A Combinatorial Photocrosslinking Method for the Preparation of Porous Structures with Widely Differing Properties

Porous photo-crosslinked hybrid networks based on poly(trimethylene carbonate-co-ε-caprolactone) and recombinant human-like collagen

Hybrid hydrogel networks were prepared from recombinant human-like collagen (rh-collagen) and poly(trimethylene carbonate-co-ε-caprolactone) (P (TMC-co-ε-CL)) to overcome the mechanical and bioactivity limitations associated with the respective individual networks. Both polymers were functionalised with methacrylic anhydride to yield photo-crosslinkable materials. Porous hybrid networks of different compositions were prepared by photo-crosslinking frozen mixtures of solutions of the functionalized polymers in acidified DMSO. After extraction with water, the obtained networks had the intended compositions, high porosities and gel content. Upon equilibration in water, the total water content was found to increase with increasing collagen content. The tensile properties and suture retention strength (SRS) of the hybrids were improved compared to a rh-collagen network. In particular, the 17:83 wt% rh-collagen:P(TMC-co-ε-CL) network had considerably higher toughness and SRS, showing promise as a hybrid hydrogel network to be further investigated for tissue engineering purposes.

Measuring the elastic modulus of soft biomaterials using nanoindentation

The measurement of the elastic modulus of soft biomaterials via nanoindentation relies on the accurate determination of the zero-point of the tip-sample interaction on which the depth of penetration into the sample is based. Non-cantilever based nanoindentation systems were originally designed for hard materials, and therefore monitoring the zero-point contact presents a significant challenge for the characterisation of very soft biomaterials. This study investigates the ability of non-cantilever based nanoindentation to differentiate between hydrogels with elastic moduli on the order of single kiloPascals (kPa) using a bespoke soft contact protocol and low flexural stiffness of instrument. Polyethylene glycol (PEG) hydrogels were fabricated as a model system with a range of elastic moduli by varying the polymer concentration and degree of crosslinking. Elastic modulus values were calculated using the Oliver-Pharr method, Hertzian contact model, as well as a viscoelastic model to account for the time-dependent behaviour of the gels. The stiffness measurements were validated by measuring cantilever beams with the equivalent flexural stiffness to that of the PEG hydrogels being tested. The results demonstrated a high repeatability of the measurements, enabling differentiation between hydrogels with elastic moduli in the single kPa to hundreds of kPa range.

One-Step Fabrication of Porous Membrane-Based Scaffolds by Air-Water Interfacial Phase Separation: Opportunities for Engineered Tissues

Common methods for fabricating membrane-based scaffolds for tissue engineering with (hydrophobic) polymers include thermal or liquid-phase inversion, sintering, particle leaching, electrospinning and stereolithography. However, these methods have limitations, such as low resolution and pore interconnectivity and may often require the application of high temperatures and/or toxic porogens, additives or solvents. In this work, we aim to overcome some of these limitations and propose a one-step method to produce large porous membrane-based scaffolds formed by air-water interfacial phase separation using water as a pore-forming agent and casting substrate. Here, we provide proof of concept using poly (trimethylene carbonate), a flexible and biocompatible hydrophobic polymer. Membrane-based scaffolds were prepared by dropwise addition of the polymer solution to water. Upon contact, rapid solvent-non-solvent phase separation took place on the air-water interface, after which the scaffold was cured by UV irradiation. We can tune and control the morphology of these scaffolds, including pore size and porosity, by changing various parameters, including polymer concentration, solvent type and temperature. Importantly, human hepatic stellate cells cultured on these membrane-based scaffolds remained viable and showed no signs of pro-inflammatory stress. These results indicate that the proposed air-water interfacial phase separation represents a versatile method for creating porous membrane-based scaffolds for tissue engineering applications.

Tough combinatorial poly(urethane-isocyanurate) polymer networks and hydrogels synthesized by the trimerization of mixtures of NCO-prepolymers

The development of tough hydrogels is an essential but challenging topic in biomaterials research that has received much attention over the past years. By the combinatorial synthesis of polymer networks and hydrogels based on prepolymers with different properties, new materials with widely varying characteristics and unexpected properties may be identified. In this paper, we report on the properties of combinatorial poly(urethane-isocyanurate) (PUI) type polymer networks that were synthesized by the trimerization of mixtures of NCO-functionalized poly(ethylene glycol) (PEG), poly(propylene gylcol) (PPG), poly(ε-caprolactone) (PCL) and poly(trimethylene carbonate) (PTMC) prepolymers in solution. The resulting polymer networks showed widely varying material properties. Combinatorial PUI networks containing at least one hydrophilic PEG component showed high water uptakes of >100 wt%. The resulting hydrogels demonstrated elastic moduli of up to 10.1 MPa, ultimate tensile strengths of up to 9.8 MPa, elongation at break values of up to 624.0% and toughness values of up to 53.4 MJ m^-3. These values are exceptionally high and show that combinatorial PUI hydrogels are among the toughest hydrogels reported in the literature. Also, the simple two-step synthesis and wide range of suitable starting materials make this synthesis method more versatile and widely applicable than the existing methods for synthesizing tough hydrogels. An important finding of this work is that the presence of a hydrophobic network component significantly enhances the toughness and tensile strength of the combinatorial PUI hydrogels in the hydrated state. This enhancement is the largest when the hydrophobic network component is crystallizable in nature. In fact, the PUI hydrogels containing a crystallizable hydrophobic network component are shown to be semi-crystalline in the water-swollen state. Due to their high toughness values in the water-swollen state together with their water uptake values, elastic moduli and ultimate tensile strengths, the developed hydrogels are expected to be promising materials for biomedical coating- and adhesive applications, as well as for tissue-engineering. STATEMENT OF SIGNIFICANCE: The development of tough hydrogels is a challenging topic that has received much attention over the past years. At present, double network type hydrogels are considered state-of-the-art in the field, demonstrating toughness values of several tens of MJ m^-3. However, in terms of ease and versatility of the synthesis method, the possibilities are limited using a double network approach. In this work, we present combinatorial poly(urethane-isocyanurate) type polymer networks and hydrogels, synthesized by the trimerization of mixtures of NCO-functionalized prepolymers. The resulting hydrogels demonstrate exceptionally high toughness values of up to 53 MJ m^-3, while the synthesis method is versatile and widely applicable. This new class of hydrogels is therefore considered highly promising in the future development of load-bearing biomaterials.

Triblock Copolymers Based on ε-Caprolactone and Trimethylene Carbonate for the 3D Printing of Tissue Engineering Scaffolds

Background

Biodegradable PCL- b-PTMC- b-PCL triblock copolymers based on trimethylene carbonate (TMC) and ε-caprolactone (CL) were prepared and used in the 3D printing of tissue engineering scaffolds. Triblock copolymers of various molecular weights containing equal amounts of TMC and CL were prepared. These block copolymers combine the low glass transition temperature of amorphous PTMC (approximately -20°C) and the semi-crystallinity of PCL (glass transition approximately -60°C and melting temperature approximately 60°C).

Methods

PCL- b-PTMC- b-PCL triblock copolymers were synthesized by sequential ring opening polymerization (ROP) of TMC and ε-CL. From these materials, films were prepared by solvent casting and porous structures were prepared by extrusion-based 3D printing.

Results

Films prepared from a polymer with a relatively high molecular weight of 62 kg/mol had a melting temperature of 58°C and showed tough and resilient behavior, with values of the elastic modulus, tensile strength and elongation at break of approximately 120 MPa, 16 MPa and 620%, respectively. Porous structures were prepared by 3D printing. Ethylene carbonate was used as a crystalizable and water-extractable solvent to prepare structures with microporous strands. Solutions, containing 25 wt% of the triblock copolymer, were extruded at 50°C then cooled at different temperatures. Slow cooling at room temperature resulted in pores with widths of 18 ± 6 μm and lengths of 221 ± 77 μm, rapid cooling with dry ice resulted in pores with widths of 13 ± 3 μm and lengths of 58 ± 12 μm. These PCL- b-PTMC- b-PCL triblock copolymers processed into porous structures at relatively low temperatures may find wide application as designed degradable tissue engineering scaffolds.

Conclusions

In this preliminary study we prepared biodegradable triblock copolymers based on 1,3-trimethylene carbonate and ε-caprolactone and assessed their physical characteristics. Furthermore, we evaluated their potential as melt-processable thermoplastic elastomeric biomaterials in 3D printing of tissue engineering scaffolds.