Analysis of photoinitiated polymerization in a membrane mimetic film using infrared spectroscopy and near-IR Raman microscopy

Biomimetic design and performance of polymerizable lipids

Bilayer lipid membranes (BLMs) have received significant attention over the past several decades because of their applications in biological and material sciences. BLMs consist of two amphiphilic lipid layers arranged with their hydrophilic head region exposed to the surrounding aqueous environment and hydrophobic domains in the core. In biology, lipid membranes confine and support the cell structure while selectively controlling the diffusion of ions and proteins between the intra- and extracellular matrix (ECM). Naturally derived lipid monomers spontaneously self-assemble to develop smart gateways that recognize and incorporate desired protein transporters or ion channels. BLMs are useful research models of lamellar lipid assemblies and associated protein receptors in cell membranes. The transport properties of lipid membranes can be tuned through careful consideration of the solution medium, transporter functionality, and pH, as well as other environmental conditions. BLMs are of particular interest in the design of biofunctional coatings, controlled release technologies, and biosensors; however, high-performance applications require lipid membranes to remain stable under harsh denaturing conditions. Accordingly, synthetic strategies are often proposed to increase the chemical and mechanical stability of lipid assemblies. The polymerization of self-assembled lipid structures is a strategy that results in robust biocompatible architectures, and diverse reactive functional groups are available for the synthesis of monomeric lipids. The selection of the polymerizable functionality and its precise location within the lipid assembly influences the ultimate supramolecular microstructure and polymerization efficiency. The biomimetic potential of polymerized lipids depends on the stability and robustness of the self-assembled membranes, and it is essential that the polymerizable functionality not disturb the amphiphilic nature of the lipid to maintain biocompatibility. Innovative applications are the motivational force for the development of durable polylipid compositions. Surface modification with biocompatible polylipids provides the opportunity for specific binding of biological molecules for applications as sensors or controlled release delivery vehicles. The ability to create stable lipid assemblies requires a comprehensive understanding of the mechanism of lipid polymerization in confined supramolecular geometries. The future is exciting as researchers begin to fully understand the morphology of polylipids in an effort to successfully produce naturally derived sustainable materials. In this Account, we highlight recent efforts to covalently stabilize lipid membranes and discuss emerging applications of mechanically robust self-assembled lipid architectures.

In vivo biocompatibility and stability of a substrate-supported polymerizable membrane-mimetic film

The cell membrane establishes an important paradigm for the molecular engineering of coatings for implantable devices because of its intrinsic biocompatibility and ability to act as a template for the assembly of diverse membrane-associated macromolecules. A stabilized membrane-mimetic film was assembled on alginate/Ca(2+) hydrogel microcapsules by in situ polymerization of an acrylate functionalized phospholipid. The phospholipid monomer was prepared as unilamellar vesicles and fused onto octadecyl chains that were components of an amphiphilic terpolymer anchored onto a polyelectrolyte multilayer (PEM) by electrostatic interactions. Microcapsules coated with a membrane-mimetic film were implanted into the peritoneal cavity of C57BL/6 mice, and the short-term biostability and biocompatibility of membrane-mimetic films assembled on two different alginate/poly(l-lysine) PEM cushions were compared. The nature of the underlying PEM support had a profound impact on the biocompatibility of the membrane-mimetic film, as the percentage of retrieved microcapsules completely overgrown with host cells shifted from 66+/-5.9% to less than 1% when modifications to the PEM were made. When assembled on the appropriate PEM support, biocompatibility of membrane-mimetic-coated microspheres was high wherein 87.5+/-5.7% of the implanted microspheres were retrieved 4 weeks after implantation and 92.6+/-6.4% of the retrieved capsules were free of cell adhesion or fibrotic overgrowth. Finally, 4 weeks after implantation, microspheres coated with a Texas red-labeled membrane-mimetic film were imaged with confocal microscopy and exhibited a uniform film around the periphery of the implant, indicating a high degree of film biostability. Hence, membrane-mimetic films provide a new route for generating robust, biocompatible, and biochemically heterogeneous coatings for implantable devices through molecular self-assembly.

Fabrication of a phospholipid membrane-mimetic film on the luminal surface of an ePTFE vascular graft

A stabilized, membrane-mimetic film was produced on the luminal surface of an ePTFE vascular graft by in situ photopolymerization of an acrylate functonalized phospholipid using a fiber optic diffusing probe. The phospholipid monomer was synthesized, prepared as unilamellar vesicles, and fused onto close-packed octadecyl chains that were components of an amphiphilic terpolymer anchored onto the polyelectrolyte multilayer (PEM) by electrostatic interactions. Scanning electron microscopy (SEM) confirmed that gelatin impregnation of the graft followed by the subsequent biomimetic film coating filled in the fibril and node structure of the luminal surface of the ePTFE graft and was smooth. The lipid film displayed an initial advancing contact angle of 44 degrees , which increased to 55 degrees after being subjected to a wall shear rate of 500s(-1) for 24h at 37 degrees C in phosphate buffered saline (PBS). Fourier transform (FT-IR) spectroscopy was used to characterize the stages of biomimetic film assembly and confirmed the stability of the film under shear flow conditions. In vivo assessment using a baboon femoral arteriovenous shunt model demonstrated minimal platelet and fibrinogen deposition over a 1-h blood-contacting period. The results of this study confirm the versatility of a biomimetic film coating system by successfully transferring the methodology previously developed for planar substrates to the luminal surface of an ePTFE vascular graft.