Materials with tunable stiffness responsive to stimuli are useful in a wide range of fields, such as soft robotics, reconfigurable structures and biomedical engineering. People have been pursuing stiffness variable materials for larger Young’s modulus change and faster response rate. However, the broad transition temperature range of stiffness variable polymers means that excessive energy consumption is needed to heat and trigger the softening. And limited smart microelectronic devices were developed based on stiffness variable polymers.We developed a series of stiffness variable polymers with narrow transition temperature range, high moduli change ratio. The transition temperature can be tuned to body temperature range, which provides opportunities to build thermal-responsive, mechanically adaptive devices to be used on skin or in tissue. We also demonstrated the materials used as functional actuation materials. In Chapter 1 and Chapter 2, we discussed the basics of stiffness variable materials. And we formulated a series of stiffness variable polymers with transition temperatures covering 30 ℃ to 43 ℃, the Young’s moduli of which can drop three to four orders of magnitude. The polymers’ thermal and mechanical properties were characterized and discussed. Because the transition can be triggered by body temperature, they can be used for thermal-responsive, mechanically adaptive neural interfaces, which are rigid before implantation and become soft after implantation. In Chapter 3, based on these polymers, the application as neural interfaces were explored. Intracortical probes were fabricated on the softening polymer showing great promise to provide tissue-like contact and reduce the micromotion around the tip. Based on the same polymer, we designed a multimodal neural probe that can simultaneously sense serotonin signal and electrophysiological signal. In vitro and ex vivo characterization of the neural probe were performed. In Chapter 4, the softening polymer was used as a substrate to achieve a stretchable epidural electrode array for spinal cord. The silver nanowire and the wrinkled Parylene-C enabled the 20% stretchability for this device. In Chapter 5, based on the stiffness variable polymer with transition temperature 43 ℃, we designed and prototyped a Braille display. Carbon nanotubes were micropatterned as Joule heating electrode to trigger the softening of the polymer membrane, allowing for localized pneumatic actuation in the softened area. These stiffness variable polymers with the transition temperature ranging from 30 ℃ to 43 ℃ demonstrated the promise to fabricate tissue-like electronics. These polymers can tolerate microfabrication process, and their sharp moduli change enabled a series of smart structures and devices to better adapt to the change of working conditions.