Stimuli-responsive materials as sensors and actuators in microfluidic devices

The integration of stimuli-responsive materials into microfluidic systems can provide a means for external control over fluid flow and can reduce the overall complexity of microfluidic devices. Herein we present several approaches for introducing fluid movement and sensing using stimuli-responsive materials. The first approach comprises the use of adaptive nanostructured coatings for direct sensing of flow in continuous flow mode. For this, the inner walls of micro-capillaries and micro-channels were coated with polymeric materials that can be used to detect a variety of target species. Two types of adaptive coatings will be discussed. The first one is based on the conductive polymer polyaniline (PAni) [1,2] while the second consists of polymeric brushes based on spiropyran [3,4]. Using the “grafting” approach homogeneous coatings were obtained on the micro-channel/micro-capillary surface that retained their inherent nano-morphology. The optical proprieties of these coatings change in response to a variety of target analytical species (divalent metal ions, solvents of different polarities, ammonia, H+) passing through the microfluidic device in continuous flow mode. The grafting approach can provide nanostructured to microstructured coatings that combine small diffusion paths with relatively thick optical pathlengths, thereby providing sensitive and fast optical responses to the target analytes. The second approach comprises the use of porous photo-actuated hydrogels as photo-controlled micro-valves in microfluidic systems for repeatable ON/OFF flow modulation in flowing streams over a wide range of pH values (acidic to ca. pH 7.0). Incorporation of such stimuli-controlled structures in microfluidic devices offers unprecedented versatility and external flow control. We envision using these systems to create a new generation of sustainable, low-cost, photonically-controlled and self-reporting fluidic systems

Solvato-morphologically controlled photo-responsive hydrogels for micro-valve applications

In recent literature, photo-responsive hydrogels have been synthesised by co-polymerisation of N-isopropylacrylamide (NIPAAM) with spiropyran (SP) derivatives. This process demands external protonation of the hydrogels to induce re-swelling, typically by immersing the hydrogel in strong acidic environments. Also the re-swelling times are long, typically up to several hours. These disadvantages have restricted the use of photo-actuated hydrogels to single-use applications. Recently, we reported that the addition of acrylic acid (AA), copolymerised within the hydrogel, provides an internal source of protons that allows photo-actuation in neutral pH environments. [Bartosz, et al. 1] The polymerisation solvent influences the morphology of the hydrogel, by producing porous hydrogels of different pore sizes. This impacts the diffusion path length for water molecules moving in/out of the hydrogel matrix, thus altering the swelling and shrinking kinetics of the hydrogel.[2] In this study photo-actuated hydrogels were generated using p(NIPAAM-co-SP-co-AA) copolymer, in a 100-1-5 mole ratio. Different ratios of water: organic solvent (tetrahydrofran (THF), dioxane and acetone) were used as the polymerisation solvent. This resulted in hydrogels with different pore sizes and different swelling/shrinking and actuation kinetics. By using THF:water (4:1 v:v) as the polymerization solvent, a remarkable contraction in hydrogel size of up to 50% was obtained after four minutes of white light irradiation. Optimising these hydrogels by varying the polymerization solvent has resulted in faster and reproducible shrinking and reswelling cycles. Using the different polymerisation solvents, hydrogel microstructures were photo-polymerised around pillars in-situ inside PDMS/glass microfluidic channels for valve applications. When actuated, the valve contracted thus opening the channel and allowing fluid to flow. The opposite was seen when the valve was kept in the dark. Thus demonstrating successful photo-controlled valves in microfluidic systems

Reversible photo-actuated hydrogels for micro-valve applications

In recent years, a popular way of photo-modulating flow control in microfluidic channels has been through the use of acidified spiropyran (SP) hydrogels that needed to be externally protonated with HCl solutions.1,2 In the swollen protonated merocyanine (MCH+) form, the hydrogel blocks the channels and prevents flow. When exposed to white light, the positively charged MCH+ is converted to the uncharged SP form, triggering shrinking of the hydrogel, and the channel opens. The addition of acrylic acid copolymerised within the hydrogel provides an internal source of protons that allows repeatable photo- actuation in neutral pH environments. Here we report the effect of the polymerization solvent on the shrinking and swelling kinetics of the photo-responsive hydrogel. Using this approach, reversible fast photo-actuated hydrogels have been obtained and have been successfully used for micro-valves applications in micro-fluidic channels

Photo-responsive soft actuators based on spiropyran functionalised hydrogels

Hydrogels are a class of polymeric materials, which swell upon hydration. Hydrogels have been used in many areas such as microfluidic systems where they have the function of pumps or valves [1]. Incorporation of photochromic units in hydrogel materials offers new possibilities for the production of photo-responsive soft actuators[2]. Spiropyrans are one of the most popular classes of photochromic compounds [3]. In acidic environments, the spiropyran gets protonated to form the hydrophilic protonated merocyanine (MCH+) that can be reversed back to the closed hydrophobic spiropyran (SP) by irradiation with white light. Here we demonstrate that the size and volume of a hydrogel comprising a copolymer of acrylated spiropyran can be reduced when exposed to white lig ht irradiation. This causes the gel to shrink and water is expelled. Different ratios of the SP derivative were investigated in order to achieve increased degree of shrinking and improved kinetics. Finally, the newly obtained photo-actuators were polymerized “in situ” in microfluidic channels to obtain photo-actuated microfluidic valves suitable for biomedical applications

Stimuli-controlled fluid movement at the microscale

The integration of stimuli-responsive materials into microfluidic system provides external control over fluid flow and can reduce the over-all complexity of the microfluidic device [1, 2]. In this work, we present two main approaches for stimuli-controlled fluid movement at the microscale. The first approach comprises the use of photo-actuated hydrogels as micro-valves, while the second approach involves stimuli-controlled movement of synthetic micrometre size droplets. 1. Photo-actuator hydrogels were developed using copolymers of N-isopropylacrylamide, acrylated spiropyran and acrylic acid. In water, the acrylic acid comonomer dissociates, resulting in the protonation of the photochromic spiropyran (SP) to protonated merocyanine (MC-H+). This form is hydrophilic, allowing the hydrogel to swell. Exposure to white light promotes isomerisation of the MC-H+ form to the hydrophobic SP form, which triggers contraction of the hydrogel. In this manner, reversible photo-control over the volume of the hydrogel was achieved. It was found that different polymerization solvents directly influenced the morphology of the resulting hydrogels, and in particular, the porosity. This in turn influences the diffusion pathlength of water into/out of the gel, which has a dramatic impact on the swelling and shrinking kinetics of the hydrogel. Microstructures composed of optimised hydrogels were photopolymerised within microfluidic channels, and their application as photo-controlled valves demonstrated. These photo actuators are of great interest as they can be controlled using light, in a noncontact manner. They are also of great interest for biological applications as they can operate in neutral solutions, in contrast to previous formulations [3]. 2. For the second approach, micrometre size droplets were designed to move in an open fluidic channel. The motion of these discrete droplets was controlled by the triggered release of surfactant molecules, which were contained within the droplet. Once released, the surfactant altered the surface tension of the aqueous phase. As the solubility of the droplet-contained surfactant varied with the pH of the aqueous solution, these droplets were guided to specific destinations in fluidic channels through the use of pH gradients. These gradients can be created on demand within microfluidic channels using light due to the photoconversion of sulphonated (water soluble) MC-H+ to SP. This increases the local acidity due to the release of free protons from MC-H+, which triggers the release of surfactant, and stimulates the photo-controlled movement of the droplets

Novel photo-responsive structures for microSensors and microActuators

The continuing interest in stimuli-responsive materials has yielded quite an expansive variety of smart materials that respond to a wide range of stimuli such as electrical current, pH and light, among others [1]. A subclass of this family is comprised of stimuli-responsive hydrogels that are three-dimensional, hydrophilic, polymer networks capable of large water intake. Incorporation of responsive units in such polymeric networks allows for their use as micro-machines capable of doing mechanical work in response to the chosen stimulus. The application of smart materials offers tangible solutions in the field of actuators for microfluidic valves, artificial muscles and biomimetic robots [2-5]. Moreover, new capabilities such as motility, switchable selective uptake and release of molecular agents, sensing, signalling and seeking, will enable microstructures and micro-vehicles to manifest many of the features of biological entities. Herein we explore several bioinspired stimuli-responsive microstructures for actuation and sensing. A particular focus will be the emphasis on the important role of light as a means to enable control and interrogate stimuli-responsive materials, and exploration on how these might provide initial building blocks for creating futuristic microsystems

pH-induced shrinking and swelling of hydrogels based on copolymers of acrylic acid and acrylamide

Hydrogels are three-dimensional polymeric networks that can absorb and retain large quantities of water in relation to their physical size. By incorporating stimuli-responsive units into the gel structure, hydrogel materials can be actuated by external stimuli such as photo, thermal, electro and chemical (e.g. pH). In this paper, we demonstrate that the size and volume of a pH sensitive hydrogel based on acrylic acid (AA) and acrylamide (Am) can change when exposed to different pH environments. The pH responsive hydrogels that were developed used copolymers of AA and Am in different molar ratios 30:70, 50:50 and 70:30, respectively. At a pH value above the pKa of AA (pH > 4.5) the AA dissociates to the more hydrophilic acrylate anion (A-) triggering swelling of the hydrogel. In contrast, at pH < 4.5, the hydrogel contracts due to the formation of the less hydrophilic AA form in the polymer backbone, which triggers the release of water from the gel causing it to physically contract. The hydrogel samples were photo-polymerised using a photo-mask with 1mm diameter circles exposed. Each of the hydrogel samples was placed in pH solutions varying from pH 1-14. The hydrogels with 50:50 molar ratio of Am:AA in the polymer backbone produced hydrogels with the highest relative pH response when compared with the other molar ratios, having a large diameter increase from pH 2 (~0.57mm) to pH 10 (~3.27mm). Successive changes of the solution pH showed that the pH-induced actuation is a reversible process with no detectable hysteresis

Stimuli-responsive hydrogels based on acrylic acid and acrylamide

Hydrogels are three-dimensional polymeric networks that can absorb and retain large quantities of water in relation to their physical size. By incorporating stimuli-responsive units into the gel structure, hydrogels can be actuated by external stimuli such as light[1], temperature[2] and pH[3], among others. In this study pH responsive hydrogels were developed using copolymers of acrylic acid (AA) and acrylamide (Am) in different molar ratios (30:70, 50:50 and 70:30, respectively). In order to turn this pH response into a photo-response, a reversible photo-acid generator, namely spiropyran acrylate (SP-A), was copolymerised in the polymer backbone

pH and photo-responsive hydrogels based on acrylic acid and acrylamide

Hydrogels are three-dimensional polymeric networks that can absorb and retain large quantities of water in relation to their physical size. By incorporating stimuli-responsive units into the gel structure, hydrogel materials can be actuated by external stimuli such as photo, thermal, electro and pH, respectively. In this study, pH responsive hydrogels were developed by using copolymers of acrylic acid (AA) and acrylamide (Am) in different molar ratios (30:70, 50:50 and 70:30, respectively). At pH above the pKa of AA (pH>4.5) the AA dissociates to the more hydrophilic acrylate (A-) form triggering swelling of the hydrogel. In contrast, at pH < 4.5, the hydrogel contracts due to the formation of the hydrophilic AA form in the polymer backbone, which triggers release of water from the gel. In order to turn this pH response into a photo-response, a reversible photo-acid generator, spiropyran acrylate (SP-A), was copolymerised in the polymer backbone. In acidic environments, the SP-A will spontaneously convert to the protonated hydrophilic merocyanine (MC-H+) form and switch back to the hydrophobic SP-A when exposed to white light, expelling a proton in the process. The switching between these two forms can be used to trigger LCST behaviour in the gel, leading to photo-controlled swelling/contraction due to water uptake and release. The composition used for the photo responsive hydrogel was AA: Am: SP-A in a 10:10:1 molar ratio. When the hydrogel is immersed in water, in the dark, the AA dissociates and the proton is taken by the SP-A to form MC-H+, which gives the hydrogel a yellow colour. Under these conditions (A-, MC-H+) the polymer chains are more hydrophilic causing the hydrogel to expand (Fig. 1, initial point). However, when exposed to white light, the MC-H+ is converted back to the SP-A form (colourless) expelling a proton, decreasing the local pH, and protonating the AA. This makes the polymer chain less hydrophilic and the hydrogel contracts (Fig. 1, 0-10 min). As seen in Fig. 1, this process is reversible and with the initial photo-contraction complete in seconds. After ca. 10 min, the white light is switched off, and the hydrogel reswells to about 95% of its fully hydrated size after ca. 15 min in the dark