Bioinspired synthesis of optically and thermally responsive nanoporous membranes

Abstract

The development of porous membranes that can rapidly change flow rates in response to external, noninvasive stimuli has broad technological applications for areas ranging from biomedical devices to architecture. Environmentally responsive membranes have a fundamental role in the development of devices used in chemical sensors, biological sorters, sequencing, separations, high-throughput medical devices and labs on a chip. The design and engineering of these responsive porous membranes have been achieved through the coupling of porous membranes with polymeric materials that can change their physical conformation in response to pressure, heat, pH or different chemical entities. Inspired by the phototropic growth of coleoptiles and the light-mediated mechanism that plants use to open their stomata, in this work, light-responsive porous membranes were engineered, mathematically modeled and synthesized. This biologically inspired approach led to a state-of-the-art design technique and a device that outperforms its natural counterpart and is capable of reversibly controlling flow rates from 0.001 to 0.035 ml s−1 cm−2 in less than a few minutes using the noninvasive stimulus of light. We envision that the polymeric responsive membranes and the platform synthesis technique employed in this manuscript for their fabrication could be utilized in a broad range of applications and will have a great impact on the fields of fluid handling, biomedical high-throughput devices, sensors, medicine and other fields of chemistry, biology and mechanical engineering. Membranes whose permeability depends on certain environmental conditions — such as the temperature, pH or glucose concentration — are attractive materials for use in sensing, separation or drug delivery systems. Such membranes already exist but typically they are slow to respond and require a change in their entire environment. Taking inspiration from plants, whose stomatal pores in the leaf epidermis open in the light to allow gas exchange and close in the dark, for example, J Rubén Morones-Ramírez (www.rubenmorones.com) from the Universidad Autónoma de Nuevo León in México has now devised a membrane that is both light and temperature responsive. The polymer (poly(N-isopropylacrylamide)) was first grafted onto the surface of a porous membrane and gold nanoparticles were subsequently incorporated. Exposure to heat or irradiation with light — as the nanoparticles convert light to heat — rapidly induced a change in the polymer's conformation and, in turn, the opening of the membrane's pores, which closed again when the stimulus was removed. The system shows promise for optothermally switchable valves or flow control. Natural systems employ energy converters in the form of chlorophores or chemical entities to transform light into a mechanical response, which allows them to open and close pores. Inspired by nature, in this work, we used metallic nanoparticles as opto-thermal energy converters to switch thermally responsive polymers incorporated into nanporous membranes to open and close fluid flow.