Solution-based fabrication of mechanically transformative materials for implantable applications

Implantable probes and needles represent multifunctional biomedical platforms by integrating sensing, stimulation, and drug delivery capabilities. Conventional rigid probes often result in inflammatory responses due to large mechanical mismatch with soft biological tissues, whereas soft probes with improved long-term performances are difficult to be inserted deep into the compliant biological tissues. An emerging class of mechanically transformative materials addresses the challenge by embedding a phase-change material of gallium within an elastomeric matrix. These materials exhibit high stiffness under ambient conditions to enable facile insertion and compliant mechanical properties after implantations. The widespread implementation of mechanically transformative materials is primarily hindered by the lack of facile fabrication techniques for delicate gallium features. In this study, we introduce a solution-based approach for scalable fabrication of gallium-based mechanically transformative materials, which exhibit bistable mechanical properties with large modulations in the modulus by five orders of magnitude. In a solution-based coating process, gallium features are created based on a patterned copper film and then encapsulated with elastomers to form mechanically transformative materials. The height profile of the gallium feature is controlled by the two-dimensional design of the copper pattern, which provides access to delicate and complex three-dimensional features as exemplified by mechanically transformative indwelling needles with sharp tips. The practical suitability is demonstrated by the in vivo implementation of the indwelling needles for long-term chemotherapy. The excellent biocompatibility enables applications of mechanically transformative biomedical devices in chronic implantable systems.