Michael J. Serpe

Stimuli-responsive polymers and their applications

Responsive polymer-based materials are capable of altering their chemical and/or physical properties upon exposure to external stimuli. These materials have been intensively studied over the years for a diverse range of applications, e.g., for on-demand drug delivery, tissue generation/repair, biosensing, smart coatings, and artificial muscles. Here, we review recent advances in the areas of sensing and biosensing, drug delivery, and actuators. Specific examples are given in each of these areas, and we highlight our groups work on poly(N-isopropylacrylamide)-based microgels and assemblies.

Reflection order selectivity of color-tunable poly(N-isopropylacrylamide) microgel based etalons.

Materials that exhibit color as a result of refractive index periodicity in the matter they are composed of are typically referred to as photonic materials; the periodicity can be in one, two, or three dimensions. Bragg mirrors, Bragg refl ectors, Bragg stacks, and Fabry–Pérot etalons and interferometers are the simplest photonic materials and rely on periodic spacing between dielectrics in only one dimension. [ 1 ] A Fabry–Pérot etalon or interferometer (or simply etalon) is an optical device that consists of a dielectric layer between two refl ective surfaces or mirrors. When light impinges on an etalon, the contrast between the mirrors and the separating dielectric layer, as well as the refl ection at the mirror/dielectric interfaces, causes interference of the light, which results in a characteristic refl ectance spectrum. The spectrum that results can be described by the equation for the interference of light in an optical cavity, and the maxima of refl ected light intensity is described by the following equation:

Polymer‐Based Muscle Expansion and Contraction

Polymer-based stimuli-responsive polymers and materials have been of considerable interest for many years owing to their ability to convert a chemical or physical stimulus into an observable change in a system. Hydrogel-based thin films, assemblies, and particles (microgels and nanogels) have been designed to respond to a variety of stimuli for a number of potential applications in tissue engineering, artificial muscles, valves, and actuators. Hydrogels are of particular interest owing to their mechanical properties, chemical diversity, and hydration properties, which allow them to interface well with biological systems. Recently, responsive hydrogels and polymer-based thin films have been developed as programmable soft matter or motors by exploiting conformational changes of the polymer that affects the system. 6] Of specific interest to the investigation here are responsive polymer-based systems that are able to do work, that is, lift a mass. These systems, often referred to as artificial muscles, have been the subject of intense research owing to their potential to control movements in mechanical motors. One of the most well studied responsive polymers to date is poly(N-isopropylacrylamide) (pNIPAm), which shows random-coil-to-globule transition at temperatures below 32 8C. 15] Charged pNIPAm-based microgels have been synthesized and used for various applications. By far, the most common chemical functionality added to pNIPAm-based microgels is acrylic acid (AAc). AAc is a weak acid, having a pKa of approximately 4.25, therefore at pH> 4.25 the AAc groups are deprotonated, thus making the microgels negatively charged (polyanionic), while they are neutral at pH< 4.25 owing to AAc protonation. At high pH values the microgels swell because of the charge–charge (Coulombic) repulsion in the polymer network of the microgel. Herein we present a pNIPAm-microgel-based device that is able to do work and lift masses many times the mass of the device, in response to simple changes in the humidity of its environment. The device is constructed by depositing a monolayer of poly(N-isopropylacrylamide)-co-acrylic acid, pNIPAm-co-AAc microgels on an Au-coated substrate. Once deposited, the microgels form a homogenous layer, with the thickness defined by their diameter in solution. The pNIPAm-co-AAc microgels used herein had a solution diameter of (1548 69) nm (measured using differential interference contrast microscopy); the films typically have a thickness of approximately 0.5 of the solution diameter. Subsequently, a solution containing oppositely charged poly(diallyldimethylammonium chloride) (pDADMAC) is added to the microgel-coated substrate and allowed to dry. The pDADMAC solution had a pH of 6.55, which rendered the microgel layer polyanionic. The pDADMAC layer contracts when it dries, owing to water evaporation enhancing hydrophobic interactions between the pDADMAC chains. Since the electrostatic interactions between pDADMAC and the microgels, and the interaction between microgels and the Au layer, are strong, the substrate bends when the polymer dries (Figure 1).

Optical Devices Constructed from Multiresponsive Microgels

Novel multiresponsive microgels based on poly(N-isopropylacrylamide) were synthesized to contain triphenylmethane leucohydroxide, and used to construct etalons. The optical properties of the resultant etalons were investigated, and their response to ultraviolet and visible irradiation, solution pH changes, and the presence of a mimic of the nerve agent Tabun characterized. We clearly show that the optical properties of the device depended dramatically on these stimuli. This investigation illustrates the versatility of the microgel-based etalon structure, and showcases the clear utility of such devices for remote actuation, color tunable optics, sensing, and potential remotely triggered drug delivery applications.

Poly (N-isopropylacrylamide) microgel-based optical devices for sensing and biosensing.

Responsive polymer-based materials have found numerous applications due to their ease of synthesis and the variety of stimuli that they can be made responsive to. In this review, we highlight the groups efforts utilizing thermoresponsive poly (N-isopropylacrylamide) (pNIPAm) microgel-based optical devices for various sensing and biosensing applications.

Responsive polymers for analytical applications: a review.

Stimuli-responsive polymers are capable of translating changes in their local environment to changes in their chemical and/or physical properties. This ability allows stimuli-responsive polymers to be used for a wide range of applications. In this review, we highlight the analytical applications of stimuli-responsive polymers that have been published over the past few years with a focus on their applications in sensing/biosensing and separations. From this review, we hope to make clear that while the history of using stimuli-responsive polymers for analytical applications is rich, there are still a number of directions to explore and exciting advancements to be made in this flourishing field of research.

Polyelectrolyte mediated intra and intermolecular crosslinking in microgel-based etalons for sensing protein concentration in solution

Biotin modified polycationic polymers are capable of penetrating the Au overlayer of poly(N-isopropylacrylamide)-co-acrylic acid microgel-based etalons. Once penetrated, the polycations crosslink the polyanionic microgels, causing them to collapse, resulting in a concomitant blue shift of the spectral peaks in the reflectance spectrum. We show that the magnitude of the blue shift depends on the concentration of the biotinylated polycation solution exposed to the etalon. This behavior is subsequently used for biosensing applications.

Detecting solution pH changes using poly (N-isopropylacrylamide)-co-acrylic acid microgel-based etalon modified quartz crystal microbalances.

Poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgel-based etalons have been shown to have visible color and unique spectral properties, which both depend on solution temperature and pH. In this investigation, pNIPAm-co-AAc microgel-based etalons were fabricated on the Au electrode of a quartz crystal microbalance (QCM), and the resonant frequency of the QCM monitored as a function of temperature, at pH 3.0. Furthermore, the resonant frequency at either pH 3.0 or 7.0 was monitored while keeping the solution temperature constant at various temperatures. In all cases, when the solution temperature was below the collapse transition for the microgels (∼32°C), the resonant frequency at pH 3.0 was lower than at pH 7.0, which we attribute to the film transitioning from a deswollen to swollen state, respectively. It was observed that the magnitude of the resonant frequency change increased as the solution temperature approached the collapse temperature for the microgels. The overall sensitivity to pH was determined to be 1.3×10(-8)M [H(+)]Hz(-1) and a theoretical detection limit of 390nM was obtained. This sensitivity will be exploited further for future biosensing applications.

Deswelling Kinetics of Color Tunable Poly(N-Isopropylacrylamide) Microgel-Based Etalons

Poly(N-isopropylacrylamide) (pNIPAm) microgel-based etalons are optical materials fabricated by depositing a monolithic layer of microgels on a semitransparent Au film adhered to a glass coverslip, followed by the deposition of a second semitransparent Au layer over the microgel layer (overlayer). These materials exhibit characteristic colors and multipeak reflectance spectra, both of which depend on the distance between the Au surfaces (mediated by the microgel diameter) and the refractive index of the microgel layer. In this submission, the deswelling kinetics of pNIPAm microgel-based etalons are investigated by inducing microgel deswelling through exposure to a 30% methanol/H(2)O solution. Exposed to this solvent system, the transition temperature of the microgels is lowered to a temperature below the experimental temperature and the microgels comprising the etalon collapse. This collapse induces an etalon color change, which is observed as a blue shift in the reflectance spectrum. The kinetics of deswelling were shown to be strongly dependent on the thickness of the Au overlayer, e.g., thicker overlayers slow the solvent exchange and the resultant deswelling kinetics. Additionally, for thicker overlayers, the rate of deswelling increases with decreasing etalon size. Taken together, these results suggest that the kinetics depend strongly on the ability of the solvent to exchange from/to the microgel layer. For example, if the Au overlayer is thin, more solvent can exchange through the overlayer in a given amount of time compared to an etalon composed of a thick overlayer. Likewise, etalons of smaller dimensions have faster deswelling kinetics due to the shorter distance the solvent needs to travel laterally through the microgel layer to exchange. The results from this study are of fundamental importance but will be used to develop sensors with fast response times for point-of-care diagnostics.

Stimuli-responsive microgel-based etalons for optical sensing

Responsive polymer-based materials (or smart materials) have found numerous uses over the years due to their ease of synthesis and various responsivities/functionalities. Of them, stimuli-responsive hydrogel particles (microgels) have generated a lot of interest, and have been used for a number of applications, most importantly for this submission is their use as components of photonic materials. In this review, we highlight a few key examples of the use of stimuli-responsive materials for sensing applications, with a particular focus on our work with microgel-based etalons.