Poster O5 - Molecular Design Mediated Simultaneous Control of Mechanics and Degradation Enabling Multifunctional Citrate-Based Biomaterials

Introduction: The entry of poly(octamethylene citrate) (POC) to the biomedical device market as the latest in only 9 FDA approved resorbable polymers represents a new step in development of proregenerative biomaterials; however, balancing biocompatibility, mechanics, and resorption rates within a single platform to meet the widely divergent properties of biological tissues remains a critical challenge for both citrate-based materials and the biopolymer field at large. Herein molecular design enables mechanics spanning multiple orders of magnitude (exceeding POC by >100x in strength and modulus) from soft elastomers to hard plastics via concomitant control of polymer chain branching, crosslink density, and inter/intramolecular hydrogen bonding within a single step synthesis utilizing commercially available and biocompatible polyacid (citrate), diol (1,8 octanediol), and polyol (xylitol) monomers. Simultaneously, polyol feeding ratio and thermal crosslinking mediated variation of polymer hydrophilicity and crosslink density afford tunable degradation rates ( < 1 week - >12 months). Strong inter/intramolecular interactions further enable intrinsically band shifting fluorescence spanning blue to red and glass transition-based shape memory spanning 0°C to 90°C. Hydroxyl group dependent modulation of surface chemistry increased cell proliferation while released citrate offers innate antimicrobial effects. In an exemplary application, strong interactions between citrate and hydroxyapatite are utilized to fabricate orthopedic composites surpassing the mechanics of cortical bone (~300 MPa compressive stress) while maintaining variable degradation (6 weeks - > 1 year), enhancing biocompatibility and osteogenic properties, and exceeding clinically used PLGA/HA in cranial bone regeneration in vivo, establishing a multipotent material via molecular structure-based tailoring of physical properties spanning skin to bone.

Materials and

Methods: Highly tunable citrate/xylitol-based polymers (CXBPs) were synthesized via a single step, catalyst free polycondensation reaction of citric acid, 1,8 octanediol, and xylitol, varying xylitol molar feeding ratios from 0 to 100% of total diol/polyol content. Resulting branched prepolymers were purified via precipitation in DI water followed by lyophilization and dissolution in organic solvent. Synthesized prepolymers were characterized to assess the effect of feeding ratio on molecular weight, degree of branching, and functional group content via MALDI-MS, GPC, and NMR/FTIR. Films, porous scaffolds, and composites were fabricated via solvent casting, porogen leaching, and physical mixing with hydroxyapatite ceramic, respectively, followed by thermal crosslinking to obtain the final materials. Effect of monomer feeding ratio, thermal crosslinking time, and ceramic content on polymer hydrophilicity, mechanics, swelling, and in vitro degradation rates was thoroughly studied while polymer structure and inter/intramolecular bonding was assessed via AFM and FTIR. Optical and fluorescence properties were studied via UV-VIS and fluorometer while shape memory and thermal properties were studied using DMA and DSC/TGA. In vitro citrate release was quantified via HPLC. Biocompatibility and cell proliferation was assessed on films using L929 cells and on composites using human mesenchymal stem cells. Osteoconductivity and osteogenic potential were assessed via in vitro mineralization, ALP detection, and RT-PCR. Antimicrobial potential was assessed against s. Au and e. Coli. Finally, bone regeneration in vivo was assessed in a rat critical size cranial defect model using composite microparticles compared to empty defect and PLGA/HA controls, with bone regeneration measured via uCT and histological staining.

Results, Conclusions, and Discussions: Harnessing the interplay between polymer network branching, hydrogen bonding, and hydrophilicity/hydrophobicity resulting from the carboxyl groups, aliphatic carbon chains, and multiple hydroxyl groups of citrate, 1,8-octanediol, and xylitol, respectively, a novel class of biodegradable thermoset biopolymer (CXBPs) was developed. Molecular design via modulation of diol/polyol ratios in combination with variation of thermal crosslinking conditions resulted in peak tensile stresses from 1.5 to 110MPa coinciding with tensile moduli ranging from 50MPa to 3GPa and tensile strains from 2 to >500%, matching biological tissues ranging from skin and muscle to bone. Concurrently, degradation rates were varied from 10 days to >12 months. Further, utilizing the effects of diol/polyol ratio and thermal crosslinking on both mechanics and degradation (wherein the opposing effects of crosslink density and hydrophilicity can be readily modulated), combinations of elastic/plastic mechanics and rapid/slow degradation can be chosen to suit tissue specific material properties and resorption rates. Further, CXBPs display full spectrum fluorescence ranging from 300nm to 650nm while also demonstrating shape memory properties over a wide range of temperatures from 0 to 90°C. Steady release of citrate results in 100% inhibition of both gram positive and gram-negative microbes, while increased hydroxyl surface groups enhance biocompatibility and cell proliferation. Further, the strong binding interactions between citrate and calcium phosphate ceramics result in high strength CXBP/hydroxyapatite composites (up to 300MPa and 2GPa compressive strength and modulus, respectively), exceeding the mechanics of native cortical bone, while maintaining tunable degradation rates (from 6 weeks to >1 year) as well as impressive biocompatibility, antimicrobial potential, and osteogenic capabilities, culminating in increased cranial bone regeneration compared to commercially utilized PLGA/HA. Critically, the above material and functional properties are obtained via a simple, single step synthesis utilizing biologically safe monomers available at commodity scale, in contrast to previous efforts necessitating multistep synthesis of starting materials and/or polymers as well as laborious post-polymerization modifications. Given the above results and in context of the recent FDA approval of similar orthopedic composites (composed of citric acid, 1,8 octanediol, and hydroxyapatite), it is expected that multifunctional CXBPs will achieve significance in academic research and clinical use in the future.

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