Hydration-induced Void-containing Hydrogels for Encapsulation and Sustained Release of Small Hydrophilic Molecules.

Efficient encapsulation and sustained release of small hydrophilic molecules from traditional hydrogel systems have been challenging due to the large mesh size of 3D networks and high water content. Furthermore, the encapsulated molecules are prone to early release from the hydrogel prior to use, resulting in a short shelf life of the formulation. Here, we present a hydration-induced void-containing hydrogel (HVH) based on hyperbranched polyglycerol-poly(propylene oxide)-hyperbranched polyglycerol (HPG-PPG-HPG) as a robust and efficient delivery system for small hydrophilic molecules. Specifically, after the HPG-PPG-HPG is incubated overnight at 4 °C in the drug solution, it is hydrated into a hydrogel containing micron-sized voids, which could encapsulate hydrophilic drugs and achieve 100% drug encapsulation efficiency. In addition, the voids are surrounded by a densely packed polymer matrix, which restricts drug transport to achieve sustained drug release. The hydrogel/drug formulation can be stored for several months without changing the drug encapsulation and release properties. HVH hydrogels are injectable due to shear thinning properties. In rats, a single injection of the HPG-PPG-HPG hydrogel containing 8 µg of tetrodotoxin (TTX) produced sciatic nerve block lasting up to 10 hours without any TTX-related systemic toxicity nor local toxicity to nerves and muscles.

Homogeneity of liquid metal polymer composites: impact on mechanical, electrical, and sensing behavior.

Liquid metal polymer composites (LMPCs) are formed by dispersing eutectic gallium-indium-tin (galinstan) droplets within a soft polymer matrix, such as polydimethylsiloxane (PDMS), resulting in an insulating composite that is suitable for dielectric applications, including wearable sensors and actuators. LMPCs offer a unique combination of robust mechanical performance and desirable electrical properties. While much research has focused on the effects of rigid fillers in polymer composites, the behavior of liquid metal fillers, particularly the impact of homogeneity, has received limited attention. The density disparity between galinstan and the polymer matrix (6.44 g cm-3 compared to 0.97 g cm-3) results in the settling of galinstan droplets before curing, especially in matrices with low viscosity, leading to an inhomogeneous composition that may affect material performance. To address this, an innovative approach was introduced that enabled a spatially uniform (homogeneous) dispersion of galinstan droplets in PDMS while preserving the non-conductive nature of the composites. Work described herein evaluates the influence of homogeneity on electrical and mechanical properties as well as performance of LMPCs as pressure sensors. It was found that homogeneity has minimal effect on permittivity and dielectric loss but exhibits a complex behavior with respect to other parameters, including dielectric strength, which is often exacerbated at higher concentrations (≥50 vol%). These findings provide valuable insight that contributes to improved control over the material properties of LMPCs and expands their potential applications in soft robotics and stretchable electronics.

Multimodal Deformation of Liquid Metal Multimaterial Composites as Stretchable, Dielectric Materials for Capacitive Pressure Sensing.

Traditional electronic devices are composed of rigid materials and components that tend to be unsuitable for soft robotic and stretchable electronic applications, such as wearable or continuous pressure sensing. However, deformable materials have the potential to improve upon traditional devices through enhanced sensitivity and responsiveness, better conformability and biocompatibility at the human-machine interface, and greater durability. This work presents deformable composite materials composed of the gallium-indium-tin alloy galinstan (GaInSn) that combines the conductivity of a metal and the intrinsic deformability of a liquid. Dispersing galinstan in an elastomer allows for the formation of deformable dielectric materials that have tunable mechanical and electrical behavior, for example, modulus and relative permittivity. Galinstan composites have been shown previously to have a minimal modulus impact on the elastomer but concurrently achieve impressive dielectric performance. However, galinstan dispersions can be costly and face challenges of mechanical and electrical reliability. Thereby, this work investigates multimaterial composites composed of galinstan and a rigid filler, either iron or barium titanate, with respect to morphology, mechanical behavior, dielectric behavior, and pressure sensing performance for the purpose of achieving a balance between a low modulus and superior electrical performance. By combining galinstan and rigid fillers, it was found that the mechanical and electrical properties, such as modulus, permittivity, loss behavior, sensitivity, and linearity of the multimaterial composites can be improved by tuning filler formulation. This suggests that these dielectric materials can be used for sensing applications that can be precisely calibrated to specific material properties and the needs of the user. These deformable multimaterial composites, found to be stretchable and highly responsive in sensing applications, will expand the current mechanical abilities of deformable dielectric materials to improve soft robotic and stretchable electronic devices.