Water-responsive supercontractile polymer films for biomedical applications

Field: Materials Science (Physical Sciences)

Stimulus-responsive contractile materials are significant both in nature and in our daily lives. Contraction of muscles is the basis of animal movement. Supercontraction of spider dragline silks is related to their superior mechanical properties. The synthetic heat shrink polymers are widely used in packaging. All these natural and commercial contractile materials exhibit large (~ 50%) and fast (within seconds) contraction, a prerequisite of their functioning. Advanced stimulus-responsive contractile materials with different stimulus like heat, light and chemicals have been developed recently. But only a few of them achieve fast and large contraction. Besides, in terms of biomedical applications, stimulus like high temperature, ultraviolet light and harmful chemicals are incompatible with vulnerable physiological environment. Body-temperature (37 °C) are suitable stimulus, but this low transition temperature increases the premature contraction risk at ambient conditions. Water is also a benign stimulus. But most of the water-responsive contractile materials are based on water-breakable hydrogen bonds as switch segments, overly dense of which hinder contraction while sparse ones are unstable under ambient humidity. This thesis designs a series water-responsive supercontractile films. These films are stable at ambient environmental conditions and exhibit large and fast contraction when wetted. Based on them, this thesis develops two new biomedical devices: shape-adaptive electronic-tissue interfaces and automatic wound closure tapes. Geometrical and mechanical match between implantable electronic devices and target tissues are significant for both the devices’ performances and the tissues’ safety. Shape-wrapping soft electrodes can construct conformal and stable electronic-tissue interface. However, their sizes and shapes need to be customized in advance. Similar issues of geometrical-match wrapping have been achieved using heat shrink films in industry packaging field, but they contract at temperatures >90 °C and are much harder than tissues. Hydrogels with tissue-like softness can realize mechanical match with tissues, but it is still challenging to combine hydrogels with other electronic materials into a feasible device, due to the hydration layer and different surface energy. In this thesis, a novel water-responsive supercontractile polymer films which are initially dry, stable and flexible, contract by > 50% within seconds (~30%/s) after wetting, and become soft (~100 kPa) and stretchable (~600%) hydrogel thin films thereafter. Such supercontraction is attributed to the films’ aligned microporous hierarchical structures, which also facilitate electronic integration. Using this film, the shape-adaptive electrode arrays that contract instantly and wrap conformally and softly around nerves and muscle of different sizes when wetted are fabricated for in vivo nerve stimulation and electrophysiological signal recording. Traditional suturing for wound closure is time-consuming, need professional skills and cause additional pain. Developing the convenient and automatic wound closure process could help patients treat the wound instantly by themselves. Recently, body-temperature responsive contractile materials are used to automatically close the wound. However, they are either not stable at ambient temperature or show slow contraction and small contraction stress, making them impractical especially for closing skin wound with large tension. This thesis develops novel supercontractile films which are stable at ambient conditions, exhibit large (> 50%) and fast (~12%/s) water-induced contraction and generate extra-large contraction stress (~6 MPa) and output work density (~1028 kJ/m3), making them can lift objects 10000 heavier than its own. Combining them with tissue adhesive develops smart tapes which realize ex vivo porcine skin wound closure and healthy human skin squeezing. Such SC tapes holds promise for a convenient and effective wound closure strategy especially for emergency and self-care/treatment applications.