Role of Mechanical Factors in Applications of Stimuli-Responsive Polymer Gels - Status and Prospects.

Due to their unique characteristics such as multifold change of volume in response to minute change in the environment, resemblance of soft biological tissues, ability to operate in wet environments, and chemical tailorability, stimuli responsive gels represent a versatile and very promising class of materials for sensors, muscle-type actuators, biomedical applications, and autonomous intelligent structures. Success of these materials in practical applications largely depends on their ability to fulfill application-specific mechanical requirements. This article provides an overview of recent application-driven development of covalent polymer gels with special emphasis on the relevant mechanical factors and properties. A short account of mechanisms of gel swelling and mechanical characteristics of importance to stimuli-responsive gels is presented. The review highlights major barriers for wider application of these materials and discusses latest advances and potential future directions toward overcoming these barriers, including interpenetrating networks, homogeneous networks, nanocomposites, and nanofilamentary gels.

Polymerized PolyHEMA photonic crystals: pH and ethanol sensor materials.

The surface of monodisperse silica particles synthesized using the Stober process were coated with a thin layer of polystyrene. Surface charge groups were attached by a grafting polymerization of styrene sulfonate. The resulting highly charged monodisperse silica particles self-assemble into crystalline colloidal arrays (CCA) in deionized water. We polymerized hydroxyethyl methacrylate (HEMA) around the CCA to form a HEMA-polymerized crystalline colloidal array (PCCA). Hydrofluoric acid was utilized to etch out the silica particles to produce a three-dimensional periodic array of voids in the HEMA PCCA. The diffraction from the embedded CCA sensitively monitors the concentration of ethanol in water because the HEMA PCCA shows a large volume dependence on ethanol due to a decreased Flory-Huggins mixing parameter. Between pure water and 40% ethanol the diffraction shifts across the entire visible spectral region. We accurately modeled the dependence of the diffraction wavelength on ethanol concentration using Flory theory. We also fabricated a PCCA (which responds to pH changes in both low and high ionic strength solutions) by utilizing a second polymerization to incorporate carboxyl groups into the HEMA PCCA. We were also able to model the pH dependence of diffraction of the HEMA PCCA by using Flory theory. An unusual feature of the pH response is a hysteresis in response to titration to higher and lower pH. This hysteresis results from the formation of a Donnan potential at high pH which shifts the ionic equilibrium. The kinetics of equilibration is very slow due to the ultralow diffusion constant of protons in the carboxylated PCCA as predicted earlier by the Tanaka group.

Rippling of polymer nanofibers.

This paper studies the evolution mechanism of surface rippling in polymer nanofibers under axial stretching. This rippling phenomenon has been detected in as-electrospun polyacrylonitrile in recent single-fiber tension tests, and in electrospun polyimide nanofibers after imidization. We herein propose a one-dimensional nonlinear elastic model that takes into account the combined effect of surface tension and nonlinear elasticity during the rippling initiation and its evolution in compliant polymer nanofibers. The polymer nanofiber is modeled as an incompressible, isotropically hyperelastic Mooney-Rivlin solid. The fiber geometry prior to rippling is considered as a long circular cylinder. The governing equation of surface rippling is established through linear perturbation of the static equilibrium state of the nanofiber subjected to finite axial prestretching. The critical stretch and ripple wavelength are determined in terms of surface tension, elastic property, and fiber radius. Numerical examples are demonstrated to examine these dependencies. In addition, a critical fiber radius is determined, below which the polymer nanofibers are intrinsically unstable. The present model, therefore, is capable of predicting the rippling condition in compliant nanofibers, and can be further used as a continuum mechanics approach for the study of surface instability and nonlinear wave propagation in compliant fibers and wires at the nanoscale.

Modeling of stimulated hydrogel volume changes in photonic crystal Pb2+ sensing materials.

We modeled the stimulated hydrogel volume transitions of a material which binds Pb2+ and is used as a photonic crystal chemical sensing material. This material consists of a polymerized crystalline colloidal array (PCCA) hydrogel which contains a crown ether molecular recognition group. The PCCA is a polyacrylamide hydrogel which embeds a crystalline colloidal array (CCA) of monodisperse polystyrene spheres of approximately 100 nm. The array spacing is set to diffract light in the visible spectral region. Changes in the hydrogel volume induced by Pb2+ binding alter the array spacing and shift the diffracted wavelength. This system allows us to sensitively follow the hydrogel swelling behavior which results from the immobilization of the Pb2+ by the crown ether chelating groups. Binding of the Pb2+ immobilizes its counterions. This results in a Donnan potential, which results in an osmotic pressure which swells the hydrogel. We continue here our development of a predictive model for hydrogel swelling based on Flory's theory of gel swelling. We are qualitatively able to model the PCCA swelling but cannot correctly model the large responsivity observed at the lowest Pb2+ concentrations which give rise to the experimentally observed low detection limits for Pb2+. These PCCA materials enable stimulated hydrogel volume transitions to be studied.

Electrochemical investigation of Pb2+ binding and transport through a polymerized crystalline colloidal array hydrogel containing benzo-18-crown-6.

The transport of Pb2+ through a sensory gel, a polymerized crystalline colloidal array hydrogel with immobilized benzo-18-crown-6, is important for understanding and optimizing the sensor. Square wave voltammetry at a Hg/Au electrode reveals many parameters. The partition coefficient for Pb2+ into a control gel (no crown ether), K(p), is 1.00 +/- 0.018 (errors reported are SEM). The porosity, epsilon, of the gel is 0.90 +/- 0.01. Log K(c) for complexation in the gel is 2.75 +/- 0.014. Log K(c) in aqueous solution for Pb2+ with the ligand 4-acryloylamidobenzo-18-crown-6 is 3.01 +/- 0.010 with dissociation rate k(d) = (8.34 +/- 0.45) x 10(2) s(-1) and association rate k(f) = (8.79 +/- 0.025) x 10(7) M(-1) s(-1). The partition coefficient of the ligand 4-acryloylamidobenzo-18-crown-6 into the control gel, K(p,L) is 2.07 +/- 0.15. The diffusion coefficient of Pb2+ in the control gel is 6.72 x 10(-6) +/- 0.12 cm(2)/s. For the sensor gel, but not control gel, diffusion coefficients are location dependent. The range of diffusion coefficients for Pb2+ in the probed locations was found to be (6.11-12.60) x 10(-7) cm(2)/s for 0.91 mM Pb2+ and (2.84-9.39) x 10(-7) cm(2)/s for 0.35 mM Pb2+. Lead binding in the sensor gel is slightly less avid than in solution. This is attributed, in part, to the demonstrated affinity of the ligand 4-acryloylamidobenzo-18-crown-6 to the gel. Diffusion coefficients determined for the sensor gel were found to be location dependent. This is attributed to heterogeneities in the crown concentration in the gel. Analysis of diffusion coefficients and rate constants show that diffusion and not chemical relaxation will limit the time response of the material.

High ionic strength glucose-sensing photonic crystal.

We demonstrate a colorimetric glucose recognition material consisting of a crystalline colloidal array embedded within a polyacrylamide-poly(ethylene glycol) (PEG) hydrogel, or a polyacrylamide-15-crown-5 hydrogel, with pendent phenylboronic acid groups. We utilize a new molecular recognition motif, in which boronic acid and PEG (or crown ether) functional groups are prepositioned in a photonic crystal hydrogel, such that glucose self-assembles these functional groups into a supramolecular complex. The formation of the complex results in an increase in the hydrogel cross-linking, which for physiologically relevant glucose concentration blue shifts the photonic crystal diffraction. The visually evident diffraction color shifts across the visible spectral region over physiologically important glucose concentration ranges. These materials respond to glucose at physiological ionic strengths and pH values and are selective in their mode of response for glucose over galactose, mannose, and fructose. Thus, we have developed a new recognition motif for glucose that shows promise for the fabrication of noninvasive or minimally invasive in vivo glucose sensing for patients with diabetes mellitus.

Photonic crystal aqueous metal cation sensing materials.

We developed a polymerized crystalline colloidal array photonic material that senses metal cations in water at low concentrations (PCCACS). Metal cations such as Cu2+, Co2+, Ni2+, and Zn2+ bind to 8-hydroxyquinoline groups covalently attached to the PCCACS. At low metal concentrations (

Photonic crystal carbohydrate sensors: low ionic strength sugar sensing.

We developed a carbohydrate sensing material, which consists of a crystalline colloidal array (CCA) incorporated into a polyacrylamide hydrogel (PCCA) with pendent boronic acid groups. The embedded CCA diffracts visible light, and the PCCA diffraction wavelength reports on the hydrogel volume. This boronic acid PCCA responds to species containing vicinal cis diols such as carbohydrates. This PCCA photonic crystal sensing material responds to glucose in low ionic strength aqueous solutions by swelling and red shifting its diffraction as the glucose concentration increases. The hydrogel swelling results from a Donnan potential due to formation of boronate anion; the boronic acid pK(a) decreases upon glucose binding. This sensing material responds to glucose and other sugars at <50 microM concentrations in low ionic strength solutions.