Polyelectrolyte nanofilm biomaterials: optimizing mechanical rigidity and bioactivity

Nanofilm biomaterials – formed by the layer-by-layer assembly of biocompatible polyelectrolytes – are important systems for a variety of cell-contacting applications. Mechanical rigidity and bioactivity are two key film properties known to significantly influence the behavior of contacting cells. Cells tend to attach more efficiently to “harder” films, and bioactivity may be conferred through the incorporation of proteins, peptides, or drugs within the film architecture. A key challenge is to realize films that are simultaneously rigid and bioactive. Chemical cross-linking of the polymer framework – the standard means of increasing a film’s rigidity – often enhances cell attachment, but can suppress bioactivity. In particular, cross-linking steps can deactivate embedded biomolecules, limit their mobility, and/or suppress film biodegradation, thus limiting the extent of cell-bioactive species interaction. We present here two strategies to decouple mechanical rigidity and bioactivity, potentially enabling nanofilm biomaterials that are both mechanically rigid and bioactive. One approach is to selectively cross-link the film’s outer “skin”, to promote cell attachment, such that the film interior remains “soft”, to promote cellular interaction with bioactive species. The other approach involves nanoparticle templates, whose removal following full-film cross-linking yields pore space capable of significant bioactive species loading. We characterize film assembly and cross-linking extent via quartz crystal microgravimetry with dissipation (QCMD) and Fourier transform infrared spectroscopy in attenuated total reflection mode (FTIR-ATR), film rigidity via nanoindentation analysis, and cell adhesiveness with respect to a system of pre-osteoblastic MC3T3-E1 cells. We show both strategies to yield films whose mechanical rigidity and cell adhesiveness are comparable to standard cross-linked (positive control) films, but whose bioactive species loading capacity is significantly enhanced. Nanofilm biomaterials that are simultaneously cell adhesive and bioactive are excellent candidates for a variety of cell contacting applications.