Increasing the pore sizes of bone-mimetic electrospun scaffolds comprised of polycaprolactone, collagen I and hydroxyapatite to enhance cell infiltration

A fast process for imprinting micro and nano patterns on electrospun fiber meshes at physiological temperatures

Electrospun fiber meshes are patterned at length scales comparable to or lower than their fiber diameter. Simple nano- and microgrooves and closed geometric shapes are imprinted in different tones using a fast imprint process at physiological temperatures. Human mesenchymal stromal cells cultured on patterned scaffolds show differences in cellular morphology and cytoskeleton organization. Microgrooved electrospun fibers support upregulation of alkaline phosphatase and bone morphogenetic protein-2 gene expression when cells are cultured in osteogenic medium.

Advancements in electrospinning of polymeric nanofibrous scaffolds for tissue engineering

Polymeric nanofibers have potential as tissue engineering scaffolds, as they mimic the nanoscale properties and structural characteristics of native extracellular matrix (ECM). Nanofibers composed of natural and synthetic polymers, biomimetic composites, ceramics, and metals have been fabricated by electrospinning for various tissue engineering applications. The inherent advantages of electrospinning nanofibers include the generation of substrata with high surface area-to-volume ratios, the capacity to precisely control material and mechanical properties, and a tendency for cellular in-growth due to interconnectivity within the pores. Furthermore, the electrospinning process affords the opportunity to engineer scaffolds with micro- to nanoscale topography similar to the natural ECM. This review describes the fundamental aspects of the electrospinning process when applied to spinnable natural and synthetic polymers; particularly, those parameters that influence fiber geometry, morphology, mesh porosity, and scaffold mechanical properties. We describe cellular responses to fiber morphology achieved by varying processing parameters and highlight successful applications of electrospun nanofibrous scaffolds when used to tissue engineer bone, skin, and vascular grafts.

3D Printing for Tissue Engineering

Tissue engineering aims to fabricate functional tissue for applications in regenerative medicine and drug testing. More recently, 3D printing has shown great promise in tissue fabrication with a structural control from micro- to macro-scale by using a layer-by-layer approach. Whether through scaffold-based or scaffold-free approaches, the standard for 3D printed tissue engineering constructs is to provide a biomimetic structural environment that facilitates tissue formation and promotes host tissue integration (e.g., cellular infiltration, vascularization, and active remodeling). This review will cover several approaches that have advanced the field of 3D printing through novel fabrication methods of tissue engineering constructs. It will also discuss the applications of synthetic and natural materials for 3D printing facilitated tissue fabrication.

Enhanced human bone marrow mesenchymal stem cell functions in novel 3D cartilage scaffolds with hydrogen treated multi-walled carbon nanotubes

Cartilage tissue is a nanostructured tissue which is notoriously hard to regenerate due to its extremely poor inherent regenerative capacity and complex stratified architecture. Current treatment methods are highly invasive and may have many complications. Thus, the goal of this work is to use nanomaterials and nano/microfabrication methods to create novel biologically inspired tissue engineered cartilage scaffolds to facilitate human bone marrow mesenchymal stem cell (MSC) chondrogenesis. To this end we utilized electrospinning to design and fabricate a series of novel 3D biomimetic nanostructured scaffolds based on hydrogen (H2) treated multi-walled carbon nanotubes (MWCNTs) and biocompatible poly(L-lactic acid) (PLLA) polymers. Specifically, a series of electrospun fibrous PLLA scaffolds with controlled fiber dimension were fabricated in this study. In vitro MSC studies showed that stem cells prefer to attach in the scaffolds with smaller fiber diameter. More importantly, the MWCNT embedded scaffolds showed a drastic increase in mechanical strength and a compressive Young's modulus matching to natural cartilage. Furthermore, our MSC differentiation results demonstrated that incorporation of the H2 treated carbon nanotubes and poly-L-lysine coating can induce more chondrogenic differentiations of MSCs than controls. After two weeks of culture, PLLA scaffolds with H2 treated MWCNTs and poly-L-lysine can achieve the highest glycosaminoglycan synthesis, making them promising for further exploration for cartilage regeneration.

Enhanced growth of endothelial precursor cells on PCG-matrix facilitates accelerated, fibrosis-free, wound healing: a diabetic mouse model

Diabetes mellitus (DM)-induced endothelial progenitor cell (EPC) dysfunction causes impaired wound healing, which can be rescued by delivery of large numbers of 'normal' EPCs onto such wounds. The principal challenges herein are (a) the high number of EPCs required and (b) their sustained delivery onto the wounds. Most of the currently available scaffolds either serve as passive devices for cellular delivery or allow adherence and proliferation, but not both. This clearly indicates that matrices possessing both attributes are 'the need of the day' for efficient healing of diabetic wounds. Therefore, we developed a system that not only allows selective enrichment and expansion of EPCs, but also efficiently delivers them onto the wounds. Murine bone marrow-derived mononuclear cells (MNCs) were seeded onto a PolyCaprolactone-Gelatin (PCG) nano-fiber matrix that offers a combined advantage of strength, biocompatibility wettability; and cultured them in EGM2 to allow EPC growth. The efficacy of the PCG matrix in supporting the EPC growth and delivery was assessed by various in vitro parameters. Its efficacy in diabetic wound healing was assessed by a topical application of the PCG-EPCs onto diabetic wounds. The PCG matrix promoted a high-level attachment of EPCs and enhanced their growth, colony formation, and proliferation without compromising their viability as compared to Poly L-lactic acid (PLLA) and Vitronectin (VN), the matrix and non-matrix controls respectively. The PCG-matrix also allowed a sustained chemotactic migration of EPCs in vitro. The matrix-effected sustained delivery of EPCs onto the diabetic wounds resulted in an enhanced fibrosis-free wound healing as compared to the controls. Our data, thus, highlight the novel therapeutic potential of PCG-EPCs as a combined 'growth and delivery system' to achieve an accelerated fibrosis-free healing of dermal lesions, including diabetic wounds.

Growth and spontaneous differentiation of umbilical-cord stromal stem cells on activated carbon cloth

We have investigated the capacity of activated carbon cloth to support the growth and differentiation of human mesenchymal umbilical-cord stromal stem cells. Our results demonstrate that this scaffold provides suitable conditions for the development of cell-derived matrix proteins and facilitates the growth of undifferentiated stem cells with the ability to induce osteogenic and chondrogenic differentiation. Immunoflourescence staining revealed extensive expression of collagen in all the samples, and collagen type II and osteopontin within the samples cultivated in specific differentiation-inducing media. Cell growth and the formation of natural collagen, calcium-magnesium carbonate and hydroxyapatite crystals, together with the self-assemblage of collagen to produce suprafibrillar arrangements of fibrils all occur simultaneously and can be studied together ex vivo under physiological conditions. Furthermore, the spontaneous differentiation of stem cells cultured on activated carbon cloth with no osteogenic supplements opens up new possibilities for bone-tumour engineering and treatment of traumatic and degenerative bone diseases.

The influence of prefreezing temperature on pore structure in freeze-dried beta-TCP scaffolds

A combined method of tricalcium phosphate (TCP) scaffold production, which comprised negative mold and scaffold fabrication, was reported in this study. The negative mold structure was designed by computer and fabricated by fused deposition modeling (FDM) technology, while the TCP scaffold was produced by freeze-drying technology under different prefreezing temperatures of -10 degrees C, -30 degrees C, and -86 degrees C and thermal treatment to get beta-TCP. The scaffold structure was evaluated with X-ray, scanning electron microscopy (SEM), compressive mechanical testing, and micro-computerized tomography (micro-CT). The cell-scaffold interaction was studied by culturing dog bone marrow stromal cells (BMSCs) on the scaffolds and assessing differentiated BMSC function by measuring cellular alkaline phosphatase (ALP) activity. The results showed good interconnectivity and good pore distribution with the pore size ranging from 50 to 250 microm and compressive modulus of 1.18 MPa at a prefreezing temperatures of -10 degrees C. In vitro cell culture results indicated that the porous scaffolds were not toxic to bone cells. These results demonstrate that rapid prototyping and freeze-drying technologies for creating beta-TCP scaffolds are promising for bone tissue engineering.

Engineering bone tissue using human dental pulp stem cells and an osteogenic collagen-hydroxyapatite-poly (L-lactide-co-ε-caprolactone) scaffold

The aim of this study was to design a new natural/synthetic bioactive bone scaffold for potential use in bone replacement applications. We developed a tri-component osteogenic composite scaffold made of collagen (Coll), hydroxyapatite (HA) and poly(l-lactide-co-ε-caprolactone) (PLCL). This Coll/HA/PLCL composite scaffold was combined with human osteoblast-like cells obtained by differentiation of dental pulp stem cells (DPSCs) to engineer bone tissue in vitro. Results show that the 3D Coll/HA/PLCL composite scaffold was highly porous, thereby enabling osteoblast-like cell adhesion and growth. Cultured in the Coll/HA/PLCL scaffold, the osteoblast-like cells expressed different osteogenic genes, produced alkaline phosphatase and formed nodules more than did PLCL alone. Micro-CT analyses revealed a significant (30%) increase of tissue mineralisation on the surface as well as inside of the Coll/HA/PLCL scaffold, thus confirming its effectiveness as a bone regeneration platform.

Creation of macropores in electrospun silk fibroin scaffolds using sacrificial PEO-microparticles to enhance cellular infiltration

Electrospun scaffolds are widely used in tissue engineering; however, a common problem is the poor cell infiltration because of the small pore size and tightly packed structure of these fibrous scaffolds. To address this issue, a novel technique was developed to fabricate electrospun silk fibroin (SF) scaffolds with rather macropores and high porosity using electrospraying-generated PEO microparticles as porogen. The morphology and pore size of MPES scaffolds were evaluated by scanning electron microscopy. It was revealed that MPES scaffold had a relatively loose structure with an increase of mean pore size (i.e., approx. 30 μm of MPES vs. approx. 5 μm of traditional electrospun scaffolds (TES) and porosity (i.e., 95% vs. 84% of TES). Culture of mouse 3T3 fibroblast in TES and MPES scaffold revealed that both scaffolds could support cell attachment, spread and proliferation. Yet, cell inflitration in vitro under the static culture condition only occurred in the MPES scaffold. Subcutaneous implantation of scaffolds in rats further confirmed that the tissue ingrowth was more efficient in the MPES scaffold compared to TES scaffold. Thus, the use of PEO microparticles as porogen was a feasible and effective method for creating macroporous electrospun SF scaffold, which provided an alternative to address the limitation of cell infiltration associated with electrospun fibrous scaffold.

Properties and modification of porous 3-D collagen/hydroxyapatite composites

A freeze drying technique was used to form porous three-dimensional collagen matrixes modified by the addition of a variable amount of nano-hydroxyapatite. For chemical cross-linking EDC/NHS were used. Physical cross-linking was achieved by dehydrothermal treatment. Mechanical properties, morphology, dissolution, porosity, density, enzymatic degradation and swelling properties of materials have been studied after cross-linking. The density of scaffolds and its compressive modulus increased with an increasing amount of hydroxyapatite and collagen concentration in the composite scaffold, while the swelling ratio and porosity decreased. The studied scaffolds dissolved slowly in PBS solution. DHT cross-linked collagen matrices showed a much faster degradation rate after exposure to collagenase than the EDC cross-linked samples.

Recent advances in bone tissue engineering scaffolds

Bone disorders are of significant concern due to increase in the median age of our population. Traditionally, bone grafts have been used to restore damaged bone. Synthetic biomaterials are now being used as bone graft substitutes. These biomaterials were initially selected for structural restoration based on their biomechanical properties. Later scaffolds were engineered to be bioactive or bioresorbable to enhance tissue growth. Now scaffolds are designed to induce bone formation and vascularization. These scaffolds are often porous, made of biodegradable materials that harbor different growth factors, drugs, genes, or stem cells. In this review, we highlight recent advances in bone scaffolds and discuss aspects that still need to be improved.

Supramolecular polycaprolactone nanocomposite based on functionalized hydroxyapatite

Arms bearing ureido-pyrimidinone functional groups with self-association capability (through quadruple hydrogen bonds) were successfully grafted onto hydroxyapatite nanoparticles. The supramolecularly modified nanoparticles (nHApUPy) exhibited enhanced colloidal stability compared to the original hydroxyapatite nanoparticles and were uniformly dispersed in supramolecular polycaprolactone in PCL(UPy)2/HApUPy nanocomposites at different filler loadings. The combined atomic force microscopy, mechanical, and rheological analyses confirmed a high degree of compatibility of HApUPy nanoparticles with the polymer matrix. The temperature dependence of the supramolecular structure in PCL(UPy)2/HApUPy nanocomposites was determined from dynamic rheological measurements at two different temperatures, 60°C and 85°C. The osteocompatibility of the nanocomposite containing HApUPy nanoparticles was compared to the pure polymer. The preliminary cell results clearly confirm that the supramolecular nanocomposites are nontoxic and biocompatible. Therefore, it is postulated that supramolecular nanocomposites provide a new way of tuning the mechanical properties of the supramolecular polymers, particularly supramolecular polycaprolactones.

Delivery of platelet-derived growth factor as a chemotactic factor for mesenchymal stem cells by bone-mimetic electrospun scaffolds

The recruitment of mesenchymal stem cells (MSCs) is a vital step in the bone healing process, and hence the functionalization of osteogenic biomaterials with chemotactic factors constitutes an important effort in the tissue engineering field. Previously we determined that bone-mimetic electrospun scaffolds composed of polycaprolactone, collagen I and nanohydroxyapatite (PCL/col/HA) supported greater MSC adhesion, proliferation and activation of integrin-related signaling cascades than scaffolds composed of PCL or collagen I alone. In the current study we investigated the capacity of bone-mimetic scaffolds to serve as carriers for delivery of an MSC chemotactic factor. In initial studies, we compared MSC chemotaxis toward a variety of molecules including PDGF-AB, PDGF-BB, BMP2, and a mixture of the chemokines SDF-1α, CXCL16, MIP-1α, MIP-1β, and RANTES. Transwell migration assays indicated that, of these factors, PDGF-BB was the most effective in stimulating MSC migration. We next evaluated the capacity of PCL/col/HA scaffolds, compared with PCL scaffolds, to adsorb and release PDGF-BB. We found that significantly more PDGF- BB was adsorbed to, and subsequently released from, PCL/col/HA scaffolds, with sustained release extending over an 8-week interval. The PDGF-BB released was chemotactically active in transwell migration assays, indicating that bioactivity was not diminished by adsorption to the biomaterial. Complementing these studies, we developed a new type of migration assay in which the PDGF-BB-coated bone-mimetic substrates were placed 1.5 cm away from the cell migration front. These experiments confirmed the ability of PDGF-BB-coated PCL/col/HA scaffolds to induce significant MSC chemotaxis under more stringent conditions than standard types of migration assays. Our collective results substantiate the efficacy of PDGF-BB in stimulating MSC recruitment, and further show that the incorporation of native bone molecules, collagen I and nanoHA, into electrospun scaffolds not only enhances MSC adhesion and proliferation, but also increases the amount of PDGF-BB that can be delivered from scaffolds.

Improved cellular infiltration in electrospun fiber via engineered porosity

Small pore sizes inherent to electrospun matrices can hinder efficient cellular ingrowth. To facilitate infiltration while retaining its extracellular matrix-like character, electrospinning was combined with salt leaching to produce a scaffold having deliberate, engineered delaminations. We made elegant use of a specific randomizing component of the electrospinning process, the Taylor Cone and the falling fiber beneath it, to produce a uniform, well-spread distribution of salt particles. After 3 weeks of culture, up to 4 mm of cellular infiltration was observed, along with cellular coverage of up to 70% within the delaminations. To our knowledge, this represents the first observation of extensive cellular infiltration of electrospun matrices. Infiltration appears to be driven primarily by localized proliferation rather than coordinated cellular locomotion. Cells also moved from the salt-generated porosity into the surrounding electrospun fiber matrix. Given that the details of salt deposition (amount, size, and number density) are far from optimized, the result provides a convincing illustration of the ability of mammalian cells to interact with appropriately tailored electrospun matrices. These layered structures can be precisely fabricated by varying the deposition interval and particle size conceivably to produce in vivo-like gradients in porosity such that the resulting scaffolds better resemble the desired final structure.