Solvent-free fabrication of three dimensionally aligned polycaprolactone microfibers for engineering of anisotropic tissues

Fabrication and Characterization of Zn Particle Incorporated Fibrous Scaffolds for Potential Application in Tissue Healing and Regeneration

Zinc (Zn) metal and its alloys have received a lot of interest in biomedical applications due to their biodegradability, biocompatibility, antimicrobial activity, and ability to stimulate tissue regeneration. Bulk Zn has been successfully utilized in a variety of implant applications, most notably as bioabsorbable cardiac stents and orthopedic fixation devices, where it provides adequate mechanical properties while also releasing helpful Zn ions (Zn2+) during degradation. Such beneficial ions are dose-dependent and, when released in excess, can induce cellular toxicity. In this study, we hypothesize that embedding Zn metal particles into a polymer nanofibrous scaffold will enable control of the degradation and time release of the Zn2+. We designed and fabricated two polymer scaffolds, polycaprolactone (PCL) and polycaprolactone-chitosan (PCL-CH). Each scaffold had an increasing amount of Zn. Several physicochemical properties such as fiber morphology, crystallinity, mechanical strength, hydrophilicity, degradation and release of Zn2+, thermal properties, chemical compositions, and so forth were characterized and compared with the PCL fibrous scaffold. The biological properties of the scaffolds were evaluated in vitro utilizing direct and indirect cytotoxicity assays and cell viability. All the data show that the addition of Zn changed various physical properties of the PCL and PCL-CH scaffolds except their chemical structure. Further investigation reveals that the PCL-CH scaffolds degrade the Zn particles relatively faster than the PCL because the presence of the hydrophilic CH influences the faster release of Zn2+ in cell culture conditions as compared to the PCL fibrous scaffold. The combined advantages of CH and Zn in the PCL scaffold enriched 3T3 fibroblast cells' survival and proliferation except the ones with the higher concentration of Zn particles. These new composite scaffolds are promising and can be further considered for tissue healing and regeneration applications.

Biofabrication of Composite Tendon Constructs with the Fibrous Arrangement, High Cell Density, and Enhanced Cell Alignment

Current tissue-engineered tendons are mostly limited to the replication of fibrous organizations of native tendons, which lack the biomimicry of a densely packed cell arrangement. In this study, composite tendon constructs (CTCs) with fibrous arrangement, high cell density, and enhanced cell alignment were developed by integrating the electrohydrodynamic jet 3D printing (e-jetting) technique and the fabrication of tissue strands (TSs). A tubular polycaprolactone (PCL) scaffold was created using e-jetting, followed by coating a thin layer of alginate. Human mesenchymal stem cells were then microinjected into the PCL scaffolds, aggregated into TSs, and formed CTCs with a core-shell structure. Owing to the presence of TSs, CTCs demonstrated the anatomically relevant cell density and morphology, and cells migrated from the TSs onto e-jetted scaffolds. Also, the mechanical strength of CTCs approached that of native tendons due to the existence of e-jetted scaffolds (Young's modulus: ∼21 MPa, ultimate strength: ∼5 MPa). During the entire culture period, CTCs maintained high survival rates and good structural integrity without the observation of necrotic cores and disintegration of two portions. In addition, CTCs that were cultured with uniaxial cyclic stretching revealed not only the increased expression of tendon-related proteins but also the enhanced cellular orientation. The promising results demonstrated the potential of this novel biofabrication strategy for building tissue-engineered tendon constructs with the proper biological, mechanical, and histological relevance..

Muscle-like Scaffolds for Biomechanical Stimulation in a Custom-Built Bioreactor

Tissue engineering aims to develop in-vitro substitutes of native tissues. One approach of tissue engineering relies on using bioreactors combined with biomimetic scaffolds to produce study models or in-vitro substitutes. Bioreactors provide control over environmental parameters, place and hold a scaffold under desired characteristics, and apply mechanical stimulation to scaffolds. Polymers are often used for fabricating tissue-engineering scaffolds. In this study, polycaprolactone (PCL) collagen-coated microfilament scaffolds were cell-seeded with C2C12 myoblasts; then, these were grown inside a custom-built bioreactor. Cell attachment and proliferation on the scaffolds were investigated. A loading pattern was used for mechanical stimulation of the cell-seeded scaffolds. Results showed that the microfilaments provided a suitable scaffold for myoblast anchorage and that the custom-built bioreactor provided a qualified environment for the survival of the myoblasts on the polymeric scaffold. This PCL-based microfilament scaffold located inside the bioreactor proved to be a promising structure for the study of skeletal muscle models and can be used for mechanical stimulation studies in tissue engineering applications.

Evaluation of Biomechanical and Chemical Properties of Gamma-Irradiated Polycaprolactone Microfilaments for Musculoskeletal Tissue Engineering Applications

An appropriate and reliable sterilization technique is crucial for tissue engineering scaffolds. Skeletal muscle scaffolds are often fabricated using microfilaments of a wide variety of polymers. One method for sterilization is 25 kGy of gamma irradiation. In addition, sterilization through irradiation should administer a dose within a specific range. Radiation directly affects the chemical and mechanical properties of scaffolds. The accuracy and effects of irradiation are often not considered during sterilization procedures; however, these are important since they provide insight on whether the sterilization procedure is reliable and reproducible. This study focused on the chemical and mechanical characterization of 25 kGy gamma-irradiated scaffold. The accuracy and uncertainty of the irradiation procedure were also obtained. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses were performed to determine whether the crystallinity of the polymer changed after irradiation and whether gamma rays influenced its thermal properties. The tensile parameters of the microfilaments were analyzed by comparing irradiated and nonirradiated scaffolds to determine whether gamma radiation changed their elastic behavior. Dose distribution and uncertainty were recorded with several dosimeters. The results showed that the irradiation process slightly affected the mechanical parameters of the scaffold; however, it did not modify its crystallinity or thermal properties. The irradiation was uniform, since the measured uncertainty was low. The scaffold was pathogen-free after 7 days; this meant sterilization was achieved. These results indicated that gamma-sterilized scaffolds were a promising material for use as a skeletal muscle analog material for tissue-engineering applications because they can be sterilized with gamma rays without changing their chemical structure and mechanical properties. This study provided the dose distribution measurement and uncertainty calculations for the sterilization procedure.

Self-powered portable melt electrospinning for in situ wound dressing

Background

Electrospun (e-spun) nanofibers for wound dressing have attracted wide attention due to its large specific surface area, large porosity and breathability. Compared with solution electrospinning (e-spinning), melt e-spinning is more bio-friendly without toxic solvent participation, which provides the possibility of in situ e-spinning on wounds directly. However, previously reported melt e-spinning devices were usually bulky and cumbersome due to their necessary heating unit, and different components were separated to avoid electrostatic interference.

Results

In this article, we report on a self-powered hand-held melt e-spinning gun which can work without any external power supply (outdoors). The problem of electrostatic interference for this integrated device was solved by using a special high heat transfer insulation unit. The apparatus is easy and safe to operate by a single hand due to its small volume (24 × 6 × 13 cm³) and light weight (about 450 g). Some biodegradable polymers, for example, polycaprolactone (PCL) fibers were successful e-spun onto wounds directly by using this dressing gun.

Conclusions

PCL fibrous membrane has good biocompatibility and can be in situ electrospun to wound surface as a wound dressing by the portable melt e-spinning gun. Besides wound dressing, this hand-held melt e-spinning gun may be used in 3D printing and experimental teaching demonstration aids.

Morphology, Thermal and Mechanical Properties of Co-Continuous Porous Structure of PLA/PVA Blends by Phase Separation

Poly (lactic acid) (PLA) was blended with poly (vinyl alcohol) (PVA) in the composition of 70/30 (L7V3), 60/40 (L6V4), and 50/50 (L5V5) wt.%. L7V3 exhibits a sea-island morphology, while L6V4 and L5V5 show co-continuous phase morphologies. These polymers exhibited a solitary glass transition temperature, which obeyed the Fox equation. Thereafter, the blends were made porous by an etching process in hot water (35 °C) for 0-7 days, to remove PVA. The maximum etched PVA content of L7V3, L6V4, and L5V5 was 0.5%, 13.4%, and 36.1%, respectively; hence, L5V5 exhibited a co-continuous porous morphology with the porosity of 43.4%, the degree of swelling of 47.5%, and the pore size of 2 µm. The degree of crystallinity of PLA, exposed PLA, and L7V3 showed an insignificant change. L5V5, having the highest porosity, demonstrated the highest increase in the degree of crystallinity of approximately two times, because water induced the crystallization of PLA. The high porosity of L5V5 exhibited an excellent absorption property by increasing absorption energy more than two times, as obtained by micro indention. It had the maximum indentation depth more than 250 µm. Flexural and tensile properties considerably decreased with an increase in the porosity.

Micro/nano-hierarchical scaffold fabricated using a cell electrospinning/3D printing process for co-culturing myoblasts and HUVECs to induce myoblast alignment and differentiation

Human skeletal muscle is composed of intricate anatomical structures, including uniaxially arranged myotubes and widely distributed blood capillaries. In this regard, vascularization is an essential part of the successful development of an engineered skeletal muscle tissue to restore its function and physiological activities. In this paper, we propose a method to obtain a platform for co-culturing human umbilical vein endothelial cells (HUVECs) and C2C12 cells using cell electrospinning and 3D bioprinting. To elaborate, on the surface of mechanical supporters (polycaprolactone and collagen struts) with a topographical cue, HUVECs-laden alginate bioink was uniaxially electrospun. The electrospun HUVECs showed high cell viability (90%), homogeneous cell distribution, and efficient HUVEC growth. Furthermore, the myoblasts (C2C12 cells), which were seeded on the vascularized structure (HUVECs-laden fibers), were co-cultured to facilitate myoblast regeneration. As a result, the scaffold that included myoblasts and HUVECs represented a high degree of the myosin heavy chain (MHC) with striated patterns and enhanced myogenic-specific gene expressions (MyoD, troponin T, MHC and myogenin) as compared to the scaffold that included only myoblasts. STATEMENT OF SIGNIFICANCE: Cell electrospinning is an advanced electrospinning method that improves cell-matrix interactions by embedding cells directly into micro/nanofibers. Here, cell electrospinning was employed to achieve not only the homogeneous human umbilical vein endothelial cells (HUVECs) distribution with a high cell-viability (~90%), but also highly aligned topographical cue. Moreover, the uniaxially micropatterned PCL/collagen struts as a physical support were generated using three-dimensional (3D) printing, and was covered with HUVEC-laden micro/nanofibers. This hierarchical structure provided meaningful mechanical stability, homogeneous cell distribution, and HUVEC transformation into a narrow, elongated structure. Furthermore, the myoblasts (C2C12 cells) were seeded on the HUVECs-laden fibers and cocultured to facilitate myogenesis. In brief, a myosin heavy chain with striated patterns and enhanced myogenic specific gene expressions were represented.

Response of Dermal Fibroblasts to Biochemical and Physical Cues in Aligned Polycaprolactone/Silk Fibroin Nanofiber Scaffolds for Application in Tendon Tissue Engineering

Silk fibroin (SF) and fiber alignment were introduced into polycaprolactone (PCL)-based electrospun nanofibers as chemical and physical cues for tendon tissue engineering applications. The physicochemical properties of random PCL (RP) nanofibers, random PCL/SF (RPSF) nanofibers and aligned PCL/SF (APSF) nanofibers were characterized for fiber orientation and SF blending effects. An in vitro cell culture with rabbit dermal fibroblasts (RDFBs) on nanofibers indicated that SF promotes cell proliferation to a higher extent than fiber alignment. Cells aligned in the direction of fiber axes could be confirmed through scanning electron microscopy (SEM) observation and cytoskeleton staining. The quantitative real-time polymerase chain reaction (qRT-PCR) experiments indicated up-regulated gene expression of tendon marker proteins (type I collagen (Col I), fibronectin and biglycan) on APSF nanofibers and tendon reconstruction was confirmed from Col III gene expression. Animal experiments with Achilles tendon defect repairs in rabbits were carried out with RPSF and APSF scaffolds. The beneficial effects of fiber alignment were verified from histological and immunohistochemical staining, where cell migration and extracellular matrix protein deposition tend to stretch in a parallel direction along the axial direction of APSF nanofibers with enhanced Col I and tenascin C production. Biomechanical testing indicated the tensile stiffness and maximum load of cell-seeded APSF scaffolds were 60.2 and 81.3% of normal tendon values, respectively, which are significantly higher than cell-seeded RPSF or acellular APSF and RPSF scaffolds. These results suggest that APSF nanofiber scaffolds combined with RDFBs have the potential to repair the gap defects of Achilles tendons in vivo and to effectively restore the function and structure of tendons.

Additive Manufacturing of Patient-Customizable Scaffolds for Tubular Tissues Using the Melt-Drawing Method

Polymeric fibrous scaffolds for guiding cell growth are designed to be potentially used for the tissue engineering (TE) of tubular organs including esophagi, blood vessels, tracheas, etc. Tubular scaffolds were fabricated via melt-drawing of highly elastic poly(l-lactide-co-ε-caprolactone) (PLC) fibers layer-by-layer on a cylindrical mandrel. The diameter and length of the scaffolds are customizable via 3D printing of the mandrel. Thickness of the scaffolds was varied by changing the number of layers of the melt-drawing process. The morphology and tensile properties of the PLC fibers were investigated. The fibers were highly aligned with a uniform diameter. Their diameters and tensile properties were tunable by varying the melt-drawing speeds. These tailorable topographies and tensile properties show that the additive-based scaffold fabrication technique is customizable at the micro- and macro-scale for different tubular tissues. The merits of these scaffolds in TE were further shown by the finding that myoblast and fibroblast cells seeded onto the scaffolds in vitro showed appropriate cell proliferation and distribution. Human mesenchymal stem cells (hMSCs) differentiated to smooth muscle lineage on the microfibrous scaffolds in the absence of soluble induction factors, showing cellular shape modulation and scaffold elasticity may encourage the myogenic differentiation of stem cells.

Winner of the Young Investigator Award of the Society for Biomaterials (USA) for 2016, 10th World Biomaterials Congress, May 17-22, 2016, Montreal QC, Canada: Aligned microribbon-like hydrogels for guiding three-dimensional smooth muscle tissue regeneration

Smooth muscle tissue is characterized by aligned structures, which is critical for its contractile functions. Smooth muscle injury is common and can be caused by various diseases and degenerative processes, and there remains a strong need to develop effective therapies for smooth muscle tissue regeneration with restored structures. To guide cell alignment, previously cells were cultured on 2D nano/microgrooved substrates, but such method is limited to fabricating 2D aligned cell sheets only. Alternatively, aligned electrospun nanofiber has been employed as 3D scaffold for cell alignment, but cells can only be seeded post fabrication, and nanoporosity of electrospun fiber meshes often leads to poor cell distribution. To overcome these limitations, we report aligned gelatin-based microribbons (µRBs) as macroporous hydrogels for guiding smooth muscle alignment in 3D. We developed aligned µRB-like hydrogels using wet spinning, which allows easy fabrication of tissue-scale (cm) macroporous matrices with alignment cues and supports direct cell encapsulation. The macroporosity within µRB-based hydrogels facilitated cell proliferation, new matrix deposition, and nutrient diffusion. In aligned µRB scaffold, smooth muscle cells showed high viability, rapid adhesion, and alignment following µRB direction. Aligned µRB scaffolds supported retention of smooth muscle contractile phenotype, and accelerated uniaxial deposition of new matrix (collagen I/IV) along the µRB. In contrast, cells encapsulated in conventional gelatin hydrogels remained round with matrix deposition limited to pericellular regions only. We envision such aligned µRB scaffold can be broadly applicable in growing other anisotropic tissues including tendon, nerves and blood vessel.

Smooth Muscle Cell Alignment and Phenotype Control by Melt Spun Polycaprolactone Fibers for Seeding of Tissue Engineered Blood Vessels

A method has been developed to induce and retain a contractile phenotype for vascular smooth muscle cells, as the first step towards the development of a biomimetic blood vessel construct with minimal compliance mismatch. Melt spun PCL fibers were deposited on a mandrel to form aligned fibers of 10 μm in diameter. The fibers were bonded into aligned arrangement through dip coating in chitosan solution. This formed a surface of parallel grooves, 10 μm deep by 10 μm across, presenting a surface layer of chitosan to promote cell surface interactions. The aligned fiber surface was used to culture cells present in the vascular wall, in particular fibroblasts and smooth muscle cells. This topography induced “surface guidance” over the orientation of the cells, which adopted an elongated spindle-like morphology, whereas cells on the unpatterned control surface did not show such orientation, assuming more rhomboid shapes. The preservation of VSMC contractile phenotype on the aligned scaffold was demonstrated by the retention of α-SMA expression after several days of culture. The effect was assessed on a prototype vascular graft prosthesis fabricated from polylactide caprolactone; VSMCs aligned longitudinally along a fiberless tube, whereas, for the aligned fiber coated tubes, the VSMCs aligned in the required circumferential orientation.

In vivo evaluation of electrospun polycaprolactone graft for anterior cruciate ligament engineering

The anterior cruciate ligament (ACL) is critical for the structural stability of the knee and its injury often requires surgical intervention. Because current reconstruction methods using autograft or allograft tissue suffer from donor-site morbidity and limited supply, there has been emerging interest in the use of bioengineered materials as a platform for ligament reconstruction. Here, we report the use of electrospun polycaprolactone (PCL) scaffolds as a candidate platform for ACL reconstruction in an in vivo rodent model. Electrospun PCL was fabricated and laser cut to facilitate induction of cells and collagen deposition and used to reconstruct the rat ACL. Histological analysis at 2, 6, and 12 weeks postimplantation revealed biological integration, minimal immune response, and the gradual infiltration of collagen in both the bone tunnel and intra-articular regions of the scaffold. Biomechanical testing demonstrated that the PCL graft failure load and stiffness at 12 weeks postimplantation (13.27±4.20N, 15.98±5.03 N/mm) increased compared to time zero testing (3.95±0.33N, 1.95±0.35 N/mm). Taken together, these results suggest that electrospun PCL serves as a biocompatible graft for ACL reconstruction with the capacity to facilitate collagen deposition.

Electrospun anisotropic architectures and porous structures for tissue engineering

Recent advances in electrospun anisotropic architectures and porous structures, as well as their applications in tissue engineering, are presented.

Biodegradable and biocompatible poly(ethylene glycol)-based hydrogel films for the regeneration of corneal endothelium

Corneal endothelial cells (CECs) are responsible for maintaining the transparency of the human cornea. Loss of CECs results in blindness, requiring corneal transplantation. In this study, fabrication of biocompatible and biodegradable poly(ethylene glycol) (PEG)-based hydrogel films (PHFs) for the regeneration and transplantation of CECs is described. The 50-μm thin hydrogel films have similar or greater tensile strengths to human corneal tissue. Light transmission studies reveal that the films are >98% optically transparent, while in vitro degradation studies demonstrate their biodegradation characteristics. Cell culture studies demonstrate the regeneration of sheep corneal endothelium on the PHFs. Although sheep CECs do not regenerate in vivo, these cells proliferate on the films with natural morphology and become 100% confluent within 7 d. Implantation of the PHFs into live sheep corneas demonstrates the robustness of the films for surgical purposes. Regular slit lamp examinations and histology of the cornea after 28 d following surgery reveal minimal inflammatory responses and no toxicity, indicating that the films are benign. The results of this study suggest that PHFs are excellent candidates as platforms for the regeneration and transplantation of CECs as a result of their favorable biocompatibility, degradability, mechanical, and optical properties.

Solvent-free Fabrication of Tissue Engineering Scaffolds with Immiscible Polymer Blends

A completely organic solvent-free fabrication method is developed for tissue engineering scaffolds by gas foaming of immiscible polylactic acid (PLA) and sucrose blends, followed by water leaching. PLA scaffolds with above 90% porosity and 25-200 μm pore size were fabricated. The pore size and porosity was controlled with process parameters including extrusion temperature and foaming process parameters. Dynamic mechanical analysis showed that the extrusion temperature could be used to control the scaffold strength. Both unfoamed and foamed scaffolds were used to culture glioblastoma (GBM) cells M059K. The results showed that the cells grew better in the foamed PLA scaffolds. The method presented in the paper is versatile and can be used to fabricate tissue engineering scaffolds without any residual organic solvents.