Achieving Interconnected Pore Architecture in Injectable PolyHIPEs for Bone Tissue Engineering

Abstract

Template polymerization of a high internal phase emulsion (polyHIPE) is a relatively new method to produce tunable high-porosity scaffolds for tissue regeneration. This study focuses on the development of biodegradable injectable polyHIPEs with interconnected porosity that have the potential to fill bone defects and enhance healing. Our laboratory previously fabricated biodegradable polyHIPEs that cure in situ upon injection; however, these scaffolds possessed a closed-pore morphology, which could limit bone ingrowth. To address this issue, HIPEs were fabricated with a radical initiator dissolved in the organic phase rather than the aqueous phase of the emulsion. Organic-phase initiation resulted in macromer densification forces that facilitated pore opening during cure. Compressive modulus and strength of the polyHIPEs were found to increase over 2 weeks to 43 -12 MPa and 3 -0.2 MPa, respectively, properties comparable to cancellous bone. The viscosity of the HIPE before cure (11.0 -2.3 Pa$s) allowed for injection and filling of the bone defect, retention at the defect site during cure under water, and microscale integration of the graft with the bone. Precuring the materials before injection allowed for tuning of the work and set times. Furthermore, storage of the HIPEs before cure for 1 week at 4°C had a negligible effect on pore architecture after injection and cure. These findings indicate the potential of these emulsions to be stored at reduced temperatures and thawed in the surgical suite before injection. Overall, this work highlights the potential of interconnected propylene fumarate dimethacrylate polyHIPEs as injectable scaffolds for bone tissue engineering.

Osteoinductive PolyHIPE Foams as Injectable Bone Grafts

“…[54][55][56] However, we have previously demonstrated that pore size of these injectable polyHIPEs can be readily tuned by altering the stability of the emulsion (surfactant concentration, aqueous phase volume fraction, fabrication conditions) to achieve pore sizes greater than 100 mm. 7,8 Given that our current studies demonstrate that the surface modification had minimal effect on pore size, we expect to be able to utilize this technique to modify polyHIPE grafts with larger pores for investigation as bone grafts in upcoming in vivo studies.…”

“…8 Briefly, the diester 1,2 hydroxypropyl fumarate was synthesized by adding propylene oxide dropwise to a solution of fumaric acid and pyridine in 2-butanone (2.3:1.0:0.033 mol) and refluxing at 75°C for 18 h. Residual reagents were removed by distillation, and the product redissolved in dichloromethane before removing acidic byproducts with washing. The diester product was then dried under vacuum before endcapping with methacrylate groups using methacryloyl chloride in the presence of triethylamine.…”

“…We previously demonstrated that the polyHIPE can effectively space-fill an irregularly shaped defect and cure under physiological conditions (37°C and water) with microscale integration with the surrounding bone. 8 Notably, the redox initiation utilized in these grafts does not require external stimuli to initiate crosslinking, which can limit the size of constructs to the effective depth of penetration. Despite these promising findings, there was no confirmation that this new material could support osteoblastic differentiation of hMSCs before this study.…”

“…Altering compositional and processing parameters results in monoliths with a range of pore sizes, porosities, pore morphologies (open vs. closed), compressive properties, and cure times. [1][2][3][4][5][6][7][8][9][10][11] However, a major challenge in designing functional bone grafts is determining a material composition that promotes ingrowth of surrounding bone and proliferation of cells (osteoconduction), the differentiation of progenitor cells down an osteoblastic lineage (osteoinduction), and osseointegration with the native bone. Various scaffold chemistries and bioactive agents are currently being studied to determine a matrix that presents cues to stimulate osteoblastic differentiation of human mesenchymal stem cells (hMSCs) and promote integration and healing.…”

“…7,8,37 This platform technology provides a tunable system for fabricating high porosity scaffolds with properties comparable to the trabecular bone that can be used to fill irregularly shaped defects directly in the surgical suite without the need for prefabrication based on computer-aided design. The present study expands upon our injectable polyHIPE platform technology to fabricate monoliths with osteoinductive character to promote osteogenesis and integration.…”