Thermal and Light Dual Responsive Membrane for Passive Temperature Regulation

Membranes that respond to the environment without requiring an internal power source enable autonomic control of transport of ions and water, energy harvesting, evaporative cooling, separation processes, and devices for haptic feedback. Because no internal power system is required, the smart membranes require no wires or other external connectors. Of particular interest are membranes that respond to light, heat, humidity, ionic strength, and combinations of these stimuli.

Here, we present a passive temperature regulative membrane by emulating the structure of leaf stomata through thermally and light dual responsive materials coated on a membrane. This is achieved by coating poly(N-isopropylacrylamide) and spiropyran hydrogels on a nylon membrane via surface initiated free radical polymerization. Because of the dimension switchability of the PNIPAm hydrogel near the lower critical solution temperature (LCST), i.e., swells below LCST and de-swells above LCST, we are able to tune the pore sizes of the membrane under the different temperatures. Similarly, we are able to tune the pore sizes of the membrane when light is on or off due to the photo switchability of the spiropyran hydrogel. With such open/close leaf stomata structure, the membrane allows dynamic control of fluid transport during either the temperature or light triggering conditions. We have observed ~6 times faster flow rate of water through the coated membrane when pores are open at the testing highest temperature and light condition as compared to the control membrane with closed pores at lower temperature and dark condition. With a higher flow rate, the membrane subsequently draws the local temperature down due to a higher heat loss, demonstrating a cooling effect. This dual responsive membrane will be a potential material system for autonomic thermal regulations for reducing energy consumption in various applications including vehicles, buildings, storage tanks, reversible heat pumps and personal thermal management.