Molecular Design of Light-Responsive Hydrogels, For in Situ Generation of Fast and Reversible Valves for Microfluidic Applications

Abstract

Light--responsive hydrogel valves with enhanced response characteristics compatible with microfluidics have been obtained by optimization of molecular design of spiropyran photoswitches and gel composition. Self--protonating gel formulations were exploited, wherein acrylic acid was copolymerized in the hydrogel network as an internal proton do--nor, to achieve a swollen state of the hydrogel in water at neutral pH. Light--responsive properties were endowed upon the hydrogels by copolymerization of spiropyran chromophores, using electron withdrawing and donating groups to tune the gel--swelling rate. Faster macroscopic swelling of the hydrogels was obtained by changing an ester to an ether at the 6' position (factor of 4) or shifting the ether group to the 8' position of the spiropyran (factor of 2.5) producing a 10 fold increase in total. The effect was also visible in the swelling behavior of the corresponding hydrogel valves, where the ob--served macroscopic changes were reversible and reproducible and in agreement with the molecular kinetics. Gel--valves integrated within microfluidic channels have been fabricated and allow reversible and repeatable operation, with opening of the valve effected in 1 minute, while closing takes around 5.5 minutes.

Microcantilever arrays functionalised with spiropyran photoactive moieties as systems to measure photo-induced surface stress changes

“…Previous work on spiropyran photochromic systems include their use for photo-induced optical sensing of metal ions, continuous flow optical sensing of solvents of different polarity [23,24], reversible photo-actuators for microfluidic applications [25][26][27], photo-chemopropulsion of microdroplets [28], and self-assembly [29,30], among others. More recently it was demonstrated that the conversion between the SP and MC form can be achieved also mechanically under tensile loading, showing the capability to create mechanochromic materials using SP containing polymers [31].…”

Design and Applications of Photoresponsive Hydrogels

“…UV light irradiation (365 nm) causes the azobenzene units to change to the cis state, disrupting the host-guest complexes and expanding the network. [131] Photothermal actuators are particularly inter esting, where actuation with harmless irradiation (e.g., NIR) is required. Gels immersed in water were found to increase up to 124 wt% in weight upon UV irradiation and decrease back to 104 wt% upon visible light irradiation.…”

Stimuli-Controlled Fluid Control and Microvehicle Movement in Microfluidic Channels

“…Many photochromic molecules have been studied, but three of the best-known are spirobenzopyrans (SP) [54-56, 59, 60], azobenzenes [61,62], and diarylethenes [63], all of which show reversible photochromic behaviour, transforming from one isomer to another when exposed to different wavelengths of light. Much research has been carried out into optimising the photo-response of these materials when incorporated into hydrogel matrices, for exploitation in microfluidic systems as valves, pumps, mixers and alternating the topography of surfaces [64][65][66][67]. In addition to the development of the materials themselves (e.g.…”

“…Building on this research, Schiphorst J. et al [67] further improved the actuation kinetics by varying the side chain on the spiropyran derivative. The improved formulation was then polymerised in-situ in a microfluidic channel to create a valve structure and exposed to blue light.…”

“…[9] While there are still many challenges facing this field, there have been a number of revolutionary approaches, one of the most exciting of which is the incorporation of stimuli-responsive actuators within the fluidics, to achieve integrated fluid handling on-chip. This approach can result in flow control, [10,11] mixing, [12] flow sorting, [13] pumping [14] and even sensing components fully integrated into the microfluidics chip [15]. Recent exciting developments in stimuli-responsive materials, and particularly stimuli responsive polymers and surfaces make microfluidics a great platform for demonstrating the capabilities and highlighting the functionality of these materials.…”