliability and stability of hydrogel sensors are dependent on their network structures and mechanical properties. This review focuses on tough conductive hydrogels for flexible sensors. Representative strategies to prepare stretchable, strong, tough and self-healing hydrogels are briefly reviewed since these strategies are illuminating for the development of tough conductive hydrogels. Then, a general account on various conductive hydrogels is presented and discussed. Recent advances in tough conductive hydrogels with well designed network structures and their sensory performance are discussed in detail. A series of conductive hydrogel sensors and their application in wearable devices are reviewed. Some perspectives on flexible conductive hydrogel sensors and their applications are presented at the end.