Emulsion Inks for 3D Printing of High Porosity Materials.

Photocurable emulsion inks for use with solid freeform fabrication (SFF) to generate constructs with hierarchical porosity are presented. A high internal phase emulsion (HIPE) templating technique was utilized to prepare water-in-oil emulsions from a hydrophobic photopolymer, surfactant, and water. These HIPEs displayed strong shear thinning behavior that permitted layer-by-layer deposition into complex shapes and adequately high viscosity at low shear for shape retention after extrusion. Each layer was actively polymerized with an ultraviolet cure-on-dispense (CoD) technique and compositions with sufficient viscosity were able to produce tall, complex scaffolds with an internal lattice structure and microscale porosity. Evaluation of the rheological and cure properties indicated that the viscosity and cure rate both played an important role in print fidelity. These 3D printed polyHIPE constructs benefit from the tunable pore structure of emulsion templated material and the designed architecture of 3D printing. As such, these emulsion inks can be used to create ultra high porosity constructs with complex geometries and internal lattice structures not possible with traditional manufacturing techniques.

Injectable polyHIPEs as high-porosity bone grafts.

Polymerization of high internal phase emulsions (polyHIPEs) is a relatively new method for the production of high-porosity scaffolds. The tunable architecture of these polyHIPE foams makes them attractive candidates for tissue engineered bone grafts. Previously studied polyHIPE systems require either toxic diluents or high cure temperatures which prohibit their use as an injectable bone graft. In contrast, we have developed an injectable polyHIPE that cures at physiological temperatures to a rigid, high-porosity foam. First, a biodegradable macromer, propylene fumarate dimethacrylate (PFDMA), was synthesized that has appropriate viscosity and hydrophobicity for emulsification. The process of surfactant selection is detailed with particular focus on the key structural features of both polymer (logP values, hydrogen bond acceptor sites) and surfactant (HLB values, hydrogen bond donor sites) that enable stable HIPE formation. Incubation of HIPEs at 37 °C was used to initiate radical cross-linking of the unsaturated double bond of the methacrylate groups to polymerize the continuous phase and lock in the emulsion geometry. The resulting polyHIPEs exhibited ~75% porosity, pore sizes ranging from 4 to 29 µm, and an average compressive modulus and strength of 33 and 5 MPa, respectively. These findings highlight the great potential of these scaffolds as injectable, tissue engineered bone grafts.

Hybrid polyurea elastomers with enzymatic degradation and tunable mechanical properties.

Herein, we report on the synthesis and characterization of enzymatically labile polyureas for use as a tissue-engineered ligament scaffold. Polyureas were selected due to their excellent tensile properties, fatigue resistance, and highly tunable nature. Incorporation of a collagenase-sensitive peptide into the backbone of the polyurea provided a means to confer cell-responsive degradation to the synthetic polymer. Chemical, morphological, and mechanical testing were used to confirm incorporation of the peptide and characterize polyurea films. Notably, the incorporation of the peptide resulted in an increase in modulus, elongation, and tensile strength. This was attributed to an increase in phase mixing and an increase in hydrogen bonding between the hard and soft segments. Candidate polyureas with varying levels of collagen-mimetic peptide (0%, 10%, 20%) were then subjected to degradation in collagenase media or buffer at 37°C over 4 weeks. Statistically significant decreases in strength and elongation were observed in polyureas with 20% peptide content after collagenase treatment, whereas specimens in phosphate-buffered saline showed no statistically significant difference. These observations confirmed that enzyme-specific degradation was conferred to the polyurea. Overall, these polyureas hold great promise as a material for ligament reconstruction due to the promising mechanical properties and potential for cell-mediated degradation.

Osteoinductive PolyHIPE Foams as Injectable Bone Grafts.

We have recently fabricated biodegradable polyHIPEs as injectable bone grafts and characterized the mechanical properties, pore architecture, and cure rates. In this study, calcium phosphate nanoparticles and demineralized bone matrix (DBM) particles were incorporated into injectable polyHIPE foams to promote osteoblastic differentiation of mesenchymal stem cells (MSCs). Upon incorporation of each type of particle, stable monoliths were formed with compressive properties comparable to control polyHIPEs. Pore size quantification indicated a negligible effect of all particles on emulsion stability and resulting pore architecture. Alizarin red calcium staining illustrated the incorporation of calcium phosphate particles at the pore surface, while picrosirius red collagen staining illustrated collagen-rich DBM particles within the monoliths. Osteoinductive particles had a negligible effect on the compressive modulus (∼30 MPa), which remained comparable to human cancellous bone values. All polyHIPE compositions promoted human MSC viability (∼90%) through 2 weeks. Furthermore, gene expression analysis indicated the ability of all polyHIPE compositions to promote osteogenic differentiation through the upregulation of bone-specific markers compared to a time zero control. These findings illustrate the potential for these osteoinductive polyHIPEs to promote osteogenesis and validate future in vivo evaluation. Overall, this work demonstrates the ability to incorporate a range of bioactive components into propylene fumarate dimethacrylate-based injectable polyHIPEs to increase cellular interactions and direct specific behavior without compromising scaffold architecture and resulting properties for various tissue engineering applications.