Paul D. Dalton

Direct writing by way of melt electrospinning.

Melt electrospun fibers of poly(ϵ-caprolactone) are accurately deposited using an automated stage as the collector. Matching the translation speed of the collector to the speed of the melt electrospinning jet establishes control over the location of fiber deposition. In this sense, melt electrospinning writing can be seen to bridge the gap between solution electrospinning and direct writing additive manufacturing processes.

Design, fabrication and characterization of PCL electrospun scaffolds—a review

The expanding interest in electrospinning fibers for bioengineering includes a significant use of polyesters, including poly(3-caprolactone) (PCL). This review summarizes literature on PCL and selected blends, and provides extensive descriptions of the broad range of parameters used in manufacturing such electrospun fibers. Furthermore the chemical, physical and biological approaches for characterizing the electrospun material are described and opinions offered on important information to include in future publications with this electrospun material.

Reinforcement of hydrogels using three-dimensionally printed microfibres

Despite intensive research, hydrogels currently available for tissue repair in the musculoskeletal system are unable to meet the mechanical, as well as the biological, requirements for successful outcomes. Here we reinforce soft hydrogels with highly organized, high-porosity microfibre networks that are 3D-printed with a technique termed as melt electrospinning writing. We show that the stiffness of the gel/scaffold composites increases synergistically (up to 54-fold), compared with hydrogels or microfibre scaffolds alone. Modelling affirms that reinforcement with defined microscale structures is applicable to numerous hydrogels. The stiffness and elasticity of the composites approach that of articular cartilage tissue. Human chondrocytes embedded in the composites are viable, retain their round morphology and are responsive to an in vitro physiological loading regime in terms of gene expression and matrix production. The current approach of reinforcing hydrogels with 3D-printed microfibres offers a fundament for producing tissue constructs with biological and mechanical compatibility.

Synthetic Hydrogel Guidance Channels Facilitate Regeneration of Adult Rat Brainstem Motor Axons after Complete Spinal Cord Transection

Synthetic guidance channels or tubes have been shown to promote axonal regeneration within the spinal cord from brainstem motor nuclei with the inclusion of agents such as matrices, cells, or growth factors to the tube. We examined the biocompatibility and regenerative capacity of synthetic hydrogel tubular devices that were composed of poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) (PHEMA-MMA). Two PHEMA-MMA channels, having a mean elastic modulus of either 177 or 311 kPa were implanted into T8-transected spinal cords of adult Sprague Dawley rats. The cord stumps were inserted into the channels and fibrin glue was applied to the cord-channel interface. An expanded polytetrafluoroethylene (ePTFE) membrane was used for duraplasty. Controls underwent cord transection alone. Gross and microscopic examination of the spinal cords showed continuity of tissue within the synthetic guidance channels between the cord stumps at 4 and 8 weeks. There was a trend towards an increased area and width of bridging neural tissue in the 311-kPa guidance channels compared to the 177-kPa channels. Neurofilament stained axons were visualized within the bridging tissue, and serotonergic axons were found to enter the 311-kPa channel. Retrograde axonal tracing revealed regeneration of axons from reticular, vestibular, and raphe brainstem motor nuclei. For both channels, there was minimal scarring at the channel-cord interface, and less scarring at the channel-dura interface compared to that observed next to the ePTFE. The present study is the first to show that axons from brainstem motor nuclei regenerated in unfilled synthetic hydrogel guidance channels after complete spinal cord transection.

Biofabrication: reappraising the definition of an evolving field

Biofabrication is an evolving research field that has recently received significant attention. In particular, the adoption of Biofabrication concepts within the field of Tissue Engineering and Regenerative Medicine has grown tremendously, and has been accompanied by a growing inconsistency in terminology. This article aims at clarifying the position of Biofabrication as a research field with a special focus on its relation to and application for Tissue Engineering and Regenerative Medicine. Within this context, we propose a refined working definition of Biofabrication, including Bioprinting and Bioassembly as complementary strategies within Biofabrication.

Electrospinning and additive manufacturing: converging technologies

A well-engineered scaffold for regenerative medicine, which is suitable to be translated from the bench to the bedside, combines inspired design, technical innovation and precise craftsmanship. Electrospinning and additive manufacturing are separate approaches to manufacturing scaffolds for a variety of tissue engineering applications. A need to accurately control the spatial distribution of pores within scaffolds has recently resulted in combining the two processing methods, to overcome shortfalls in each technology. This review describes where electrospinning and additive manufacturing are used together to generate new porous structures for biological applications.

Design and fabrication of tubular scaffolds via direct writing in a melt electrospinning mode.

Flexible tubular structures fabricated from solution electrospun fibers are finding increasing use in tissue engineering applications. However it is difficult to control the deposition of fibers due to the chaotic nature of the solution electrospinning jet. By using non-conductive polymer melts instead of polymer solutions the path and collection of the fiber becomes predictable. In this work we demonstrate the melt electrospinning of polycaprolactone in a direct writing mode onto a rotating cylinder. This allows the design and fabrication of tubes using 20 μm diameter fibers with controllable micropatterns and mechanical properties. A key design parameter is the fiber winding angle, where it allows control over scaffold pore morphology (e.g. size, shape, number and porosity). Furthermore, the establishment of a finite element model as a predictive design tool is validated against mechanical testing results of melt electrospun tubes to show that a lesser winding angle provides improved mechanical response to uniaxial tension and compression. In addition, we show that melt electrospun tubes support the growth of three different cell types in vitro and are therefore promising scaffolds for tissue engineering applications. Electronic supplementary material The online version of this article (doi:10.1007/s13758-011-0013-7) contains supplementary material, which is available to authorized users.

Patterned melt electrospun substrates for tissue engineering

Tissue engineering scaffolds can be built with patterning techniques that allow discrete placement of structures. In this study, electrospun fibres are collected in focused spots; the patterning and drawing of a cell adhesive scaffold is shown. Blends of biodegradable poly(ethylene glycol)-block-poly(epsilon-caprolactone) (PEG-b-PCL) and PCL were melt electrospun onto glass collectors, and the optimal electrospinning parameters determined. The quality of the fibre was largely influenced by the flow rate of the melt to the spinneret; however, this can be adjusted with the voltage. A collection distance between 3 cm and 5 cm was optimal, and at 10 cm the fibres became unfocused in their deposition although the diameter remained similar (0.96 +/- 0.19 microm). Aligned lines of electrospun fibres 200-400 microm in width could be applied onto the slide with an x-y stage, continuously and discretely. Lines of electrospun fibres could be applied on top of one another and were very uniform in diameter. Fibroblasts adhered primarily in the fibre region, due to the poor cell adhesion to the PEG substrate. Improvements in depositing hydrophilic electrospun fibres that wet and adhere to in vitro substrates and the use of stage automation for the writing interface could provide scaffold-building devices suitable for tissue engineering applications.

Control of Protein Adsorption on Functionalized Electrospun Fibers

Electrospun fibers that are protein resistant and functionalized with bioactive signals were produced by solution electrospinning amphiphilic block copolymers. Poly (ethylene glycol)‐block‐poly(D,L‐lactide) (PEG‐b‐PDLLA) was synthesized in two steps, with a PEG segment of 10 kDa, while the PDLLA block ranged from 20 to 60 kDa. Depending on the PEG and PDLLA segment ratio, as well as solvent selection, the hydrophilicity and protein adsorption could be altered on the electrospun mesh. Furthermore, an α‐acetal PEG‐b‐PDLLA was synthesized that allowed the conjugation of active molecules, resulting in surface functionalization of the electrospun fiber. Electrospun material with varying morphologies and diameter were electrospun from 10, 20, and 30 wt.% solutions. Sessile drop measurements showed a reduction in the contact angle from 120° for pure poly(D,L‐lactide) with increasing PEG/PDLLA ratio. All electrospun block PEG‐b‐PDLLA fibers had hydrophilic properties, with contact angles below 45°. The fibers were collected onto six‐arm star‐poly(ethylene glycol) (star‐PEG) coated silicon wafers and incubated with fluorescently labeled proteins. All PEG‐b‐PDLLA fibers showed no detectable adsorption of bovine serum albumin (BSA) independent of their composition while a dependence between hydrophobic block length was observed for streptavidin adsorption. Fibers of block copolymers with PDLLA blocks smaller than 39 kDa showed no adsorption of BSA or streptavidin, indicating good non‐fouling properties. Fibers were surface functionalized with Nε‐(+)‐biotinyl‐L‐lysine (biocytin) or RGD peptide by attaching the molecule to the PEG block during synthesis. Protein adsorption measurements, and the controlled interaction of biocytin with fluorescently labeled streptavidin, showed that the electrospun fibers were both resistant to protein adsorption and are functionalized. Fibroblast adhesion was contrasting between the unfunctionalized and RGD‐coupled electrospun fabrics, confirming that the surface of the fibers was functionalized. The PEG‐b‐PDLLA surface functionalized electrospun fibers are promising substrates for controlling cell–material interactions, particularly for tissue‐engineering applications. Biotechnol. Bioeng. 2008;101: 609–621.

Using extracellular matrix for regenerative medicine in the spinal cord

Regeneration within the mammalian central nervous system (CNS) is limited, and traumatic injury often leads to permanent functional motor and sensory loss. The lack of regeneration following spinal cord injury (SCI) is mainly caused by the presence of glial scarring, cystic cavitation and a hostile environment to axonal growth at the lesion site. The more prominent experimental treatment strategies focus mainly on drug and cell therapies, however recent interest in biomaterial-based strategies are increasing in number and breadth. Outside the spinal cord, approaches that utilize the extracellular matrix (ECM) to promote tissue repair show tremendous potential for various application including vascular, skin, bone, cartilage, liver, lung, heart and peripheral nerve tissue engineering (TE). Experimentally, it is unknown if these approaches can be successfully translated to the CNS, either alone or in combination with synthetic biomaterial scaffolds. In this review we outline the first attempts to apply the potential of ECM-based biomaterials and combining cell-derived ECM with synthetic scaffolds.