Localized flow control by photothermal actuation of pNIPAAm hydrogel brushes in a macroporous silicon membrane

We present the control of liquid flow through arrayed micron-sized pores in a macroporous silicon membrane. The pores are coated with about 150 nm polymer N-isopropylacrylamide (pNIPAAm) hydrogel brushes and 200 nm polypyrrole layer, which works as photothermal actuator. The size of pore openings is controlled by utilizing the swelling and de-swelling behavior of temperature-sensitive pNIPAAm brushes, and the temperature on pNIPAAm brushes is changed by 815 nm near infra-red (NIR) illumination to polypyrrole photothermal element layer. The dimension change of the pore openings is investigated by observing the transmitted light and fluorescence signal intensity through the pores in the membrane while changing the ambient temperature. It has shown that the intensity of transmitted light can be controlled by adjusting the ambient temperature across the low critical solution temperature (LCST) of the hydrogel brushes. The localized control of liquid flow through the pores is demonstrated by the diffusion of fluorescein dye from the bottom of the membrane to the surface of the membrane using pulsed NIR light illumination. Fast dynamic response of fluorescein dye diffusion upon the illumination of NIR light suggests that the presented photothermal actuation approach could be applied to diverse biomedical applications such as a localized drug release system.