2, Northwestern University, Chicago, Illinois, United States
3, Northwestern University, Evanston, Illinois, United States
The extracellular matrix (ECM) provides tissues and organs suitable mechanical properties and chemical signals to trigger cell adhesion and differentiation. In tissue engineering, biomaterials provide the cells with an appropriate environment. Fibrillar proteins from the ECM (collagen, fibronectin) and synthetic peptide-based biopolymers (e.g. Peptide Amphiphile (PA)) are widely used as structuring scaffold. PAs that self-assemble into microfibers are biocompatible, biodegradable and can incorporate signaling motifs from the ECM within their peptide sequence. This way the bioactive signals are displayed to cells to control their behavior.
Biological events are dictated by spatiotemporal heterogeneities of biomacromolecules, which means that the variation of the local concentration of biomolecules is a key signal to trigger a given cell behavior. It is then necessary to engineer biomaterials with well-defined clusters. This can be achieved by a composite approach where bio-inspired polymers are combined with nanoparticles. To this aim, silica particles (SiNP) are very interesting candidates due to their low cost, biocompatibility, easy synthesis and surface functionalization. Through the incorporation of bioconjugated SiNP into PA scaffolds, we aim at providing a unique way of tuning the scaffold bioactivity with improved modularity, by adjusting (i) the grafting of a bioactive peptide , (ii) the concentration of SiNP within the matrix, and (iii) by combining SiNP grafted with different peptide epitopes.
In this work we show how the 3D spatial positioning of the fibronectin derived epitope RGDS affects cell response. Fibroblasts cultured on RGDS-grafted SiNP@PA matrix where found to be more efficiently spread compared to RGDS-presenting PA nanofiber. Indeed, the required amount of RGDS signal was four times lower for RGDS-SiNPs@PA than for the RGDS-PA for a similar cell spreading. This is attributed to the clustering of the bioactive epitope at the SiNP surface. In addition, the hybrid SiNPs@PA material offers the possibility to incorporate several biological signals to act synergistically. This is particularly interesting when working with the ECM motifs RGDS and PHSRN that operate in a spacing-dependent manner to promote cell adhesion and spreading. We show that when the two peptides are on the PA matrix or grafted on two different populations of SiNPs, fibroblasts show no spreading. This can be improved when one is on SiNP and the second one on PA ensuring the close interaction between RGSD and PHSRN. The synergic effect is further improved when both peptides are grafted on the same particle.
This 3D biocompatible material displays all necessary signals in a controlled way without altering the self-assembly and mechanical properties of the biopolymer, being a unique model to mimic the natural ECM in a tissue engineering point of view.
 Hartgerink, J. D. et al. Science 2001, 294, 1684
 Aimé, C. et al. Nanoscale, 2012, 4, 7127