The development of soft electronics that can interact with biological tissues is crucial for understanding biological systems and mimicking their functions. These tools require biointerfacing materials that match the softness of biological tissue while maintaining sufficient electrical conductivity for bioelectronic signal recording and reading. However, existing intrinsically soft and stretchable materials often contain solvents that limit long-term stability or exhibit low electronic conductivity. An ultrasoft (Young's modulus <30 kPa), conductive, and solvent-free elastomer has been lacking. This paper addresses this gap by introducing a solvent-free, ultrasoft, and conductive PDMS bottlebrush elastomer (BBE) composite with single-walled carbon nanotubes (SWCNTs) as conductive fillers. The increasing demand for ultrasoft electronics in diverse applications, such as deep-sea robotic actuators, artificial robotic skins, and human-machine interfaces, highlights the importance of this research. Conventional inorganic electronics, with their high Young's modulus, create a mechanical mismatch with biological tissues, leading to device failure and hindering long-term use. While strategies like serpentine and mesh structures improve elasticity, their limited stretchability and higher modulus can cause unintended biological responses. Intrinsically stretchable materials like hydrogels and ionogels offer an alternative, but often suffer from solvent limitations or high cost. Bottlebrush elastomers (BBEs) present a solution; their highly branched architecture results in ultralow Young's modulus without solvents. However, their non-conductive nature (e.g., PDMS-based BBEs) restricts their use in electronics. The incorporation of conductive fillers into BBEs is a promising approach, but previous attempts have resulted in materials with moduli higher than those of soft tissues or unsatisfactory conductivity. This study aims to overcome these limitations by developing a conductive BBE with improved properties.

Previous research has explored various approaches to create soft and stretchable electronics, focusing on mimicking the mechanical properties of biological systems. The use of serpentine and mesh structures in device architectures improved elasticity but were limited in stretchability. Intrinsically stretchable materials, such as hydrogels and ionogels, provided softness but often lacked stability due to high water content or the use of expensive ionic liquids. Ultrathin polymer sheets or nanomeshes showed promise in reducing mechanical constraints but required complex fabrication. Bottlebrush elastomers (BBEs), with their unique highly branched architecture, offered a potential solution by achieving ultra-low Young’s modulus without the use of solvents. However, existing BBEs were primarily non-conductive, limiting their applicability in soft electronics. A recent study reported a conductive BBE based on poly(4-methylcaprolactone) and carbon nanotubes (CNTs), but it had a modulus (66 kPa) higher than the range of soft tissues (0-30 kPa) and a relatively low conductivity (0.01 S/m). This work aimed to improve upon these shortcomings by creating a conductive BBE that meets the required softness and conductivity levels for various applications.

The researchers prepared solvent-free, conductive PDMS BBEs by incorporating single-walled carbon nanotubes (SWCNTs) as conductive fillers. Commercially available PDMS monomers (MCR-M11), crosslinkers (DMS-R22), and SWCNTs were used to simplify the synthesis process. The crosslinking ratio (molar ratio of monomer:crosslinker, MM:CL) was adjusted to control the softness of the BBEs. The mechanical properties, including Young's modulus and elasticity, were characterized using cyclic tensile testing. Adhesion properties were evaluated through lap shear testing on various substrates. The electrical conductivity of the SWCNT/BBEs was measured using a source meter, and the strain sensing characteristics were determined by measuring resistance changes under tensile strain and pressure. Electrochemical impedance spectroscopy (EIS) was used to characterize the material's impedance. Environmental stability was assessed by storing the samples in different solutions and environments. Cytotoxicity was evaluated by culturing human dermal fibroblasts on the materials. For device fabrication, laser cutting and 3D printing techniques were employed to pattern the conductive and non-conductive BBEs. Laser cutting created single-layer patterns, while 3D printing allowed for the fabrication of multilayer structures. These devices were then tested in various applications.

The study successfully synthesized a solvent-free conductive PDMS BBE composite with SWCNTs. The SWCNT/BBE, with a filler concentration of 0.4–0.6 wt%, exhibited an ultralow Young's modulus (2.98–10.65 kPa), significantly lower than previously reported conductive BBEs and within the range of soft biological tissues. The conductivity ranged from 2.06–17.84 S/m. The material also demonstrated satisfactory adhesion to various substrates. Laser cutting and 3D printing methods were successfully used to pattern the SWCNT/BBE into functional devices. Laser-cut strain sensors showed excellent performance when attached to both rigid and soft-bodied robots and even a hornworm, demonstrating the material's ultra-softness and minimal impact on the subject's movement. 3D-printed multilayer touch pads demonstrated high sensitivity, enabling detection of very light touches. Finally, 3D-printed ECG electrodes demonstrated performance comparable to commercial hydrogel electrodes, suggesting potential for use in wearable electrophysiological measurements. The overall findings suggest that this new material platform is suitable for a wide range of applications in soft robotics and bioelectronics.

The development of a solvent-free, ultrasoft, and conductive elastomer addresses a significant challenge in the field of soft electronics. The results demonstrate that the unique combination of ultra-low Young's modulus and high conductivity makes this material exceptionally suitable for creating devices that seamlessly integrate with biological systems. The success in patterning the material using both laser cutting and 3D printing techniques opens up possibilities for creating complex and highly functional devices. The applications demonstrated—from soft robotics to wearable sensing and electrophysiological measurements—highlight the material's versatility and potential impact. The excellent biocompatibility and environmental stability of the material further enhance its practical utility. Compared to existing conductive elastomers, the significantly lower Young's modulus and comparable or improved conductivity represent a major advance. This work lays the groundwork for future research in developing more sophisticated soft electronics with improved sensitivity and performance.

This research successfully developed a novel conductive and elastic bottlebrush elastomer (SWCNT/BBE) with superior properties for ultrasoft electronics. The material's ultralow Young's modulus, high conductivity, solvent-free nature, and excellent biocompatibility make it a promising platform for various applications in soft robotics, wearable sensors, and electrophysiological measurements. Future research could explore optimizing the SWCNT dispersion for even higher conductivity and investigating the long-term in vivo performance and biocompatibility of the material in different biological environments. Exploring other conductive fillers and BBE chemistries could lead to further improvements in material properties.

While the SWCNT/BBE shows promising results, some limitations exist. The conductivity, while satisfactory for many applications, could be further improved. The cytotoxicity tests, while indicating relatively low cytotoxicity, suggest further investigation into the potential effects of the SWCNTs and any proprietary compounds present in them might be needed for a comprehensive understanding. Long-term in vivo studies are necessary to fully assess the biocompatibility and long-term stability in different biological contexts.