Biofriendly, Stretchable, and Reusable Hydrogel Electronics as Wearable Force Sensors.

The ever-growing overlap between stretchable electronic devices and wearable healthcare applications is igniting the discovery of novel biocompatible and skin-like materials for human-friendly stretchable electronics fabrication. Amongst all potential candidates, hydrogels with excellent biocompatibility and mechanical features close to human tissues are constituting a promising troop for realizing healthcare-oriented electronic functionalities. In this work, based on biocompatible and stretchable hydrogels, a simple paradigm to prototype stretchable electronics with an embedded three-dimensional (3D) helical conductive layout is proposed. Thanks to the 3D helical structure, the hydrogel electronics present satisfactory mechanical and electrical robustness under stretch. In addition, reusability of stretchable electronics is realized with the proposed scenario benefiting from the swelling property of hydrogel. Although losing water would induce structure shrinkage of the hydrogel network and further undermine the function of hydrogel in various applications, the worn-out hydrogel electronics can be reused by simply casting it in water. Through such a rehydration procedure, the dehydrated hydrogel can absorb water from the surrounding and then the hydrogel electronics can achieve resilience in mechanical stretchability and electronic functionality. Also, the ability to reflect pressure and strain changes has revealed the hydrogel electronics to be promising for advanced wearable sensing applications.

Capillary Origami Inspired Fabrication of Complex 3D Hydrogel Constructs.

Hydrogels have found broad applications in various engineering and biomedical fields, where the shape and size of hydrogels can profoundly influence their functions. Although numerous methods have been developed to tailor 3D hydrogel structures, it is still challenging to fabricate complex 3D hydrogel constructs. Inspired by the capillary origami phenomenon where surface tension of a droplet on an elastic membrane can induce spontaneous folding of the membrane into 3D structures along with droplet evaporation, a facile strategy is established for the fabrication of complex 3D hydrogel constructs with programmable shapes and sizes by crosslinking hydrogels during the folding process. A mathematical model is further proposed to predict the temporal structure evolution of the folded 3D hydrogel constructs. Using this model, precise control is achieved over the 3D shapes (e.g., pyramid, pentahedron, and cube) and sizes (ranging from hundreds of micrometers to millimeters) through tuning membrane shape, dimensionless parameter of the process (elastocapillary number Ce ), and evaporation time. This work would be favorable to multiple areas, such as flexible electronics, tissue regeneration, and drug delivery.

Effective mechanical properties of frozen hydrogel with ice inclusions.

Hydrogel exhibits attractive mechanical properties that can be regulated to be extremely tough, strong and resilient, adhesive and fatigue-resistant, thus enabling diverse applications ranging from tissue engineering scaffolds, flexible devices, to soft machines. As a liquid-filled porous material composed of polymer networks and water, the hydrogel freezes at subzero temperatures into a new material composed of polymer matrix and ice inclusions: the frozen hydrogel displays dramatically altered mechanical properties, which can significantly affect its safety and reliability in practical applications. In this study, based upon the theory of homogenization, we predicted the effective mechanical properties (e.g., Young's modulus, shear modulus, bulk modulus and Poisson ratio) of a frozen hydrogel with periodically distributed longitudinal ice inclusions. We firstly estimated its longitudinal Young's modulus, longitudinal Poisson ratio and plane strain bulk modulus using the self-consistent method, and then its longitudinal and transverse shear modulus using the generalized self-consistent method; further, the results were employed to calculate its transverse Young's modulus and transverse Poisson ratio. We validated the theoretical predictions against both finite element (FE) simulation and experimental measurement results, with good agreement achieved. We found that the estimated transverse Poisson ratio ranges from 0.3 to 0.53 and, at low volume fraction of ice inclusions, exhibits a value larger than 0.5 that exceeds the Poisson ratios of both the polymer matrix and the ice inclusion (typically 0.33-0.35). Compared with other homogenization methods (e.g., the rule of mixtures, the Halpin-Tsai equations, and the Mori-Tanaka method), the present approach is more accurate in predicting the effective mechanical properties (in particular, the transverse Poisson ratio) of frozen hydrogel. Our study provides theoretical support for the practical applications of frozen liquid-saturated porous materials such as hydrogel.

Bioinspired Microstructure Platform for Modular Cell-Laden Microgel Fabrication.

Cell-laden microgels have attracted increasing interest in various biomedical fields, as living building blocks to construct spatially organized multicellular structures or complex tissue features (e.g., cell spheroids and aligned cells/fibers). Although numerous approaches have been developed to tailor cell-laden microgels, there is still an unmet need for modular, versatile, convenient, and high-throughput methods. In this study, as inspired by the phenomena of water droplet manipulation from natural microstructures, a novel platform is developed to manipulate microscale hydrogel droplets and fabricate modular cell-laden microgels. First, taking antenna-like trichome as a template, catcher-like bioinspired microstructures are fabricated and hydrogel droplets are manipulated modularly in a versatile, convenient, and high-throughput manner, which is compatible with various types of hydrogels (e.g., photo-cross-linking, thermal-cross-linking, and ion-cross-linking). It is demonstrated that this platform can manipulate cell-laden microgels as modular units, such as two or more cell-laden microgels on one single catcher-like structure and different structures on one single chip. The authors also demonstrate the application of this platform on constructing complex tissue features like myocardial fibrosis tissue models to study cardiac fibrosis. The developed platform will be a powerful tool for engineering various in vitro tissue models for widespread biomedical applications.