Devin G. Barrett

Design and Applications of Biodegradable Polyester Tissue Scaffolds Based on Endogenous Monomers Found in Human Metabolism

Synthetic polyesters have deeply impacted various biomedical and engineering fields, such as tissue scaffolding and therapeutic delivery. Currently, many applications involving polyesters are being explored with polymers derived from monomers that are endogenous to the human metabolism. Examples of these monomers include glycerol, xylitol, sorbitol, and lactic, sebacic, citric, succinic, α-ketoglutaric, and fumaric acids. In terms of mechanical versatility, crystallinity, hydrophobicity, and biocompatibility, polyesters synthesized partially or completely from these monomers can display a wide range of properties. The flexibility in these macromolecular properties allows for materials to be tailored according to the needs of a particular application. Along with the presence of natural monomers that allows for a high probability of biocompatibility, there is also an added benefit that this class of polyesters is more environmentally friendly than many other materials used in biomedical engineering. While the selection of monomers may be limited by nature, these polymers have produced or have the potential to produce an enormous number of successes in vitro and in vivo.

Aliphatic polyester elastomers derived from erythritol and α,ω-diacids

Soft polyester elastomers have emerged as a promising family of biodegradable materials for drug delivery and tissue engineering. Specifically referring to soft tissue engineering, potential biomaterials should be elastic and flexible, so as to mimic the mechanical properties of natural tissue. Herein, we report the design of several elastomers based on the polycondensation of erythritol, a sugar substitute that is approved for human consumption by the Food and Drug Administration, and one of eight dicarboxylic acids: glutaric, adipic, pimelic, suberic, azelaic, sebacic, dodecanedioic, and tetradecanedioic acids. By varying the length of the diacid and the curing conditions, several elastomers were designed with a range of physical and mechanical properties. Poly(erythritol glutarate), poly(erythritol adipate), poly(erythritol pimelate), poly(erythritol suberate), poly(erythritol azelate), poly(erythritol sebacate) poly(erythritol dodecanedioate), and poly(erythritol tetradecanedioate) achieved Youngs modulus, ultimate tensile stress, and rupture strain values of 0.08–80.37 MPa, 0.14–16.65 MPa, and 22–466%, respectively. Additionally, as tissue engineering may require the use of complex 3-dimensional designs, embossed films and porous films were designed in order to demonstrate the ease of processing. Hydrolytic degradation rates ranging from 100% in 3 weeks to 6.4% in 6 weeks were obtained in phosphate-buffered saline solutions at 37 °C. Finally, in vitro cytotoxicity was studied with Swiss albino 3T3 fibroblasts and human mesenchymal stem cells. Based on these results, we believe that the poly(erythritol dicarboxylate) series are excellent candidates for potential soft biomaterials.

Microfluidic Lithography of SAMs on Gold to Create Dynamic Surfaces for Directed Cell Migration and Contiguous Cell Cocultures

A straightforward, flexible, and inexpensive method to create patterned self-assembled monolayers (SAMs) on gold using microfluidics-microfluidic lithography-has been developed. Using a microfluidic cassette, alkanethiols were rapidly patterned on gold surfaces to generate monolayers and mixed monolayers. The patterning methodology is flexible and, by controlling the solvent conditions and thiol concentration, permeation of alkanethiols into the surrounding PDMS microfluidic cassette can be advantageously used to create different patterned feature sizes and to generate well-defined SAM surface gradients with a single microfluidic chip. To demonstrate the utility of microfluidic lithography, multiple cell experiments were conducted. By patterning cell adhesive regions in an inert background, a combination of selective masking of the surface and centrifugation achieved spatial and temporal control of patterned cells, enabling the design of both dynamic surfaces for directed cell migration and contiguous cocultures. Cellular division and motility resulted in directed, dynamic migration, while the centrifugation-aided seeding of a second cell line produced contiguous cocultures with multiple sites for heterogeneous cell-cell interactions.

Preparation of a Class of Versatile, Chemoselective, and Amorphous Polyketoesters

A straightforward and versatile strategy for preparing a class of biodegradable and amorphous polyketoesters is reported. A series of ketone-containing diesters and diacids were combined with di(ethylene glycol) through condensation polymerization, achieving values of up to 10.1 x 10(3) g/mol. Glass transition temperatures ranged from -41 to -6 degrees C, rendering all of the materials liquid at room temperature. By including ketone groups in the repeat unit, facile postpolymerization modifications were possible by reaction with oxyamine-tethered ligands through the formation of an oxime linkage. Upon reaction with molecules that contain oxyamines, under mild conditions, these polymers can easily have a diverse set of side chains appended without coreagents or catalysts. The chemoselective oxime-forming coupling strategy is compatible with physiological conditions and can be done in the presence of a wide range of functional groups and biomolecules, including proteins and nucleic acids. We demonstrate the utility of this strategy by immobilizing a cell adhesive peptide (H2NO-RGD) to polyketoester films, creating cell adhesive elastomers. This immobilization strategy is synthetically flexible for designing and tailoring polymers for targeted biological applications.

Microfluidic etching and oxime-based tailoring of biodegradable polyketoesters.

A straightforward, flexible, and inexpensive method to etch biodegradable poly(1,2,6-hexanetriol alpha-ketoglutarate) films is reported. Microfluidic delivery of the etchant, a solution of NaOH, can create micron-scale channels through local hydrolysis of the polyester film. In addition, the presence of a ketone in the repeat unit allows for prior or post chemoselective modifications, enabling the design of functionalized microchannels. Delivery of oxyamine tethered ligands react with ketone groups on the polyketoester to generate covalent oxime linkages. By thermally sealing an etched film to a second flat surface, poly(1,2,6-hexanetriol alpha-ketoglutarate) can be used to create biodegradable microfluidic devices. In order to determine the versatility of the microfluidic etch technique, poly(epsilon-caprolactone) was etched with acetone. This strategy provides a facile method for the direct patterning of biodegradable materials, both through etching and chemoselective ligand immobilization.

Thermosets synthesized by thermal polyesterification for tissue engineering applications

Synthetic polyesters have become an integral part of various biomedical and engineering fields, such as tissue scaffolding and therapeutic delivery. While polymer and copolymers derived from lactide, glycolide, e-caprolactone, and p-dioxanone still dominate the biomedical industry, many new polymerization strategies and materials are being explored, including thermal polycondensation. Adjusting curing conditions and monomer feed ratios allows for easy control over the macromolecular properties of polymers resulting from thermal polyesterification. In terms of mechanical versatility, crystallinity, hydrophobicity, and biocompatibility, polyesters synthesized thermally have displayed a wide range of properties. These properties allow for materials to be tailored according to the needs of a particular application. Additionally, several natural metabolites—some endogenous to human biochemical pathways—are able to serve as precursors for a significant number of these polyester thermosets. While many starting materials have been reused and reformulated to produce novel polyesters, new small molecules are continually being introduced to the materials community as promising monomer candidates to continue the growth of this research area. To date, polyesters synthesized with thermal polycondensation have already produced a tremendous amount of in vitro and in vivo success.

Pressure perturbation calorimetry of helical peptides

Pressure perturbation calorimetry quantifies the temperature dependence of a solutes thermal expansion coefficient, providing information about solute–solvent interactions. We tested the idea that pressure perturbation calorimetry can provide information about solvent‐accessible surface area by studying peptides with different secondary structures. The peptides comprised two host–guest series: one predominately an α‐helix, the other predominately a polyproline II helix. In aqueous buffer, we find a correlation between the amount of secondary structure as assessed by circular dichroism spectropolarimetry and the pressure perturbation calorimetry data. We conclude that pressure perturbation calorimetry can provide information about the exposure of polar and nonpolar surface area. Data acquired in a buffered urea solution, however, are not as easily interpreted. Proteins 2006.

A Tunable, Chemoselective, and Moldable Biodegradable Polyester for Cell Scaffolds

A tremendous amount of research has gone into generating many different types of biodegradable materials for applications ranging from tissue engineering and drug delivery to green chemistry plastics. For biological applications, one of the most studied classes of biodegradable materials is polyesters, due to their versatility in synthetic design and their low cost. Although there have been a few successful applications, the majority of potential polymer candidates are unable to function predictably in a complex and evolving biological environment. If both the material and biological criteria of a polymer are taken into consideration during the design phase, the probability of achieving a particular biological application will be enhanced. For more sophisticated biological applications, there still remain several challenges in using biodegradable polyesters. We believe that, in order to generate a more flexible biodegradable material for a diverse set of cell biological and tissue engineering applications, the materials must be designed to possess the following criteria : the polymer should 1) be easily functionalized, 2) not elicit a cytotoxic response, 3) be moldable to generate a variety of 3D structures and features, and 4) offer the potential to tune the degradation rates. An important characteristic of polymers for in vivo studies is the inherent cytotoxicity of the material and its degradation byproducts. In synthesizing the material, two approaches relating to cytotoxicity exist. The first and more common approach is to design a material with the anticipation that it will not affect cells negatively. This strategy usually requires little or no prior knowledge of the material’s behavior in a biological environment or of the intricate biological processes of cells and tissues. A second, less common strategy involves using natural products as monomers. This reduces the potential cytotoxicity of some of the degradation products as they are naturally occurring and are involved in normal cellular metabolic functions and pathways. Another challenge in polymer design has been the development of the capability to functionalize biodegradable materials easily, mildly, and specifically. Related to the concept of facile functionalization is the possibility of temporal control of ligand conjugation. Some methods are able to introduce functionality during the polymerization process; this precludes opportunities for introducing chemical groups as a function of time and also for functionalizing selected regions while others remain unmodified. To overcome this limitation, alternate strategies have incorporated functional handles capable of post-polymerization, chemoselective modifications. While these examples are chemoselective, they usually require extra reagents that are often necessary to enable the coupling chemistry. These extra reactions and compounds could lead to the degradation of polymer chains or could introduce compounds that may be cytotoxic and not amenable to cell culture conditions. One of the most successful subclasses of biodegradable materials has been based on modified poly(e-caprolactone) systems, which exhibit several advantageous properties. However, some difficulties concerning processing and degradation arise, due to the inherent crystallinity of the polymers. In addition, when macromolecules are designed on the basis of modified lactones, multiple challenges present themselves: 1) a new monomer is required for each new functional group that is introduced, 2) the synthesis of modified rings can be challenging, and 3) post-polymerization deprotection is often required, which can degrade the polymer backbone. An important feature that would add to the flexibility of biodegradable materials, and therefore increase the scope of applications, would be the ability to mold an amorphous polymer chemically. Cross-linking of a polymer by the introduction of heat or light would allow thermosets to be designed in conjunction with a range of fabrication techniques, which can lead to materials with a spectrum of feature sizes and geometries. This capability would increase the versatility of a polymer and allow the scaling of the material from the millimeter regime, to support cells, to the microand nanoscales, for delivery agents of therapeutics to target specific cells and tissues. The ideal polymer for biological applications would therefore be degradable, noncytotoxic, amorphous, and easily functionalizable with a wide variety of ligands. Also, through the use of condensation polymerization, thermal, mechanical, and solubility properties can be controlled and tuned easily. In this manner, polymers could be designed to investigate many different research areas in biology: geometry effects, size effects, ligand density studies, surface interactions, and cell permeability requirements. A moldable and chemoselective bioACHTUNGTRENNUNGdegradable polymer would potentially be able to address a wide range of issues from tissue engineering to microparticles for drug delivery in biotechnology. Here we report the synthesis of a poly(ester ether) that serves as a flexible and straightforward material for biological applications. Our design is based on the chemoselective coupling reaction between ketones and oxyamines. Through the condensation polymerization of tetra(ethylene glycol) and aketoglutaric acid, a natural product, a biodegradable polymer containing a ketone in each repeat unit was synthesized (Scheme 1). Upsetting the stoichiometry allowed the polyester [a] D. G. Barrett, Prof. M. N. Yousaf Department of Chemistry and the Carolina Center for Genome Science University of North Carolina at Chapel Hill Chapel Hill, North Carolina 27599-3290 (USA) Fax: (+1)919-962-2388 E-mail : mnyousaf@email.unc.edu Supporting information for this article is available on the WWW under http://www.chembiochem.org or from the author.

Developing chemoselective and biodegradable polyester elastomers for bioscaffold application

Thermal polyesterification has emerged as a successful method for synthesizing polyesters for biomedical applications. However, to date, no general functionalization strategy has been incorporated into materials designed by the thermal polycondensation of polyacids and polyols. Herein, we report the design of several elastomers based on the thermal polycondensation of 4-ketopimelic acid, citric acid, and one of two diols: 1,6-hexanediol or 1,4-cyclohexanedimethanol. By varying the diol and the curing conditions, several elastomers were designed with a range of physical and mechanical properties. Poly(diol 4-ketopimelate-co-diol citrate) achieved Youngs modulus, ultimate tensile stress, and rupture strain values of 0.39-1.13 MPa, 0.27-1.04 MPa, and 108-426%, respectively. Additionally, the incorporation of the ketone from 4-ketopimelic acid gave these materials two advantageous characteristics: a site for covalent functionalization through oxime formation and the ability to covalently bond to the surrounding tissue through imine linkages. Biocompatibility was studied both in vitro and in vivo in order to gain a complete understanding as to how biological systems respond to these novel materials. Based on preliminary results, we believe that poly(diol 4-ketopimelate-co-diol citrate) polyketoesters are excellent candidates for biomaterials.

Role of surface chemistry and topology of chemoselectively tailored embossed films on shear adhesion

Surface topology has been shown to play a crucial role in the adhesive ability of gecko-inspired, embossed films. Herein, we report the use of Pattern Replication In Non-wetting Templates (PRINT) to design micro- and nano-embossed films in order to explore the relationship between surface chemistry, feature size, feature geometry, and shear adhesion. Ketone-containing elastomers were synthesized from various amounts of poly(ethylene glycol) diacrylate, 2-hydroxyethyl methacrylate, and 2-(methacryloyloxy)ethyl acetoacetate. Due to the presence of the ketone, elastomer functionalization was demonstrated with various oxyamine-, hydrazine-, and hydrazide-terminated ligands. Films were also shown to be non-cytotoxic for future applications in biomedically related fields. This system may allow for the design of flexible adhesives that can be selectively tailored for a range of applications.