RESUMO
Peptoid nanosheets are novel protein-mimetic materials that form from the supramolecular assembly of sequence-defined peptoid polymers. The component polymer chains organize themselves via a unique mechanism at the air-water interface, in which the collapse of a compressed peptoid monolayer results in free-floating, bilayer nanosheets. To impart functionality into these bilayer materials, structural engineering of the nanosheet-forming peptoid strand is necessary. We previously synthesized a series of peptoid analogues with modifications to the hydrophobic core in order to probe the nanosheet tolerance to different packing interactions. Although many substitutions were well-tolerated, routine surface pressure measurements and monolayer collapse isotherms were insufficient to explain which molecular processes contributed to the ability or inability of these peptoid analogues to form nanosheets. Here, we show that surface dilational rheology measurements of assembled peptoid monolayers at the air-water interface provide great insight into their nanosheet-forming ability. We find that a key property required for nanosheet formation is the ability to assemble into a solidlike monolayer in which the residence time of the peptoid within the monolayer is very long and does not exchange rapidly with the subphase. These collapse-competent monolayers typically have a characteristic time of diffusion-exchange values, τD, of >5000 s. Thus, rheological measurements provide an efficient method for assessing the nanosheet-forming ability of peptoid analogues. Results from these studies can be used to guide the rational design of peptoids for assembly into functional nanosheets.
RESUMO
The elastic modulus of the extracellular matrix is a dynamic property that changes during various biological processes, such as disease progression or wound healing. Most cell culture platforms, however, have traditionally exhibited static properties, making it necessary to replate cells to study the effects of different elastic moduli on cell phenotype. Recently, much progress has been made in the development of substrates with mechanisms for either increasing or decreasing stiffness in situ, but there are fewer examples of substrates that can both stiffen and soften, which may be important for simulating the effects of repeated ECM injury and resolution. In the work presented here, poly(ethylene glycol)-based hydrogels reversibly stiffen and soften with multiple light stimuli via photoisomerization of an azobenzene-containing cross-linker. Upon irradiation with cytocompatible doses of 365 nm light (10 mW/cm(2), 5 min), isomerization to the azobenzene cis configuration leads to a softening of the hydrogel up to 100-200 Pa (shear storage modulus, G'). This change in gel properties is maintained over a time scale of several hours due to the long half-life of the cis isomer. The initial modulus of the gel can be recovered upon irradiation with similar doses of visible light. With applications in mechanobiology in mind, cytocompatibility with a mechanoresponsive primary cell type is demonstrated. Porcine aortic valvular interstitial cells were encapsulated in the developed hydrogels and shown to exhibit high levels of survival, as well as a spread morphology. The developed hydrogels enable a route to the noninvasive control of substrate modulus independent of changes in the chemical composition or network connectivity, allowing for investigations of the effect of dynamic matrix stiffness on adhered cell behavior.