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The combination of recycled fiber reinforcement with recycled polymer as a feedstock material for extrusion-based additive manufacturing creates an opportunity for a more sustainable material use. In this study, recycled short carbon fibers were combined with recycled poly(ethylene terephthalate) (PET) to obtain carbon fiber-reinforced PET filaments via melt extrusion. The carbon fiber content of the extruded filaments ranged from 0.4 to 40.7 wt%. The molar mass and the degree of crystallinity after processing were determined to evaluate the influence of the extrusion process on the physico-chemical and mechanical properties of the reinforced PET filaments. Furthermore, pressure-volume-temperature measurements were carried out to investigate the influence of the carbon fibers on the shrinkage of the semi-crystalline PET. Samples were printed and their superior mechanical properties, including a 390% increase in tensile modulus, were confirmed via tensile testing. Analysis via X-ray micro-computed tomography indicated that the fiber length reduced with increasing fiber content. The high degree of fiber alignment that was observed in the extruded filaments, was slightly reduced after deposition. Scanning electron microscopy data showed that fiber pull-out was the governing failure mechanism, indicating a weak interface between the carbon fibers and the matrix. The results show the potential of extrusion-based additive manufacturing to valorize recycled PET and recycled carbon fibers.

In this featured review manuscript, the aim is to present a critical survey on the processes available for fabricating bioartificial organs (BAOs). The focus will be on hollow tubular organs for the transport of anabolites and catabolites, i.e., vessels, trachea, esophagus, ureter and urethra, and intestine. First, the anatomic hierarchical structures of tubular organs, as well as their principal physiological functions, will be presented, as this constitutes the mandatory requirements for effectively designing and developing physiologically relevant BAOs. Second, 3D bioprinting, solution electrospinning, and melt electrowriting will be introduced, together with their capacity to match the requirements imposed by designing scaffolds compatible with the anatomical and physiologically relevant environment. Finally, the intrinsic correlation between processes, materials, and cells will be critically discussed, and directives defining the strengths, weaknesses, and opportunities offered by each process will be proposed for assisting bioengineers in the selection of the appropriate process for the target BAO and its specific required functions.

Melt electrowriting (MEW) is an additive manufacturing process that produces highly defined constructs with elements in the micrometer range. A specific configuration of MEW enables printing tubular constructs to create small-diameter tubular structures. The small pool of processable materials poses a bottleneck for wider application in biomedicine. To alleviate this obstacle, an acrylate-endcapped urethane-based polymer (AUP), using a poly(epsilon-caprolactone) (PCL) (molar mass: 20 000 g mol(-1)) (AUP PCL20k) as backbone material, is synthesized and utilized for MEW. Spectroscopic analysis confirms the successful modification of the PCL backbone with photo-crosslinkable acrylate endgroups. Printing experiments of AUP PCL20k reveal limited printability but the photo-crosslinking ability is preserved post-printing. To improve printability and to tune the mechanical properties of printed constructs, the AUP-material is blended with commercially available PCL (AUP PCL20k:PCL in ratios 80:20, 60:40, 50:50). Print fidelity improves for 60:40 and 50:50 blends. Blending enables modification of the constructs' mechanical properties to approximate the range of blood vessels for transplantation surgeries. The crosslinking-ability of the material allows pure AUP to be manipulated post-printing and illustrates significant differences in mechanical properties of 80:20 blends after crosslinking. An in vitro cell compatibility assay using human umbilical vein endothelial cells also demonstrates the material's non-cytotoxicity.

Hydrogels exhibit mechanical properties and an architectural structure that mimics the native extracellular matrix. In addition, they have unique properties including swelling, diffusion, degradation and mechanical properties that can be tuned toward the targeted application, while they exhibit excellent biocompatibility. Therefore, hydrogels have been studied extensively to serve various biomedical and tissue engineering applications. In this chapter, hydrogels, their properties and their preparation methods are briefly introduced. Natural and synthetic hydrogels currently used in tissue engineering applications are also discussed, together with their advantages and limitations. Next, (conventional and co-axial) electrospinning is described as a processing technique to develop hydrogel scaffolds for tissue engineering applications. Finally, vascular tissue engineering is introduced to provide the distinct example of a biomedical application, which can be tackled using hydrogels.

A reinforced tubular, medicated electrospun construct was developed for deep flexor tendon repair. This construct combines mechanical strength with the release of anti-inflammatory and anti-adhesion drugs. In this study, the reinforced construct was evaluated using a rabbit model. It was compared to its components (a tubular, medicated electrospun polymer without reinforcement and a tubular braid as such) on the one hand to a modified Kessler suture as a control group. Forty New Zealand rabbits were randomly divided into two groups. Surgery was performed in the second and fourth deep flexor tendons of one hind paw of the rabbits in the two groups using four repair techniques. Biomechanical tensile testing and macroscopic and histological evaluations were performed at 3 and 8 weeks postoperatively. A two-way analysis of variance with pairwise comparisons revealed that the three experimental surgical techniques (a reinforced tubular medicated electrospun construct, tubular-medicated construct, and tubular braid as such) showed similar strength as that of a modified Kessler suture repair, which was characterized by a mean load at ultimate failure of 19.85 N (standard deviation [SD] 5.29 N) at 3 weeks and 18.15 N (SD 8.01 N) at 8 weeks. Macroscopically, a significantly different adhesion pattern was observed at the suture knots, either centrally or peripherally, depending on the technique. Histologically, a qualitative assessment showed good to excellent repair at the tendon repair site, irrespective of the applied technique. This study demonstrates that mechanical and biological repair strategies for flexor tendon repair can be successfully combined. ispartof: JOURNAL OF ORTHOPAEDIC RESEARCH vol:40 issue:3 pages:750-760 ispartof: location:United States status: published

This research aims at developing a more potent solution for deep flexor tendon repair by combining a mechanical and biological approach. A reinforced, multi-layered electrospun tubular construct is developed, composed of three layers: an inner electrospun layer containing an anti-inflammatory component (Naproxen), a middle layer of braided monofilament as reinforcement and an outer electrospun layer containing an anti-adhesion component (hyaluronic acid, HA). In a first step, a novel acrylate endcapped urethane-based precursor (AUP) is developed and characterized by measuring molar mass, acrylate content and thermo-stability. The AUP material is benchmarked against commercially available poly(ε-caprolactone) (PCL). Next, the materials are processed into multi-layered, tubular constructs with bio-active components (Naproxen and HA) using electrospinning. In vitro assays using human fibroblasts show that incorporation of the bio-active components is successful and not-cytotoxic. Moreover, tensile testing using ex vivo sheep tendons prove that the developed multi-layered constructs fulfill the required strength for tendon repair (i.e. 2.79-3.98 MPa), with an ultimate strength of 8.56 ± 1.92 MPa and 8.36 ± 0.57 MPa for PCL and AUP/PCL constructs respectively. In conclusion, by combining a mechanical approach (improved mechanical properties) with the incorporation of bio-active compounds (biological approach), this solution shows its potential for application in deep flexor tendon repair. ispartof: Materials Science and Engineering: C vol:119 ispartof: location:Netherlands status: Published online

The present work targets the development of collagen-based hydrogel precursors, functionalized with photo-crosslinkable methacrylamide moieties (COL-MA), for vascular tissue engineering (vTE) applications. The developed materials were physico-chemically characterized in terms of crosslinking kinetics, degree of modification/conversion, swelling behavior, mechanical properties and in vitro cytocompatibility. The collagen derivatives were benchmarked to methacrylamide-modified gelatin (GEL-MA), due to its proven track record in the field of tissue engineering. To the best of our knowledge, this is the first paper in its kind comparing these two methacrylated biopolymers for vTE applications. For both gelatin and collagen, two derivatives with varying degrees of substitutions (DS) were developed by altering the added amount of methacrylic anhydride (MeAnH). This led to photo-crosslinkable derivatives with a DS of 74 and 96% for collagen, and a DS of 73 and 99% for gelatin. The developed derivatives showed high gel fractions (i.e. 74% and 84%, for the gelatin derivatives; 87 and 83%, for the collagen derivatives) and an excellent crosslinking efficiency. Furthermore, the results indicated that the functionalization of collagen led to hydrogels with tunable mechanical properties (i.e. storage moduli of [4.8–9.4 kPa] for the developed COL-MAs versus [3.9–8.4 kPa] for the developed GEL-MAs) along with superior cell-biomaterial interactions when compared to GEL-MA. Moreover, the developed photo-crosslinkable collagens showed superior mechanical properties compared to extracted native collagen. Therefore, the developed photo-crosslinkable collagens demonstrate great potential as biomaterials for vTE applications.

In the present work, a photopolymerized urethane-based poly(ethylene glycol) hydrogel is applied as a porous scaffold material using indirect solid freeform fabrication (SFF). This approach combines the benefits of SFF with a large freedom in material selection and applicable concentration ranges. A sacrificial 3D poly(ε-caprolactone) structure is generated using fused deposition modeling and used as template to produce hydrogel scaffolds. By changing the template plotting parameters, the scaffold channel sizes vary from 280 to 360 μm, and the strut diameters from 340 to 400 μm. This enables the production of scaffolds with tunable mechanical properties, characterized by an average hardness ranging from 9 to 43 N and from 1 to 6 N for dry and hydrated scaffolds, respectively. Experiments using mouse calvaria preosteoblasts indicate that a gelatin methacrylamide coating of the scaffolds results in an increased cell adhesion and proliferation with improved cell morphology.

Varying physico-chemical properties of synthetic hydrogels and introducing additives can induce a cell interactive response of hydrogels. Hydrogels were developed based on acrylate-endcapped urethane-based poly(ethylene glycol) precursors with varying backbone molecular weight (2000–8000 g/mol), in combination with the nanoclay Laponite to explore effects on swelling, mechanical and in vitro biological properties. The combined effect of molecular weight and Laponite concentration enables the development of hybrid materials useful for different biomedical applications. Gel fractions approximating 100% along with tunable swelling (4–11 g/g polymer) and mechanical properties (Young's Modulus 0.1–0.6 MPa) are obtained. All materials are non-cytotoxic and AUPs without Laponite are non cell-interative rendering them suitable for non-adherent biomedical applications. Hydrogels composed of Laponite (0.5 or 1 wt%) and PEG backbone molecular weight of 2000 g/mol promote cell proliferation useful to function as synthetic dermal matrices or scaffolds for tissue engineering applications.