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Bioconjugates and Polymers for Biomedical Applications
A newer area of research in the Barron laboratory is focused on the development of de novo designed biopolymers and their conjugates for a number of biomedical applications. Presently, we are investigating both synthetic polymers and recombinant proteins as biomaterial building blocks to create multivalent, bioconjugate display scaffolds of polymers and small molecules, scaffolds as MRI contrast agents, and as novel hydrogels for in vitro and in vivo tissue engineering applications. At Stanford, we are working with Michael Longakerís group on bone regeneration projects, using unique animal models of super-critical bone defects developed in his laboratory (i.e., bone defects that are too large to close and heal on their own).
Bioconjugates as Multivalent Display Scaffolds
Biopolymer Conjugates Based on Protein Polymers
Monodisperse protein polymers engineered by biosynthetic techniques are well suited to serve as a basis for creating comb-like polymer architectures for biomaterial applications. We have developed a new class of linear, cationic, random-coil protein polymers designed to act as scaffolds for multivalent display. These polymers contain evenly spaced lysine residues that allow for chemical or enzymatic conjugation of pendant functional groups such as small bioactive peptides, non-fouling polymers, and small organic molecules (see Davis N, Karfeld-Sulzer L, Ding S, Barron AE, "Synthesis and characterization of a new class of cationic protein polymers for multivalent display and biomaterial applications," Biomacromolecules 2009, accepted for publication, in press. Manuscript ID: bm-2008-01348g).
Figure: Schematic to create comb-like protein polymer multivalent scaffolds by chemical or enzymatic reactions.
Biopolymer Conjugates as MRI Contrast Agents
Magnetic Resonance Imaging (MRI) is capable of providing whole-animal or -human imaging at high spatial and temporal resolution, thus providing a great tool for in vivo imaging over time. However, MRI has inherently low contrast distinction between different types of tissues, and therefore, exogenous contrast agents (CAs) are used to provide increased contrast. Although first-generation CAs consisting of a gadolinium (Gd(III)) ion chelated by a small molecule only modestly increase relaxivity, attaching many of these chelators to a larger molecular scaffold can significantly improve contrast. Certain protein polymers created in the Barron lab can be used as a reactive scaffold for multi- and polyvalent display of moieties that are useful for imaging or that are bioactive. We have taken advantage of these reactive sites to create multivalent, high-relaxivity MRI contrast agents by attaching Gd(III) chelators to the protein polymers (see Protein for the Design of a Multivalent MRI Contrast Agent," Bioconjugate Chem. 2007, 18: 1697-1700). We have also shown that Gd(III) chelator-derivatized protein polymers can be crosslinked into protein-based hydrogels, providing a hydrogel that can be observed over time using MRI, e.g., to determine the length of time that the biodegradable material persists in vivo.
Figure: MR images of (a) glutaraldehyde cross-linked hydrogel doped with protein polymer contrast agent scaffolds (b) control hydrogel without contrast agent.
Novel Synthetic Hydrogels as Biomaterials
We are developing a system of biomaterial building blocks that are highly modular and offer the ability to create custom-made hydrogels with controlled physical properties and interactions with cells. We are investigating recombinant biopolymers for a bottom-up approach to create hydrogels with potentially more uniform and controlled material properties due to the monodispersity of the protein building blocks. The hydrogels are formed by enzymatic crosslinking by tissue transglutaminase to couple two classes of protein polymers; one containing evenly spaced lysines and one with glutamines as enzymatic recognition substrates. It is our hope that with further development, these biopolymer-based hydrogels will overcome the limitations of both traditional proteins (e.g., collagen, elastin, and hyaluronic acid from animal sources) and synthetic polymer building blocks, and can be tailored with ease to specific medical applications.
Figure: (A) Reaction of tTG (B) protein polymer precursor solution before and after tTG gelation. (C) Environmental scanning electron micrograph of a protein polymer hydrogel in the hydrated state.
Bioconjugates Researchers at Stanford
Students in the Bioconjugates group