Combinations of a polyanion and a polycation, with different molar masses but narrow molar mass distributions, are employed to construct thin films by the polyelectrolyte multilayering technique. All polyelectrolytes are assembled in the presence of added salt. Even highly charged polymers in the 104 Da range of molar mass exhibit atypical multilayering characteristics. If the molar mass, MM, of either of the polymers is in this range, the thickness increment realized on addition of the shorter polymer is partially lost on exposure to the solution of longer polymer, as surface polyelectrolyte is stripped off by its oppositely charged partner. Complete loss of low MM polyanion on exposure to low MM polycation inhibits multilayer growth altogether. The role of salt is elucidated in the balance between multilayer growth, which relies on kinetically irreversible adsorption, and polyelectrolyte stripping, which produces a stable solution dispersion of polyelectrolyte complex. Subtleties in the irreversibility of commonly employed combinations of high molar mass polymers are revealed by in situ waveguide measurements.

Calorimetry and variable temperature equilibrium doping methods independently show complexation of common polyelectrolytes to be essentially entropy-driven. The complimentary techniques provide enthalpy and free energies of formation or mixing. These findings highlight the importance of polyelectrolyte complexes as ideally mixed polymer blends.

Polyelectrolyte multilayers with continuously variable amounts of ionizable weak acid functionality were prepared by blending ionizable and nonionizable polyelectrolytes in the deposition solutions. Diluting ionizable groups in this way yielded multilayers that were more structurally stable, shown by thickness and atomic force microscopy measurements, as their internal polymer charge was varied by the pH of the external solution. Multilayers prepared with opposite surface charge to that appearing within the bulk (as a result of ionization) were more stable, as were thinner films, both results suggesting the extrusion of bulk charge to the surface. These multilayers were able to control the direction and magnitude of electroosmotic flow in microfluidics systems. Multilayers bearing only one, diluted layer of ionizable material were surprisingly effective in this respect.

Natural proteins perform a variety of functions in biological systems, including actuation, catalysis, structural support, and molecular sequestering. The variety of natural protein functions suggest that they could serve as valuable and versatile building blocks for synthesis of functional materials. Based on this premise, several investigators have developed schemes to include functional proteins into hydrogel networks to take advantage of their structural, catalytic, and ligand binding properties. Natural proteins such as collagen [1] and elastin [2] have been used to develop materials with tailored structural and mechanical properties, while catalysis by enzymes (e.g., glucose oxidase [3] ) has been used to build smart, environmentally-responsive hydrogel networks. The ability of proteins to selectively bind ligands has been used to develop materials that change their degree of physical or chemical crosslinking in the presence of biological antigens, [4,5] small molecule drugs, [6] and carbohydrates, [7][8][9] resulting in environmentallyresponsive biomaterials. In each of these previous studies a nanometer-scale function associated with a specific protein was exploited to build a material that performed an intriguing macroscopic function. A function that has not been explored as extensively in materials science is the ability of proteins to undergo complex conformational changes. Proteins change conformations in response to a broad range of stimuli, including light, pH, and the binding of biological molecules. Over 200 conformational changes are well-characterized, [10] representing a vast unexplored databank of molecular building blocks for design of dynamic materials. Recent studies have demonstrated the utility of protein conformational changes in nano-scale engineered systems, including cooperative nanometer-scale motors [11] and molecular shuttles, [12] suggesting that dynamic protein molecules could also be a useful component of 3-dimensional materials. To that end, we recently reported an approach that used a dynamic protein as a partial cross-linker in a chemically cross-linked hydrogel network, and these hydrogels changed their volume in the presence of a specific protein-binding molecule.[13] Here we describe assembly of protein-based, dynamic materials using a novel photochemical approach. This approach allows for high concentrations of protein to be functionally included into a network in response to light, and the resulting materials undergo striking volume changes upon protein-ligand binding. Photochemical assembly also enables spatial control over the location of dynamic proteins in a hydrogel network, which is likely to be important in potential materials science and engineering applications.

Development of hydrogel materials that respond to specific stimuli has been of significant interest in the design of modern functional materials. A variety of previous studies have used the ligand‐binding capability of proteins to design hydrogels that change their crosslinking density in response to stimuli. However, these materials generally undergo relatively small dynamic response, with limited control over response characteristics. This manuscript describes an alternative approach that exploits the ability of proteins to undergo nanometer‐scale conformational changes in response to stimuli. We report a class of novel protein‐based hydrogel materials that undergo tunable, reversible dynamic responses with a wide dynamic range (volume decreases to ∼25–90% of initial volume). These materials also undergo tunable, reversible changes in optical transparency (optical transparency decreases to ∼35–100% of initial optical transparency), and this phenomenon is used as a mechanism for label‐free biosensing. The materials are generated by photo‐crosslinking of an engineered version of the protein calmodulin flanked on each end with poly(ethylene glycol)‐diacrylate (PEGDA) moieties. The mechanism for dynamic changes derives from calmodulin's well‐characterized “hinge motion”‐upon‐ligand binding. Variations in network parameters in these hydrogels, including the molecular weight between cross‐links and the cross‐linking density, result in systematic tuning of material responsiveness. The influence of network parameters on hydrogel dynamics reported here may serve as a guide for the design of other protein‐based, responsive materials assembled using similar principles, and these materials may be a useful platform for design of biosensors, actuators, and drug delivery systems.