Hybrid Bioinorganic Smart Membranes That Incorporate Protein-Based Molecular Switches

Polypeptide- and protein-based components have the potential to greatly enhance the functional character of hybrid organic/inorganic materials by imparting recognition, actuation, or transduction properties to these materials. In this study, we demonstrate a simple method of fabricating hybrid silica/polypeptide membranes that exhibit molecular-level control of permeability in response to an external stimulus, namely, heat. The biomolecular switches employed in this study are elastin-like polypeptides (ELPs), which were genetically engineered to allow for control of molecular size and thermal response. ELPs were chosen for incorporation into silica membranes because they exhibit a lower critical solution temperature (LCST) transition in aqueous solution. We demonstrate here that the LCST behavior of ELPs can be retained when they are entrapped in hydrated silica gels and that the LCST transition of these molecules can be used to toggle the permeability of hybrid membranes. Two different ELPs with different LCSTs and different molecular weights (60 and 13 kDa) were examined. They were incorporated into silica membranes using sol−gel synthesis conditions that resulted in the random encapsulation of the proteins in the silica matrixes such that significant porosity is present only upon hydrophobic collapse of the ELPs. Measurement of the permeation of solutions of poly(ethylene glycols) (PEGs) of various molecular weights through centrifugal and ultrafiltration membranes demonstrated that the ELPs in the hybrid membranes act as molecular switches. Below the LCSTs of the ELPs, the hybrid membranes are impermeable to all of the PEG solutions investigated, regardless of the molecular weight of the PEG; above the LCSTs, they are permeable only to those PEGs investigated with molecular weights less than 5000 Da. PEGs investigated of higher molecular weight did not permeate through the hybrid membranes. Thus, the hybrid ELP-containing membranes act as selective molecular weight cutoff filters whose permeability can be switched on and off.