(74g) Nanoporous, Optically Transparent, and Free-Standing Polyelectrolyte Multilayer Films for Applications In Tissue Engineering

The layer-by-layer assembly of alternately charged polyelectrolytes (PEs) is a simple and versatile technique to assemble polymer films with tunable properties.  The resulting polyelectrolyte multilayer (PEM) films have been used in a variety of applications including anti-bacterial coatings, drug delivery, biomaterials and tissue engineering.  The majority of the reports in the literature are on PEMs deposited and adhered to the underlying substrate.  By modulating the deposition properties of the PE and through assembly on an inert, hydrophobic substrate, we have assembled detachable PEMs for applications in tissue engineering.

Chitosan and hyaluronic acid (HA) PEMs were assembled on hydrophobic polytetrafluoroethylene substrates.  To achieve detachability, PEM assembly solution concentrations were varied from 1 to 5 mM and deposition times were varied from 20 to 80 min.  PEMs ranging from 10 to 50 bilayers were detachable.  The free-standing PEM thicknesses ranged from 0.8 to 3.5 μm and were found to be optically transparent and stable in aqueous media.  Glutaraldehyde, a crosslinker, was used to promote PEM stability and increase the Young’s modulus. The PEMs exhibited Young’s moduli ranging from 90 to 400 MPa depending upon exposure time to the cross-linker.  PEMs were exposed to acidic solutions to introduce porosity in the film. The introduction of porosity may be due to phase segregation, reorganization, or degradation within the PEM.  Exposure to acidic solutions with pH values of 3.0 and 3.5, resulted in sub-micron porous PEMs with an average pore size of 300 to 400 nm.

Free-standing, nanoporous PEMs are well suited to mimic basal membranes and the extracellular matrix due to their tunable thickness, porosity, and mechanical properties.  One such application of these free-standing PEMs is in hepatic tissue engineering.  In our research group, we have created a functional 3D liver model comprised of hepatocytes (the principal cell type of the liver) and liver sinusoidal endothelial cells (LSECs) separated by a HA and chitosan PEM interface.^1,2  The PEM mimics the Space of Disse found in vivo, a protein-rich interface between the hepatocytes and LSECs.  The thickness of the Space of Disse ranges from 0.5 to 1 μm and has an average pore size of 100 to 200 nm.  PEMs with an average thickness of 0.8 μm and porosity of 300 nm are ideal candidates to mimic the Space of Disse in vitro.  Our current studies are focused upon tuning the properties to more closely mimic the Space of Disse and incorporating the detachable PEMs into the 3D liver model and monitoring hepatic function. 

References:

1. Rajagopalan, P. et al. Tissue Eng Pt A, 2006, 12, 1553-1563.

2. Kim, Y. et al. Tissue Eng Pt A, 2010, 16, 2731-2741.