Stretching of porous poly (l-lactide-co-ε-caprolactone) membranes regulates the differentiation of mesenchymal stem cells

Background: Among a variety of biomaterials supporting cell growth for therapeutic applications, poly (l-lactide-co-ε-caprolactone) (PLCL) has been considered as one of the most attractive scaffolds for tissue engineering owing to its superior mechanical strength, biocompatibility, and processibility. Although extensive studies have been conducted on the relationship between the microstructure of polymeric materials and their mechanical properties, the use of the fine-tuned morphology and mechanical strength of PLCL membranes in stem cell differentiation has not yet been studied. Methods: PLCL membranes were crystallized in a combination of diverse solvent-nonsolvent mixtures, including methanol (MeOH), isopropanol (IPA), chloroform (CF), and distilled water (DW), with different solvent polarities. A PLCL membrane with high mechanical strength induced by limited pore formation was placed in a custom bioreactor mimicking the reproducible physiological microenvironment of the vascular system to promote the differentiation of mesenchymal stem cells (MSCs) into smooth muscle cells (SMCs). Results: We developed a simple, cost-effective method for fabricating porosity-controlled PLCL membranes based on the crystallization of copolymer chains in a combination of solvents and non-solvents. We confirmed that an increase in the ratio of the non-solvent increased the chain aggregation of PLCL by slow evaporation, leading to improved mechanical properties of the PLCL membrane. Furthermore, we demonstrated that the cyclic stretching of PLCL membranes induced MSC differentiation into SMCs within 10 days of culture. Conclusion: The combination of solvent and non-solvent casting for PLCL solidification can be used to fabricate mechanically durable polymer membranes for use as mechanosensitive scaffolds for stem cell differentiation.

Coordination of ε-Caprolactone to a Cationic Niobium(V) Alkoxide Complex: Fundamental Insight into Ring-Opening Polymerization via Coordination-Insertion

We report three niobium-based initiators for the catalytic ring-opening polymerization (ROP) of ε-caprolactone, exhibiting good activity and molecular weight control. In particular, we have prepared on the gram-scale and fully characterized a monometallic cationic alkoxo-Nb(V) ε-caprolactone adduct representing, to the best of our knowledge, an unprecedented example of a metal complex with an intact lactone monomer and a functional ROP-initiating group simultaneously coordinated at the metal center. At 80 °C, all three systems initiate the immortal solution-state ROP of ε-caprolactone via a coordination-insertion mechanism, which has been confirmed through experimental studies, and is supported by computational data. Natural bond orbital calculations further indicate that polymerization may necessitate isomerization about the metal center between the alkoxide chain and the coordinated monomer. The observations made in this work are expected to inform mechanistic understanding both of amine tris(phenolate)-supported metal alkoxide ROP initiators, including various highly stereoselective systems for the polymerization of lactides and of coordination-insertion-type ROP protocols more broadly.

Bioactive Nanostructured Scaffold-Based Approach for Tendon and Ligament Tissue Engineering

An effective therapeutic strategy to treat tendon or ligament injury continues to be a clinical challenge due to the limited natural healing capacity of these tissues. Furthermore, the repaired tendons or ligaments usually possess inferior mechanical properties and impaired functions. Tissue engineering can restore the physiological functions of tissues using biomaterials, cells, and suitable biochemical signals. It has produced encouraging clinical outcomes, forming tendon or ligament-like tissues with similar compositional, structural, and functional attributes to the native tissues. This paper starts by reviewing tendon/ligament structure and healing mechanisms, followed by describing the bioactive nanostructured scaffolds used in tendon and ligament tissue engineering, with emphasis on electrospun fibrous scaffolds. The natural and synthetic polymers for scaffold preparation, as well as the biological and physical cues offered by incorporating growth factors in the scaffolds or by dynamic cyclic stretching of the scaffolds, are also covered. It is expected to present a comprehensive clinical, biological, and biomaterial insight into advanced tissue engineering-based therapeutics for tendon and ligament repair.

Manipulation of porous poly(l-lactide-co-ε-caprolactone) microcarriers via microfluidics for C2C12 expansion

The growth and repair of skeletal muscle are due in part to activation of muscle precursor cells, commonly known as satellite cells or myoblasts. In order to acquire enough cells for neoskeletal muscle regeneration, it is urgent to develop microcarriers for skeletal myoblasts proliferation with a considerable efficiency. The current study was thus proposed to develop a microfluidic technology to manufacture porous poly(l-lactide-co-ε-caprolactone) (PLCL) microcarriers of high uniformity, and porosity was manipulated via camphene to suit the proliferation of C2C12 cells. A co-flow capillary microfluidic device was first designed to obtain PLCL microcarriers with different porosity. The attachment and proliferation of C2C12 cells on these microcarriers were evaluated and the differentiation potential of expanded cells were verified. The obtained porous microcarriers were all uniform in size with a high mono-dispersity (CV < 5 %). The content of camphene rendered effects on the size, porosity, and pore size of microcarriers, and porous structure addition produced a softening of their mechanical properties. The one of 10 % camphene (PM-10) exhibited the superior expansion for C2C12 cells with the number of cells after 5 days of culture reached 9.53 times of the adherent cells on the first day. The expanded cells from PM-10 still retained excellent myogenic differentiation performance as the expressions of MYOD, Desmin and MYH2 were intensively enhanced. Hence, the current developed porous PLCL microcarriers could offer as a promising type of substrates not only for in vitro muscular precursor cells expansion without compromising any multipotency but also have the potential as injectable constructs to mediate muscle regeneration.

Natural, synthetic and commercially-available biopolymers used to regenerate tendons and ligaments

Tendon and ligament (TL) injuries affect millions of people annually. Biopolymers play a significant role in TL tissue repair, whether the treatment relies on tissue engineering strategies or using artificial tendon grafts. The biopolymer governs the mechanical properties, biocompatibility, degradation, and fabrication method of the TL scaffold. Many natural, synthetic and hybrid biopolymers have been studied in TL regeneration, often combined with therapeutic agents and minerals to engineer novel scaffold systems. However, most of the advanced biopolymers have not advanced to clinical use yet. Here, we aim to review recent biopolymers and discuss their features for TL tissue engineering. After introducing the properties of the native tissue, we discuss different types of natural, synthetic and hybrid biopolymers used in TL tissue engineering. Then, we review biopolymers used in commercial absorbable and non-absorbable TL grafts. Finally, we explain the challenges and future directions for the development of novel biopolymers in TL regenerative treatment.

Essential Oil—Loaded Nanofibers for Pharmaceutical and Biomedical Applications: A Systematic Mini-Review

Essential oils (EOs) have been widely exploited for their biological properties (mainly as antimicrobials) in the food industry. Encapsulation of EOs has opened the way to the utilization of EOs in the pharmaceutical and biomedical fields. Electrospinning (ES) has proved a convenient and versatile method for the encapsulation of EOs into multifunctional nanofibers. Within the last five years (2017–2022), many research articles have been published reporting the use of ES for the fabrication of essential oil—loaded nanofibers (EONFs). The objective of the present mini-review article is to elucidate the potential of EONFs in the pharmaceutical and biomedical fields and to highlight their advantages over traditional polymeric films. An overview of the conventional ES and coaxial ES technologies for the preparation of EONFs is also included. Even though EONFs are promising systems for the delivery of EOs, gaps in the literature can be recognized (e.g., stability studies) emphasizing that more research work is needed in this field to fully unravel the potential of EONFs.

Periostin Contributes to Fibrocartilage Layer Growth of the Patella Tendon Tibial Insertion in Mice

Background and Objectives: The influence of periostin on the growth of the patella tendon (PT) tibial insertion is unknown. The research described here aimed to reveal the contribution of periostin to the growth of fibrocartilage layers of the PT tibial insertion using periostin knockout mice. Materials and Methods: In both the wild-type (WD; C57BL/6N, periostin +/+; n = 54) and periostin knockout (KO; periostin −/−; n = 54) groups, six mice were euthanized on day 1 and at 1, 2, 3, 4, 6, 8, 10, and 12 weeks of age. Chondrocyte proliferation and apoptosis, number of chondrocytes, safranin O-stained glycosaminoglycan (GAG) area, staining area of type II collagen, and length of the tidemark were investigated. Results: Chondrocyte proliferation and apoptosis in KO were lower than those in WD on day 1 and at 1, 4, and 8 weeks and on day 1 and at 4, 6, and 12 weeks, respectively. Although the number of chondrocytes in both groups gradually decreased, it was lower in KO than in WD on day 1 and at 8 and 12 weeks. In the extracellular matrix, the GAG-stained area in KO was smaller than that in WD on day 1 and at 1, 4, 8, 10, and 12 weeks. The staining area of type II collagen in KO was smaller than that in WD at 8 weeks. The length of the tidemark in KO was shorter than that in WD at 4 and 6 weeks. Conclusion: Loss of periostin led to decreased chondrocyte proliferation, chondrocyte apoptosis, and the number of chondrocytes in the growth process of the PT tibial insertion. Moreover, periostin decreased and delayed GAG and type II collagen production and delayed tidemark formation in the growth process of the PT tibial insertion. Periostin can, therefore, contribute to the growth of fibrocartilage layers in the PT tibial insertion. Periostin deficiency may result in incomplete growth of the PT tibial insertion.

Synergistic effects of mechanical stimulation and crimped topography to stimulate natural collagen development for tendon engineering

Suitable scaffold structures and mechanical loading are essential for functional tendon engineering. However, the bipolar fibril structure of native tendon collagen is yet to be recaptured in engineered tendons. This study compared the development of Achilles tendons of postnatal rats with and without (via surgical section) mechanical loading to define the mechanism of mechanical stimulation-mediated tendon development. The results demonstrated that the severed tendons weakened mechanically and exhibited disorganization without a bipolar fibril superstructure. Proteomic analysis revealed differentially expressed key regulatory molecules related to the collagen assembly process, including decreased fibromodulin, keratocan, fibroblast growth factor-1, and increased lumican and collagen5a1 in the severed tendons with immunohistochemical verification. Additionally, a complex regulatory network of mechanical stimulation-mediated collagen assembly in a spatiotemporal manner was also revealed using bioinformatics analysis, wherein PI3K-Akt and HDAC4 may be the predominant signaling pathways. A wavy microgrooved surface (Y = 5.47sin(0.015x)) that biomimics tendon topography was observed to enhance the expression of collagen assembly molecules under mechanical loading, and the aforementioned pathways are particularly involved and verified with their respective inhibitors of LY-294002 and LMK-235. Furthermore, an electrospun crimped nanofiber scaffold (approximately 2 μm fiber diameter and 0.12 crimpness) was fabricated to biomimic the tenogenic niche environment; this was observed to be more effective on enhancing collagen production and assembly under mechanical stimulation. In conclusion, the synergistic effect between topographical niche and mechanical stimulation was observed to be essential for collagen assembly and maturation and should be applied to functional tendon engineering in the future. STATEMENT OF SIGNIFICANCE: In biomaterial-mediated tendon regeneration, mechanical stimulation is essential for tendon collagen assembly. However, the underlying mechanisms remain not fully defined, leading to the failure of the native-like collagen regeneration. In this study, a mechanical stimulation deprivation model of rat tendon was established to reveal the mechanisms in tendon development and define the key regulatory molecules including small leucine-rich proteoglycans, lysyl oxidase and collagen V. After ensuring the importance of biomimetic structure in tendon remodeling, crimped nanofibers were developed to verify these regulatory molecules, and demonstrated that mechanical stimulation significantly enhanced collagen assembly via PIK3 and HDAC4 pathways in biomaterial-regulated tendon regeneration. This study provides more insightful perspectives in the physiologically remodeling progression of tendon collagen and design of tendon scaffolds.

Monitoring mechanical stimulation for optimal tendon tissue engineering: A mechanical and biological multiscale study

To understand the effect of mechanical stimulation on cell response, bone marrow stromal cells were cultured on electrospun scaffolds under two distinct mechanical conditions (static and dynamic). Comparison between initial and final mechanical and biological properties of the cell-constructs were conducted over 14 days for both culturing conditions. As a result, mechanically stimulated constructs, in contrast to their static counterparts, showed evident mechanical-induced cell orientation, an effective aligned collagen and tenomodulin extracellular matrix. This orientation provides clues on the importance of mechanical stimulation to induce a tendon-like differentiation. In addition, cell and collagen orientation lead to enhanced storage modulus observed under dynamic stimulation. Altogether mechanical stimulation lead to (a) cell and matrix orientation through the sense of the stretch and (b) a dominant elastic response in the cell-constructs with a minor contribution of the viscosity in the global mechanical behavior. Such a correlation could help in further studies to better understand the effect of mechanical stimulation in tissue engineering.

A combined physicochemical approach towards human tenocyte phenotype maintenance

During in vitro culture, bereft of their optimal tissue context, tenocytes lose their phenotype and function. Considering that tenocytes in their native tissue milieu are exposed simultaneously to manifold signals, combination approaches (e.g. growth factor supplementation and mechanical stimulation) are continuously gaining pace to control cell fate during in vitro expansion, albeit with limited success due to the literally infinite number of possible permutations. In this work, we assessed the potential of scalable and potent physicochemical approaches that control cell fate (substrate stiffness, anisotropic surface topography, collagen type I coating) and enhance extracellular matrix deposition (macromolecular crowding) in maintaining human tenocyte phenotype in culture. Cell morphology was primarily responsive to surface topography. The tissue culture plastic induced the largest nuclei area, the lowest aspect ratio, and the highest focal adhesion kinase. Collagen type I coating increased cell number and metabolic activity. Cell viability was not affected by any of the variables assessed. Macromolecular crowding intensely enhanced and accelerated native extracellular matrix deposition, albeit not in an aligned fashion, even on the grooved substrates. Gene analysis at day 14 revealed that the 130 kPa grooved substrate without collagen type I coating and under macromolecular crowding conditions positively regulated human tenocyte phenotype. Collectively, this work illustrates the beneficial effects of combined physicochemical approaches in controlling cell fate during in vitro expansion.

Fibrous Systems as Potential Solutions for Tendon and Ligament Repair, Healing, and Regeneration

Tendon and ligament injuries caused by trauma and degenerative diseases are frequent and affect diverse groups of the population. Such injuries reduce musculoskeletal performance, limit joint mobility, and lower people's comfort. Currently, various treatment strategies and surgical procedures are used to heal, repair, and restore the native tissue function. However, these strategies are inadequate and, in some cases, fail to re-establish the lost functionality. Tissue engineering and regenerative medicine approaches aim to overcome these disadvantages by stimulating the regeneration and formation of neotissues. Design and fabrication of artificial scaffolds with tailored mechanical properties are crucial for restoring the mechanical function of tendons. In this review, the tendon and ligament structure, their physiology, and performance are presented. On the other hand, the requirements are focused for the development of an effective reconstruction device. The most common fiber-based scaffolding systems are also described for tendon and ligament tissue regeneration like strand fibers, woven, knitted, braided, and braid-twisted fibrous structures, as well as electrospun and wet-spun constructs, discussing critically the advantages and limitations of their utilization. Finally, the potential of multilayered systems as the most effective candidates for tendon and ligaments tissue engineering is pointed out.

The synergistic effect of low oxygen tension and macromolecular crowding in the development of extracellular matrix-rich tendon equivalents

Cellular therapies play an important role in tendon tissue engineering, with tenocytes being the most prominent and potent cell population available. However, for the development of a rich extracellular matrix tenocyte-assembled tendon equivalent, prolonged in vitro culture is required, which is associated with phenotypic drift. Recapitulation of tendon tissue microenvironment in vitro with cues that enhance and accelerate extracellular matrix synthesis and deposition, whilst maintaining tenocyte phenotype, may lead to functional cell therapies. Herein, we assessed the synergistic effect of low oxygen tension (enhances extracellular matrix synthesis) and macromolecular crowding (enhances extracellular matrix deposition) in human tenocyte culture. Protein analysis demonstrated that human tenocytes at 2% oxygen tension and with 50 μg ml^-1 carrageenan (macromolecular crowder used) significantly increased synthesis and deposition of collagen types I, III, V and VI. Gene analysis at day 7 illustrated that human tenocytes at 2% oxygen tension and with 50 μg ml^-1 carrageenan significantly increased the expression of prolyl 4-hydroxylase subunit alpha 1, procollagen-lysine 2- oxoglutarate 5-dioxygenase 2, scleraxis, tenomodulin and elastin, whilst chondrogenic (e.g. runt-related transcription factor 2, cartilage oligomeric matrix protein, aggrecan) and osteogenic (e.g. secreted phosphoprotein 1, bone gamma-carboxyglutamate protein) trans-differentiation markers were significantly down-regulated or remained unchanged. Collectively, our data clearly illustrates the beneficial synergistic effect of low oxygen tension and macromolecular crowding in the accelerated development of tissue equivalents.

Tendon Tissue Engineering: Effects of Mechanical and Biochemical Stimulation on Stem Cell Alignment on Cell-Laden Hydrogel Yarns

Fiber-based approaches hold great promise for tendon tissue engineering enabling the possibility of manufacturing aligned hydrogel filaments that can guide collagen fiber orientation, thereby providing a biomimetic micro-environment for cell attachment, orientation, migration, and proliferation. In this study, a 3D system composed of cell-laden, highly aligned hydrogel yarns is designed and obtained via wet spinning in order to reproduce the morphology and structure of tendon fascicles. A bioink composed of alginate and gelatin methacryloyl (GelMA) is optimized for spinning and loaded with human bone morrow mesenchymal stem cells (hBM-MSCs). The produced scaffolds are subjected to mechanical stretching to recapitulate the strains occurring in native tendon tissue. Stem cell differentiation is promoted by addition of bone morphogenetic protein 12 (BMP-12) in the culture medium. The aligned orientation of the fibers combined with mechanical stimulation results in highly preferential longitudinal cell orientation and demonstrates enhanced collagen type I and III expression. Additionally, the combination of biochemical and mechanical stimulations promotes the expression of specific tenogenic markers, signatures of efficient cell differentiation towards tendon. The obtained results suggest that the proposed 3D cell-laden aligned system can be used for engineering of scaffolds for tendon regeneration.

Mesenchymal stem cell interacted with PLCL braided scaffold coated with poly-l-lysine/hyaluronic acid for ligament tissue engineering

The challenge of finding an adapted scaffold for ligament tissue engineering remains unsolved after years of researches. A technology to fabricate a multilayer braided scaffold with flexible and elastic poly (l-lactide-co-caprolactone) (PLCL 85/15) has been recently pioneered by our team. In this study, polyelectrolyte multilayer films (PEM) with poly-l-lysine (PLL)/ hyaluronic acid (HA) were deposited on this scaffold. After PEM modification, polygonal (PLL) and particle-like (HA) structures were present on the braided scaffold with no significant variation of fibers Young's modulus. Wharton's jelly mesenchymal stem cells (WJ-MSC) and bone marrow mesenchymal stem cells (BM-MSC) showed good metabolic activity on scaffolds. They presented a spindled shape along the fiber longitudinal direction, and crossed the fibers to form cell bridges. Collagen type I, collagen type III, and tenascin-C secreted by MSCs were detected on day 14. Moreover, one-layer modified scaffold presented increased chemotaxis. As a conclusion, our results indicate that this braided PLCL scaffold with one-layer PEM modification shows inspiring potential with satisfying mechanical properties and biocompatibility. It opens new perspectives to incorporate growth factors within PEM-modified braided PLCL scaffold for ligament tissue engineering and to recruit endogenous cells after implantation. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 3042-3052, 2018.

Flexible bipolar nanofibrous membranes for improving gradient microstructure in tendon-to-bone healing

Enthesis is a specialized tissue interface between the tendon and bone. Enthesis structure is very complex because of gradient changes in its composition and structure. There is currently no strategy to create a suitable environment and to regenerate the gradual-changing enthesis because of the modular complexities between two tissue types. Herein, a dual-layer organic/inorganic flexible bipolar fibrous membrane (BFM) was successfully fabricated by electrospinning to generate biomimetic non-mineralized fibrocartilage and mineralized fibrocartilage in tendon-to-bone integration of enthesis. The growth of the in situ apatite nanoparticle layer was induced on the nano hydroxyapatite-poly-l-lactic acid (nHA-PLLA) fibrous layer in simulated body solution, and the poly-l-lactic acid (PLLA) fibrous layer retained its original properties to induce tendon regeneration. The in vivo results showed that BFM significantly increased the area of glycosaminoglycan staining at the tendon-bone interface and improved collagen organization when compared to the simplex fibrous membrane (SFM) of PLLA. Implanting the bipolar membrane also induced bone formation and fibrillogenesis as assessed by micro-CT and histological analysis. Biomechanical testing showed that the BFM group had a greater ultimate load-to-failure and stiffness than the SFM group at 12weeks after surgery. Therefore, this flexible bipolar nanofibrous membrane improves the healing and regeneration process of the enthesis in rotator cuff repair.

STATEMENT OF SIGNIFICANCE

In this study, we generated a biomimetic dual-layer organic/inorganic flexible bipolar fibrous membrane by sequential electrospinning and in situ biomineralization, producing integrated bipolar fibrous membranes of PLLA fibrous membrane as the upper layer and nHA-PLLA fibrous membrane as the lower layer to mimic non-mineralized fibrocartilage and mineralized fibrocartilage in tendon-to-bone integration of enthesis. Flexible bipolar nanofibrous membranes could be easily fabricated with gradient microstructure for enthesis regeneration in rotator cuff tears.

Promotion of adhesion and proliferation of endothelial progenitor cells on decellularized valves by covalent incorporation of RGD peptide and VEGF

Tissue engineered heart valve is a promising alternative to current heart valve surgery, for its capability of growth, repair, and remodeling. However, extensive development is needed to ensure tissue compatibility, durability and antithrombotic potential. This study aims to investigate the biological effects of multi-signal composite material of polyethyl glycol-cross-linked decellularized valve on adhesion and proliferation of endothelial progenitor cells. Group A to E was decellularized valve leaflets, composite material of polyethyl glycol-cross-linked decellularized valves leaflets, vascular endothelial growth factor-composite materials, Arg-Gly-Asp peptide-composite materials and multi-signal modified materials of polyethyl glycol-cross-linked decellularized valve leaflets, respectively. The endothelial progenitor cells were seeded for each group, cell adhesion and proliferation were detected and neo-endothelium antithrombotic function of the multi-signal composite materials was evaluated. At 2, 4, and 8 h after the seeding, the cell numbers and 3H-TdR incorporation in group D were the highest. At 2, 4, and 8 days after the seeding, the cell numbers and 3H-TdR incorporation were significantly higher in groups C, D, and E compared with groups A and B (P < 0.05) and cell numbers and the expression of t-PA and eons in the neo-endothelium were quite similar to those in the human umbilical vein endothelial cells at 2, 4, and 8 days after the seeding. The Arg-Gly-Asp- peptides (a sequential peptide composed of arginine (Arg), glycine (Gly) and aspartic acid (Asp)) and VEGF-conjugated onto the composite material of PEG-crosslinked decellularized valve leaflets synergistically promoted the adhesion and proliferation of endothelial progenitor cells on the composite material, which may help in tissue engineering of heart valves.

Human Adipose Stem Cells Differentiated on Braided Polylactide Scaffolds Is a Potential Approach for Tendon Tissue Engineering

Growing number of musculoskeletal defects increases the demand for engineered tendon. Our aim was to find an efficient strategy to produce tendon-like matrix in vitro. To allow efficient differentiation of human adipose stem cells (hASCs) toward tendon tissue, we tested different medium compositions, biomaterials, and scaffold structures in preliminary tests. This is the first study to report that medium supplementation with 50 ng/mL of growth and differentiation factor-5 (GDF-5) and 280 μM l-ascorbic acid are essential for tenogenic differentiation of hASCs. Tenogenic medium (TM) was shown to significantly enhance tendon-like matrix production of hASCs compared to other tested media groups. Cell adhesion, proliferation, and tenogenic differentiation of hASCs were supported on braided poly(l/d)lactide (PLA) 96l/4d copolymer filament scaffolds in TM condition compared to foamed poly(l-lactide-co-ɛ-caprolactone) (PLCL) 70L/30CL scaffolds. A uniform cell layer formed on braided PLA 96/4 scaffolds when hASCs were cultured in TM compared to maintenance medium (MM) condition after 14 days of culture. Furthermore, total collagen content and gene expression of tenogenic marker genes were significantly higher in TM condition after 2 weeks of culture. The elastic modulus of PLA 96/4 scaffold was more similar to the elastic modulus reported for native Achilles tendon. Our study showed that the optimized TM is needed for efficient and rapid in vitro tenogenic extracellular matrix production of hASCs. PLA 96/4 scaffolds together with TM significantly stimulated hASCs, thus demonstrating the potential clinical relevance of this novel and emerging approach to tendon injury treatments in the future.

Microstructure-dependent mechanical properties of electrospun core-shell scaffolds at multi-scale levels

Mechanical factors among many physiochemical properties of scaffolds for stem cell-based tissue engineering significantly affect tissue morphogenesis by controlling stem cell behaviors including proliferation and phenotype-specific differentiation. Core-shell electrospinning provides a unique opportunity to control mechanical properties of scaffolds independent of surface chemistry, rendering a greater freedom to tailor design for specific applications. In this study, we synthesized electrospun core-shell scaffolds having different core composition and/or core-to-shell dimensional ratios. Two independent biocompatible polymer systems, polyetherketoneketone (PEKK) and gelatin as the core materials while maintaining the shell polymer with polycaprolactone (PCL), were utilized. The mechanics of such scaffolds was analyzed at the microscale and macroscales to determine the potential implications it may hold for cell-material and tissue-material interactions. The mechanical properties of individual core-shell fibers were controlled by core-shell composition and structure. The individual fiber modulus correlated with the increase in percent core size ranging from 0.55±0.10GPa to 1.74±0.22GPa and 0.48±0.12GPa to 1.53±0.12GPa for the PEKK-PCL and gelatin-PCL fibers, respectively. More importantly, it was demonstrated that mechanical properties of the scaffolds at the macroscale were dominantly determined by porosity under compression. The increase of scaffold porosity from 70.2%±1.0% to 93.2%±0.5% by increasing the core size in the PEKK-PCL scaffold resulted in the decrease of the compressive elastic modulus from 227.67±20.39kPa to 14.55±1.43kPa while a greater changes in the porosity of gelatin-PCL scaffold from 54.5%±4.2% to 89.6%±0.4% resulted in the compressive elastic modulus change from 484.01±30.18kPa to 17.57±1.40kPa. On the other hand, the biphasic behaviors under tensile mechanical loading result in a range from a minimum of 5.42±1.05MPa to a maximum of 12.00±1.96MPa for the PEKK-PCL scaffolds, and 10.19±4.49MPa to 22.60±2.44MPa for the gelatin-PCL scaffolds. These results suggest a feasible approach for precisely controlling the local and global mechanical characteristics, in addition to independent control over surface chemistry, to achieve a desired tissue morphogenesis using the core-shell electrospinning.

Electrostatic self-assembled graphene oxide-collagen scaffolds towards a three-dimensional microenvironment for biomimetic applications

The manipulation of the interactions between the cationic amine groups from collagen and the anionic carboxylic groups from graphene oxide mediate the synthesis of a self-assembled hydrogel capable of generate suitable 3D cellular microenvironments.

Engineering Tendon: Scaffolds, Bioreactors, and Models of Regeneration

Tendons bridge muscle and bone, translating forces to the skeleton and increasing the safety and efficiency of locomotion. When tendons fail or degenerate, there are no effective pharmacological interventions. The lack of available options to treat damaged tendons has created a need to better understand and improve the repair process, particularly when suitable autologous donor tissue is unavailable for transplantation. Cells within tendon dynamically react to loading conditions and undergo phenotypic changes in response to mechanobiological stimuli. Tenocytes respond to ultrastructural topography and mechanical deformation via a complex set of behaviors involving force-sensitive membrane receptor activity, changes in cytoskeletal contractility, and transcriptional regulation. Effective ex vivo model systems are needed to emulate the native environment of a tissue and to translate cell-matrix forces with high fidelity. While early bioreactor designs have greatly expanded our knowledge of mechanotransduction, traditional scaffolds do not fully model the topography, composition, and mechanical properties of native tendon. Decellularized tendon is an ideal scaffold for cultivating replacement tissue and modeling tendon regeneration. Decellularized tendon scaffolds (DTS) possess high clinical relevance, faithfully translate forces to the cellular scale, and have bulk material properties that match natural tissue. This review summarizes progress in tendon tissue engineering, with a focus on DTS and bioreactor systems.

Progress in cell-based therapies for tendon repair

The last decade has seen significant developments in cell therapies, based on permanently differentiated, reprogrammed or engineered stem cells, for tendon injuries and degenerative conditions. In vitro studies assess the influence of biophysical, biochemical and biological signals on tenogenic phenotype maintenance and/or differentiation towards tenogenic lineage. However, the ideal culture environment has yet to be identified due to the lack of standardised experimental setup and readout system. Bone marrow mesenchymal stem cells and tenocytes/dermal fibroblasts appear to be the cell populations of choice for clinical translation in equine and human patients respectively based on circumstantial, rather than on hard evidence. Collaborative, inter- and multi-disciplinary efforts are expected to provide clinically relevant and commercially viable cell-based therapies for tendon repair and regeneration in the years to come.

Effect of mechanical stimulation on bone marrow stromal cell-seeded tendon slice constructs: a potential engineered tendon patch for rotator cuff repair

Cell-based tissue engineered tendons have potential to improve clinical outcomes following rotator cuff repair, especially in large or massive rotator cuff tears, which pose a great clinical challenge. The aim of this study was to develop a method of constructing a functional engineered tendon patch for rotator cuff repair with cyclic mechanical stimulation. Decellularized tendon slices (DTSs) were seeded with BMSCs and subjected to cyclic stretching for 1, 3, or 7 days. The mechanical properties, morphologic characteristics and tendon-related gene expression of the constructs were investigated. Viable BMSCs were observed on the DTS after 7 days. BMSCs penetrated into the DTSs and formed dense cell sheets after 7 days of mechanical stretching. Gene expression of type I collagen, decorin, and tenomodulin significantly increased in cyclically stretched BMSC-DTS constructs compared with the unstrained control group (P < 0.05). The ultimate tensile strength and stiffness of the cyclically stretched tendon constructs were similar to the unstrained control group (P > 0.05). In conclusion, mechanical stimulation of BMSC-DTS constructs upregulated expression of tendon-related proteins, promoted cell tenogenic differentiation, facilitated cell infiltration and formation of cell sheets, and retained mechanical properties. The patch could be used as a graft to enhance the surgical repair of rotator cuff tears.

Grafting of a model protein on lactide and caprolactone based biodegradable films for biomedical applications

Thermoplastic biodegradable polymers displaying elastomeric behavior and mechanical consistency are greatly appreciated for the regeneration of soft tissues and for various medical devices. However, while the selection of a suitable base material is determined by mechanical and biodegradation considerations, it is the surface properties of the biomaterial that are responsible for the biological response. In order to improve the interaction with cells and modulate their behavior, biologically active molecules can be incorporated onto the surface of the material. With this aim, the surface of a lactide and caprolactone based biodegradable elastomeric terpolymer was modified in two stages. First, the biodegradable polymer surface was aminated by atmospheric pressure plasma treatment and second a crosslinker was grafted in order to covalently bind the biomolecule. In this study, albumin was used as a model protein. According to X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), albumin was efficiently immobilized on the surface of the terpolymer, the degree of albumin surface coverage (Γ_BSA) reached ~35%. Moreover, gel permeation chromatography (GPC) studies showed that the hydrolytic degradation kinetic of the synthesized polymer was slightly delayed when albumin was grafted. However, the degradation process in the bulk of the material was unaffected, as demonstrated by Fourier transform infrared (FTIR) analyses. Furthermore, XPS analyses showed that the protein was still present on the surface after 28 days of degradation, meaning that the surface modification was stable, and that there had been enough time for the biological environment to interact with the modified material.

Relationship between muscle fatty infiltration and the biological characteristics and stimulation potential of tenocytes from rotator cuff tears

The healing after rotator cuff surgery is still dissatisfying, and increased muscle fatty infiltration even more impairs the healing success. To achieve sufficient healing after rotator cuff reconstructions, the use of growth factors may be one possibility. The aim of the study was to identify a possible relationship between fatty infiltration of the supraspinatus muscle and cellular biological characteristics and stimulation potential of tenocyte-like cells (TLCs). TLCs of 3 donor groups differing in grade of muscle fatty infiltration were analyzed for their cellular characteristics and were stimulated with BMP-2 or BMP-7 in a 3D scaffold culture. The cell count and potency for self-renewal were significantly decreased in TLCs from donors with high muscle fatty infiltration compared to the lower fatty infiltration groups. Cell count and collagen-I expression as well as protein synthesis were stimulated by growth factors. Interestingly, TLCs of the high fatty infiltration group exhibited a weaker stimulation potential compared to the other groups. TLCs from donors with high muscle fatty infiltration generally revealed inferior characteristics compared to cells of lower fatty infiltration groups, which may be one reason for a weaker healing potential and may represent a possible starting point for the development of future treatment options.

A new generation of poly(lactide/ε-caprolactone) polymeric biomaterials for application in the medical field

Thermoplastic biodegradable polymers displaying an elastomeric behavior are greatly valued for the regeneration of soft tissues and for various medical devices. In this work, terpolymers composed of ε-caprolactone (CL), D-lactide (D-LA), and L-lactide (L-LA) were synthesized. These poly(lactide-ε-caprolactone) (PLCLs) presented an elevated randomness character (R∼1), glass transition temperatures (Tg ) higher than 20°C and adjusted L-LA content. In this way, the L-LA average sequence length (/L-LA ) was reduced to below 3.62 and showed little or no crystallization capability during in vitro degradation. As a result, the obtained materials underwent homogenous degradation exhibiting KMw ranging from 0.030 to 0.066 d(-1) and without generation of crystalline remnants in advanced stages of degradation. Mechanical performance was maintained over a period of 21 days for a rac-lactide-ε-caprolactone copolymer composed of ∼85% D,L-LA and ∼15% CL and also for a terpolymer composed of ∼72% L-LA, ∼12% D-LA and ∼16% CL. Terpolymers having L-LA content from ∼60 to 70% and CL content from ∼10 to 27% were also studied. In view of the results, those materials having CL and D-LA units disrupting the microstructural arrangement of the L-LA crystallizable chains, an L-LA content <72% and a random distribution of sequences, may display proper and tunable mechanical behavior and degradation performance for a large number of medical applications. Those with a CL content from 15 to 30% will fulfill the demand of elastomeric materials of Tg higher than 20°C whereas those with a CL content from 5 to 15% might be applied as ductile stiff materials.

The first systematic analysis of 3D rapid prototyped poly(ε-caprolactone) scaffolds manufactured through BioCell printing: the effect of pore size and geometry on compressive mechanical behaviour and in vitro hMSC viability

Novel additive manufacturing processes are increasingly recognized as ideal techniques to produce 3D biodegradable structures with optimal pore size and spatial distribution, providing an adequate mechanical support for tissue regeneration while shaping in-growing tissues. With regard to the mechanical and biological performances of 3D scaffolds, pore size and geometry play a crucial role. In this study, a novel integrated automated system for the production and in vitro culture of 3D constructs, known as BioCell Printing, was used only to manufacture poly(ε-caprolactone) scaffolds for tissue engineering; the influence of pore size and shape on their mechanical and biological performances was investigated. Imposing a single lay-down pattern of 0°/90° and varying the filament distance, it was possible to produce scaffolds with square interconnected pores with channel sizes falling in the range of 245-433 µm, porosity 49-57% and a constant road width. Three different lay-down patterns were also adopted (0°/90°, 0°/60/120° and 0°/45°/90°/135°), thus resulting in scaffolds with quadrangular, triangular and complex internal geometries, respectively. Mechanical compression tests revealed a decrease of scaffold stiffness with the increasing porosity and number of deposition angles (from 0°/90° to 0°/45°/90°/135°). Results from biological analysis, carried out using human mesenchymal stem cells, suggest a strong influence of pore size and geometry on cell viability. On the other hand, after 21 days of in vitro static culture, it was not possible to detect any significant variation in terms of cell morphology promoted by scaffold topology. As a first systematic analysis, the obtained results clearly demonstrate the potential of the BioCell Printing process to produce 3D scaffolds with reproducible well organized architectures and tailored mechanical properties.

Fabrication of electrospun poly(L-lactide-co-ε-caprolactone)/collagen nanoyarn network as a novel, three-dimensional, macroporous, aligned scaffold for tendon tissue engineering

Tissue engineering techniques using novel scaffolding materials offer potential alternatives for managing tendon disorders. An ideal tendon tissue engineered scaffold should mimic the three-dimensional (3D) structure of the natural extracellular matrix (ECM) of the native tendon. Here, we propose a novel electrospun nanoyarn network that is morphologically and structurally similar to the ECM of native tendon tissues. The nanoyarn, random nanofiber, and aligned nanofiber scaffolds of a synthetic biodegradable polymer, poly(L-lactide-co-ε-caprolactone) [P(LLA-CL)], and natural collagen I complex were fabricated using electrospinning. These scaffolds were characterized in terms of fiber morphology, pore size, porosity, and chemical and mechanical properties for the purpose of culturing tendon cells (TCs) for tendon tissue engineering. The results indicated a fiber diameter of 632 ± 81 nm for the random nanofiber scaffold, 643 ± 97 nm for the aligned nanofiber scaffold, and 641 ± 68 nm for the nanoyarn scaffold. The yarn in the nanoyarn scaffold was twisted by many nanofibers similar to the structure and inherent nanoscale organization of tendons, indicating an increase in the diameter of 9.51 ± 3.62 μm. The nanoyarn scaffold also contained 3D aligned microstructures with large interconnected pores and high porosity. Fourier transform infrared analyses revealed the presence of collagen in the three scaffolds. The mechanical properties of the sample scaffolds indicated that the scaffolds had desirable mechanical properties for tissue regeneration. Further, the results revealed that TC proliferation and infiltration, and the expression of tendon-related ECM genes, were significantly enhanced on the nanoyarn scaffold compared with that on the random nanofiber and aligned nanofiber scaffolds. This study demonstrates that electrospun P(LLA-CL)/collagen nanoyarn is a novel, 3D, macroporous, aligned scaffold that has potential application in tendon tissue engineering.

Effects of mechanical strain on human mesenchymal stem cells and ligament fibroblasts in a textured poly(L-lactide) scaffold for ligament tissue engineering

The purpose of this study was to prove the effect of cyclic uniaxial intermittent strain on the mRNA expression of ligament-specific marker genes in human mesenchymal stem cells (MSC) and anterior cruciate ligament-derived fibroblasts (ACL-fibroblasts) seeded onto a novel textured poly(L-lactide) scaffold (PLA scaffold). Cell-seeded scaffolds were mechanically stimulated by cyclic uniaxial stretching. The expression of ligament matrix gene markers: collagen types I and III, fibronectin, tenascin C and decorin, as well as the proteolytic enzymes matrix metalloproteinase MMP-1 and MMP-2 and their tissue specific inhibitors TIMP-1 and TIMP-2 was investigated by analysing the mRNA expression using reverse transcriptase polymerase chain reaction and related to the static control. In ACL-fibroblasts seeded on PLA, mechanical load induced up-regulation of collagen types I and III, fibronectin and tenascin C. No effect of mechanical stimulation on the expression of ligament marker genes was found in undifferentiated MSC seeded on PLA. The results indicated that the new textured PLA scaffold could transfer the mechanical load to the ACL-fibroblasts and improved their ligament phenotype. This scaffold might be suitable as a cell-carrying component of ACL prostheses.

In vivo lamellar bone formation in fibre coated MgCHA-PCL-composite scaffolds

Bio-inspired materials with controlled topography have gained increasing interest in regenerative medicine, because of their ability to reproduce the physical features of natural extracellular matrix, thus amplifying certain biological responses both in vitro and in vivo, such as contact guidance and differentiation. However, information on the ability to adapt this high cell potential to 3D scaffolds, effective to be implanted in clinical bone defect, is still missing. Here, we examine the pattern of bone tissue generated within the implant in an ectopic model, seeding bone marrow progenitor cells onto PCL-MgCHA scaffolds. This composite material presented a porous structure with micro/nanostructured surfaces obtained by combining phase inversion/salt leaching and electrospinning techniques. Histological analysis of grafts harvested after 1-2-6 months from implantation highlights an extent of lamellar bone tissue within interconnected pores of fibre coated PCL-MgCHA composites, whereas uncoated scaffolds displayed sparse deposition of bone. Pure PCL scaffolds did not reveal any trace of bone for the overall 6 months of observation. In conclusion, we show that a structural modification in scaffold design is able to enhance bone regeneration possibly mimicking some physiological cues of the natural tissue.

Enhanced regeneration of the ligament-bone interface using a poly(L-lactide-co-ε-caprolactone) scaffold with local delivery of cells/BMP-2 using a heparin-based hydrogel

Recently, the ligament-bone (LTB) junction has been emphasized for the effective transmission of mechanical force and the reduction in stress concentration between the soft ligament and hard bone tissue. The aim of this study was to regenerate an integrated LTB interface by inoculating LTB-relevant cells, isolated from fibrocartilage (FC) or ligament (LIG), separately into each designated region in a single porous cylindrical PLCL scaffold. An injectable, heparin-based hydrogel that has proved to be effective in the culture of chondrocytes as well as the sustained release of growth factor was employed to locally deliver fibrochondrocytes and osteoinductive bone morphogenetic protein-2 (BMP-2) into the FC region, to promote FC regeneration. In in vitro experiments the hydrogel-combined FC systems produced significantly larger amounts of calcium and glycosaminoglycans (GAGs), but less collagen and DNA than FC samples without the hydrogel and all LIG samples. After in vivo subcutaneous implantation in mice for 8 weeks the secreted calcium and GAG contents of the hydrogel-containing FC samples were superior or similar to those of the in vitro hydrogel-containing FC samples at 6 weeks. As a result of the enhanced production of calcium and GAG, the in vivo hydrogel-containing FC samples produced the highest compressive modulus among all samples. Histological and immunofluorescence analysis as well as elemental analysis also confirmed a denser and more homogeneous distribution of calcium, GAG, osteocalcin and neovascularization marker in the in vitro/in vivo hydrogel-containing FC systems than those without hydrogel. These results also show the beneficial effects of BMP-2 added using the hydrogel. In summary, the use of a heparin-based hydrogel for the local delivery of fibrochondrocytes and BMP-2 could accelerate the maturation and differentiation of LTB-specific FC tissues, and it was also possible to recreate the unique stratification of calcified FC and LIG tissues in a single porous PLCL scaffold in terms of both biochemical and biomechanical properties.