Bioactive, Elastic, and Biodegradable Emulsion Electrospun DegraPol Tube Delivering PDGF-BB for Tendon Rupture Repair

Hyaluronic acid/PEO electrospun tube reduces tendon adhesion to levels comparable to native tendons - An in vitro and in vivo study

A major problem after tendon injury is adhesion formation to the surrounding tissue leading to a limited range of motion. A viable strategy to reduce adhesion extent is the use of physical barriers that limit the contact between the tendon and the adjacent tissue. The purpose of this study was to fabricate an electrospun bilayered tube of hyaluronic acid/polyethylene oxide (HA/PEO) and biodegradable DegraPol® (DP) to improve the anti-adhesive effect of the implant in a rabbit Achilles tendon full laceration model compared to a pure DP tube. Additionally, the attachment of rabbit tenocytes on pure DP and HA/PEO containing scaffolds was tested and Scanning Electron Microscopy, Fourier-transform Infrared Spectroscopy, Differential Scanning Calorimetry, Water Contact Angle measurements, and testing of mechanical properties were used to characterize the scaffolds. In vivo assessment after three weeks showed that the implant containing a second HA/PEO layer significantly reduced adhesion extent reaching levels comparable to native tendons, compared with a pure DP implant that reduced adhesion formation only by 20 %. Tenocytes were able to attach to and migrate into every scaffold, but cell number was reduced over two weeks. Implants containing HA/PEO showed better mechanical properties than pure DP tubes and with the ability to entirely reduce adhesion extent makes this implant a promising candidate for clinical application in tendon repair.

Bletilla striata Polysaccharide-Containing Carboxymethyl Cellulose Bilayer Structure Membrane for Prevention of Postoperative Adhesion and Achilles Tendon Repair

Postoperative tissue adhesion and poor tendon healing are major clinical problems associated with tendon surgery. To avoid postoperative adhesion and promote tendon healing, we developed and synthesized a membrane to wrap the surgical site after tendon suturing. The bilayer-structured porous membrane comprised an outer layer [1,4-butanediol diglycidyl ether cross-linked with carboxymethyl cellulose (CX)] and an inner layer [1,4-butanediol diglycidyl ether cross-linked with Bletilla striata polysaccharides and carboxymethyl cellulose (CXB)]. The morphology, chemical functional groups, and membrane structure were determined. In vitro experiments revealed that the CX/CXB membrane demonstrated good biosafety and biodegradability, promoted tenocyte proliferation and migration, and exhibited low cell attachment and anti-inflammatory effects. Furthermore, in in vivo animal study, the CX/CXB membrane effectively reduced postoperative tendon-peripheral tissue adhesion and improved tendon repair, downregulating inflammatory cytokines in the tendon tissue at the surgical site, which ultimately increased tendon strength by 54% after 4 weeks.

In Vitro Assessment of Bacterial Adhesion and Biofilm Formation on Novel Bioactive, Biodegradable Electrospun Fiber Meshes Intended to Support Tendon Rupture Repair

The surgical repair of a ruptured tendon faces two major problems: specifically increased fibrous adhesion to the surrounding tissue and inferior mechanical properties of the scar tissue compared to the native tissue. Bacterial attachment to implant materials is an additional problem as it might lead to severe infections and impaired recovery. To counteract adhesion formation, two novel implant materials were fabricated by electrospinning, namely, a random fiber mesh containing hyaluronic acid (HA) and poly(ethylene oxide) (PEO) in a ratio of 1:1 (HA/PEO 1:1) and 1:4 (HA/PEO 1:4), respectively. Electrospun DegraPol (DP) treated with silver nanoparticles (DP-Ag) was developed to counteract the bacterial attachment. The three novel materials were compared to the previously described DP and DP with incorporated insulin-like growth factor-1 (DP-IGF-1), two implant materials that were also designed to improve tendon repair. To test whether the materials are prone to bacterial adhesion and biofilm formation, we assessed 10 strains of Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa, and Enterococcus faecalis, known for causing nosocomial infections. Fiber diameter, pore size, and water contact angle, reflecting different degrees of hydrophobicity, were used to characterize all materials. Generally, we observed higher biofilm formation on the more hydrophobic DP as compared to the more hydrophilic DP-IGF-1 and a trend toward reduced biofilm formation for DP treated with silver nanoparticles. For the two HA/PEO implants, a similar biofilm formation was observed. All tested materials were highly prone to bacterial adherence and biofilm formation, pointing toward the need of further material development, including the optimized incorporation of antibacterial agents such as silver nanoparticles or antibiotics.

Drug Delivery Approaches to Improve Tendon Healing

Tendon injuries disrupt the transmission of forces from muscle to bone, leading to chronic pain, disability, and a large socioeconomic burden. Tendon injuries are prevalent; there are over 300,000 tendon repair procedures a year in the United States to address acute trauma or chronic tendinopathy. Successful restoration of function after tendon injury remains challenging clinically. Despite improvements in surgical and physical therapy techniques, the high complication rate of tendon repair procedures motivates the use of therapeutic interventions to augment healing. While many biological and tissue engineering approaches have attempted to promote scarless tendon healing, there is currently no standard clinical treatment to improve tendon healing. Moreover, the limited efficacy of systemic delivery of several promising therapeutic candidates highlights the need for tendon-specific drug delivery approaches to facilitate translation. This review article will synthesize the current state-of-the-art methods that have been used for tendon-targeted delivery through both systemic and local treatments, highlight emerging technologies used for tissue-specific drug delivery in other tissue systems, and outline future challenges and opportunities to enhance tendon healing through targeted drug delivery.

Smart Strategies to Overcome Drug Delivery Challenges in the Musculoskeletal System

The musculoskeletal system (MSKS) is composed of specialized connective tissues including bone, muscle, cartilage, tendon, ligament, and their subtypes. The primary function of the MSKS is to provide protection, structure, mobility, and mechanical properties to the body. In the process of fulfilling these functions, the MSKS is subject to wear and tear during aging and after injury and requires subsequent repair. MSKS diseases are a growing burden due to the increasing population age. The World Health Organization estimates that 1.71 billon people suffer from MSKS diseases worldwide. MSKS diseases usually involve various dysfunctions in bones, muscles, and joints, which often result in pain, disability, and a decrease in quality of life. The most common MSKS diseases are osteoporosis (loss of bone), osteoarthritis (loss of cartilage), and sarcopenia (loss of skeletal muscle). Because of the disease burden and the need for treatment, regenerative drug therapies for MSKS disorders are increasingly in demand. However, the difficulty of effective drug delivery in the MSKS has become a bottleneck for developing MSKS therapeutics. The abundance of extracellular matrix and its small pore size in the MSKS present a formidable barrier to drug delivery. Differences of vascularity among various MSKS tissues pose complications for drug delivery. Novel strategies are necessary to achieve successful drug delivery in different tissues composing the MSKS. Those considerations include the route of administration, mechanics of surrounding fluids, and biomolecular interactions, such as the size and charge of the particles and targeting motifs. This review focuses on recent advances in challenges to deliver drugs to each tissue of the MSKS, current strategies of drug delivery, and future ideas of how to overcome drug delivery challenges in the MSKS.

Bioactive and Elastic Emulsion Electrospun DegraPol Tubes Delivering IGF-1 for Tendon Rupture Repair

Tendon injuries can result in two major drawbacks. Adhesions to the surrounding tissue may limit the range of motion, while fibrovascular scar formation can lead to poor biomechanical outcomes. Prosthetic devices may help to mitigate those problems. Emulsion electrospinning was used to develop a novel three-layer tube based on the polymer DegraPol (DP), with incorporated insulin-like growth factor-1 (IGF-1) in the middle layer. Scanning electron microscopy was utilized to assess the fiber diameter in IGF-1 containing pure DP meshes. Further characterization was performed with Fourier Transformed Infrared Spectroscopy, Differential Scanning Calorimetry, and water contact angle, as well as through the assessment of mechanical properties and release kinetics from ELISA, and the bioactivity of IGF-1 by qPCR of collagen I, ki67, and tenomodulin in rabbit Achilles tenocytes. The IGF-1-containing tubes exhibited a sustained release of the growth factor up to 4 days and showed bioactivity by significantly upregulated ki67 and tenomodulin gene expression. Moreover, they proved to be mechanically superior to pure DP tubes (significantly higher fracture strain, failure stress, and elastic modulus). The novel three-layer tubes intended to be applied over conventionally sutured tendons after a rupture may help accelerate the healing process. The release of IGF-1 stimulates proliferation and matrix synthesis of cells at the repair site. In addition, adhesion formation to surrounding tissue can be reduced due to the physical barrier.

Platelet-derived growth factor AA-modified electrospun fibers promote tendon healing

Platelet-derived growth factor AA (PDGF-AA) is an important promoter of tissue injury repair and might be a candidate for improving the mechanical properties of repaired tendons. Here, we designed a PDGF-AA-modified poly(lactide-co-glycolide) acid (PLGA) electrospun fibers to promote tendon rehabilitation after injury. In the present study, we grafted PDGF-AA on the surface of PLGA. In structural experiments, we found that the hydrophilicity of PLGA containing PDGF-AA (PLGA-PDGF-AA) increased, but the strength of the material did not change significantly. Moreover, no significant changes in tendon cell proliferation and viability were observed in the PLGA-PDGF-AA treatment compared with the control group. The mouse tendon injury model (n = 9) experiment illustrated that PLGA-PDGF-AA effectively promoted tendon healing, and we confirmed that PLGA-PDGF-AA promoted collagen synthesis and deposition by immunohistochemistry and RT-PCR. Moreover, the mechanical strength of PLGA-PDGF-AA-treated mouse (n = 9) tendon tissue was also higher than that of the PLGA-treated group alone. In conclusion, PLGA-PDGF-AA promoted regeneration after tendon injury and serves as a potential adjuvant material for surgical tendon injury repair.

A Promising Candidate in Tendon Healing Events—PDGF-BB

Tendon injuries are one of the most common musculoskeletal disorders for which patients seek medical aid, reducing not only the quality of life of the patient but also imposing a significant economic burden on society. The administration of growth factors at the wound site is a feasible solution for enhancing tendon healing. Platelet-derived growth factor-BB (PDGF-BB) has a well-defined safety profile compared to other growth factors and has been approved by the Food and Drug Administration (FDA). The purpose of this review is to summarize the role of PDGF-BB in tendon healing through a comprehensive review of the published literature. Experimental studies suggest that PDGF-BB has a positive effect on tendon healing by enhancing inflammatory responses, speeding up angiogenesis, stimulating tendon cell proliferation, increasing collagen synthesis and increasing the biomechanics of the repaired tendon. PDGF-BB is regarded as a promising candidate in tendon healing. However, in order to realize its full potential, we still need to carefully consider and study key issues such as dose and application time in the future, so as to explore further applications of PDGF-BB in the tendon healing process.

PDGF-loaded microneedles promote tendon healing through p38/cyclin D1 pathway mediated angiogenesis

Tendon injury is one of the most serious orthopedic diseases often leading to disability of patients. Major shortages of tendon healing are due to its multiple comorbidities, uncertainty of therapeutic efficacy and insufficient of angiogenesis. With a deeper understanding of angiogenic mechanism of tendon healing, we investigated an innovative microneedle patch loaded with platelet derived growth factor (PDGF) to achieve a constant systemic administration of PDGF to enhance topical tendon healing. Rat achilles tendon injury model was performed as in vivo animal models. Histological staining showed an enhancement of tendon healing quality, especially angiogenesis. Biomechanical studies demonstrated an increase of tendon stiffness, maximum load and maximum stress with treatment of PDGF-loaded microneedles. Furthermore, MAPK/p38/Cyclin D1 pathway and angiogenesis were found to play an important role in tendon healing process by using a biological high throughput RNA-sequence method and bioinformatic analysis. The high throughput RNA-seq tendon healing results were confirmed by histochemical staining and western blot. These results suggest the novel therapeutic potential of PDGF-loaded microneedle patch in tendon surgery.

Electrospun PEO/rGO Scaffolds: The Influence of the Concentration of rGO on Overall Properties and Cytotoxicity

Reduced graphene oxide (rGO) is one of the graphene derivatives that can be employed to engineer bioactive and/or electroactive scaffolds. However, the influence of its low and especially high concentrations on scaffolds’ overall properties and cytotoxicity has yet to be explored. In this study, polyethylene oxide (PEO)-based scaffolds containing from 0.1 to 20 wt% rGO were obtained by electrospinning. Morphological, thermal and electrical properties of the scaffolds were characterized by SEM, Raman spectroscopy, XRD, DSC and electrical measurements. The diameter of the fibers decreased from 0.52 to 0.19 µm as the concentration of rGO increased from 0.1 wt% to 20 wt%. The presence of rGO above the percolation threshold (5.7 wt%) resulted in a significantly reduced electrical resistivity of the scaffolds. XRD and Raman analysis revealed delamination of the graphene layers (interlayer spacing increased from 0.36 nm to 0.40–0.41 nm), and exfoliation of rGO was detected for the samples with an rGO concentration lower than 1 wt%. In addition, an evident trend of increasing cell viability as a function of the rGO concentration was evidenced. The obtained results can serve as further guidance for the judicious selection of the rGO content incorporated into the PEO matrix for constructing electroactive scaffolds.

Electrospun tube reduces adhesion in rabbit Achilles tendon 12 weeks post-surgery without PAR-2 overexpression

One great challenge in surgical tendon repair is the minimization of peritendinous adhesions. An electrospun tube can serve as a physical barrier around a conventionally sutured tendon. Six New Zealand White rabbits had one Achilles tendon fully transsected and sutured by a 4-strand suture. Another six rabbits had the same treatment, but with the additional electrospun DegraPol tube set around the sutured tendon. The adhesion formation to the surrounding tissue was investigated 12 weeks post-operation. Moreover, inflammation-related protease-activated receptor-2 (PAR-2) protein expression was assessed. Finally, rabbit Achilles tenocyte cultures were exposed to platelet-derived growth factor-BB (PDGF-BB), which mimicks the tendon healing environment, where PAR-2 gene expression was assessed as well as immunofluorescent staining intensity for F-actin and α-tubulin, respectively. At 12 weeks post-operation, the partially degraded DegraPol tube exhibited significantly lower adhesion formation (- 20%). PAR-2 protein expression was similar for time points 3 and 6 weeks, but increased at 12 weeks post-operation. In vitro cell culture experiments showed a significantly higher PAR-2 gene expression on day 3 after exposure to PDGF-BB, but not on day 7. The cytoskeleton of the tenocytes changed upon PDGF-BB stimulation, with signs of reorganization, and significantly decreased F-actin intensity. An electrospun DegraPol tube significantly reduces adhesion up to twelve weeks post-operation. At this time point, the tube is partially degraded, and a slight PAR-2 increase was detected in the DP treated tendons, which might however arise from particles of degrading DegraPol that were stained dark brown. PAR-2 gene expression in rabbit tenocytes reveals sensitivity at around day 10 after injury.

Biomaterials strategies to balance inflammation and tenogenesis for tendon repair

Adult tendon tissue demonstrates a limited regenerative capacity, and the natural repair process leaves fibrotic scar tissue with inferior mechanical properties. Surgical treatment is insufficient to provide the mechanical, structural, and biochemical environment necessary to restore functional tissue. While numerous strategies including biodegradable scaffolds, bioactive factor delivery, and cell-based therapies have been investigated, most studies have focused exclusively on either suppressing inflammation or promoting tenogenesis, which includes tenocyte proliferation, ECM production, and tissue formation. New biomaterials-based approaches represent an opportunity to more effectively balance the two processes and improve regenerative outcomes from tendon injuries. Biomaterials applications that have been explored for tendon regeneration include formation of biodegradable scaffolds presenting topographical, mechanical, and/or immunomodulatory cues conducive to tendon repair; delivery of immunomodulatory or tenogenic biomolecules; and delivery of therapeutic cells such as tenocytes and stem cells. In this review, we provide the biological context for the challenges in tendon repair, discuss biomaterials approaches to modulate the immune and regenerative environment during the healing process, and consider the future development of comprehensive biomaterials-based strategies that can better restore the function of injured tendon. STATEMENT OF SIGNIFICANCE: Current strategies for tendon repair focus on suppressing inflammation or enhancing tenogenesis. Evidence indicates that regulated inflammation is beneficial to tendon healing and that excessive tissue remodeling can cause fibrosis. Thus, it is necessary to adopt an approach that balances the benefits of regulated inflammation and tenogenesis. By reviewing potential treatments involving biodegradable scaffolds, biological cues, and therapeutic cells, we contrast how each strategy promotes or suppresses specific repair steps to improve the healing outcome, and highlight the advantages of a comprehensive approach that facilitates the clearance of necrotic tissue and recruitment of cells during the inflammatory stage, followed by ECM synthesis and organization in the proliferative and remodeling stages with the goal of restoring function to the tendon.

Differentiation of human adipose-derived mesenchymal stem cells toward tenocyte by platelet-derived growth factor-BB and growth differentiation factor-6

Mesenchymal Stem Cells (MSCs) are important in regenerative medicine and tissue engineering and will be a very sensible choice for repair and regeneration of tendon. New biological practices, such as cellular therapy using stem cells, are promising for facilitating or expediting tendon therapy. Before using these cells clinically, it is best to check and confirm the optimal conditions for differentiation of these cells in the laboratory. Hence, in the present study, the impacts of PDGF-BB and GDF-6 supplementation on adipose-derived MSCs (ASCs) culture were studied. The frozen ASC were recovered and expanded in basic culture medium (DMEM with 10%FBS). The cells after passage five (P5) were treated with basic medium containing L-Prolin, Ascorbic Acid and only PDGF-BB or GDF-6 (20 ng/ml) or both of them (mix) as 3 groups for 14 days to investigate efficiency of ASCs differentiation towards tenocytes. The cells culturing in basic medium were used as control group. To validate tenogenic differentiation, H&E and Sirius Red staining were used to assess cell morphology and collagen production, respectively. In addition, mRNA levels of collagen I and III, Scleraxis and Tenomodulin as tenogenic markers were analyzed using qPCR. In all test groups, cells appeared slenderer, elongated cytoplasmic attributes compared to the control cells. The intensity of Sirius Red staining was significantly higher in GDF-6, PDGF-BB alone, than in group without supplements. The optical density was higher in the GDF-6 than PDGF-BB and mix-group. QPCR results showed that Col I and III gene expression was increased in all groups compared to the control. SCX expression was significantly increased only in the PDGF-BB group. TNMD mRNA expression was not significant among groups. In this study, we have corroborated that human ASCs are reactionary to tenogenic induction by GDF-6 and PDGF-BB alone or in combination. These outcomes will help greater insight into GDF-6 and PDGF-BB driven tenogenesis of ASCs and new directions of discovery in the design of ASC-based treatments for tendon healing.

Advanced technology-driven therapeutic interventions for prevention of tendon adhesion: Design, intrinsic and extrinsic factor considerations

Tendon adhesion formation describes the development of fibrotic tissue between the tendon and its surrounding tissues, which commonly occurs as a reaction to injury or surgery. Its impact on function and quality of life varies from negligible to severely disabling, depending on the affected area and extent of adhesion formed. Thus far, treatment options remain limited with prophylactic anti-inflammatory medications and revision surgeries constituting the only tools within the doctors' armamentarium - neither of which provides reliable outcomes. In this review, the authors aim to collate the current understanding of the pathophysiological mechanisms underlying tendon adhesion formation, highlighting the significant role ascribed to the inflammatory cascade in accelerating adhesion formation. The bulk of this article will then be dedicated to critically appraising different therapeutic structures like nanoparticles, hydrogels and fibrous membranes fabricated by various cutting-edge technologies for adhesion formation prophylaxis. Emphasis will be placed on the role of the fibrous membranes, their ability to act as drug delivery vehicles as well as the combination with other therapeutic structures (e.g., hydrogel or nanoparticles) or fabrication technologies (e.g., weaving or braiding). Finally, the authors will provide an opinion as to the future direction of the prevention of tendon adhesion formation in view of scaffold structure and function designs.

Polysaccharide Extracted from Bletilla striata Promotes Proliferation and Migration of Human Tenocytes

Tendon healing after injury is relatively slow, mainly because of the weak activity and metabolic properties of tendon cells (tenocytes). Bletilla striata polysaccharide (BSP) has been reported to enhance cell proliferation. Here, we aimed to increase tendon cell proliferation by BSP treatment. We isolated tenocytes from the flexor tendon of human origin. Moreover, we improved the process of extracting BSP. When human tenocytes (HTs) were treated with 100 μg/mL BSP, the MEK/ERK1/2 and PI3K/Akt signaling pathways were activated, thereby enhancing the proliferation ability of tenocytes. BSP treatment also increased the migration of HTs and their ability to secrete the extracellular matrix (Col-I and Col-III). In conclusion, BSP was successfully extracted from a natural Chinese herbal extract and was shown to enhance tenocytes proliferation, migration and collagen release ability. This study is the first to demonstrate improved healing of tendons using BSP.

A multilayered scaffold for regeneration of smooth muscle and connective tissue layers

Tissue regeneration often requires recruitment of different cell types and rebuilding of two or more tissue layers to restore function. Here, we describe the creation of a novel multilayered scaffold with distinct fiber organizations-aligned to unaligned and dense to porous-to template common architectures found in adjacent tissue layers. Electrospun scaffolds were fabricated using a biodegradable, tyrosine-derived terpolymer, yielding densely-packed, aligned fibers that transition into randomly-oriented fibers of increasing diameter and porosity. We demonstrate that differently-oriented scaffold fibers direct cell and extracellular matrix (ECM) organization, and that scaffold fibers and ECM protein networks are maintained after decellularization. Smooth muscle and connective tissue layers are frequently adjacent in vivo; we show that within a single scaffold, the architecture supports alignment of contractile smooth muscle cells and deposition by fibroblasts of a meshwork of ECM fibrils. We rolled a flat scaffold into a tubular construct and, after culture, showed cell viability, orientation, and tissue-specific protein expression in the tube were similar to the flat-sheet scaffold. This scaffold design not only has translational potential for reparation of flat and tubular tissue layers but can also be customized for alternative applications by introducing two or more cell types in different combinations.

Histological evaluation of cellular response to a multifilament electrospun suture for tendon repair

BACKGROUND

Rotator cuff tendon repair in humans is a commonly performed procedure aimed at restoring the tendon-bone interface. Despite significant innovation of surgical techniques and suture anchor implants, only 60% of repairs heal successfully. One strategy to enhance repair is the use of bioactive sutures that provide the native tendon with biophysical cues for healing. We investigated the tissue response to a multifilament electrospun polydioxanone (PDO) suture in a sheep tendon injury model characterised by a natural history of failure of healing.

METHODOLOGY AND RESULTS

Eight skeletally mature English Mule sheep underwent repair with electrospun sutures. Monofilament sutures were used as a control. Three months after surgery, all tendon repairs healed, without systemic features of inflammation, signs of tumour or infection at necropsy. A mild local inflammatory reaction was seen. On histology the electrospun sutures were densely infiltrated with predominantly tendon fibroblast-like cells. In comparison, no cellular infiltration was observed in the control suture. Neovascularisation was observed within the electrospun suture, whilst none was seen in the control. Foreign body giant cells were rarely seen with either sutures.

CONCLUSION

This study demonstrates that a tissue response can be induced in tendon with a multifilament electrospun suture with no safety concerns.

Impact of PDGF-BB on cellular distribution and extracellular matrix in the healing rabbit Achilles tendon three weeks post-operation

Current methods for tendon rupture repair suffer from two main drawbacks: insufficient strength and adhesion formation, which lead to rerupture and impaired gliding. A novel polymer tube may help to overcome these problems by allowing growth factor delivery to the wound site and adhesion reduction, and by acting as a physical barrier to the surrounding tissue. In this study, we used a bilayered DegraPol® tube to deliver PDGF-BB to the wound site in a full-transection rabbit Achilles tendon model. We then performed histological and immunohistochemical analysis at 3 weeks postoperation. Sustained delivery of PDGF-BB to the healing Achilles tendon led to a significantly more homogenous cell distribution within the healing tissue. Lower cell densities next to the implant material were determined for +PDGF-BB samples compared to -PDGF-BB. PDGF-BB application increased proteoglycan content and reduced alpha-SMA+ areas, clusters of different sizes, mainly vessels. Finally, PDGF-BB reduced collagens I and III in the extracellular matrix. The sustained delivery of PDGF-BB via an electrospun DegraPol® tube accelerated tendon wound healing by causing a more uniform cell distribution with higher proteoglycan content and less fibrotic tissue. Moreover, the application of this growth factor reduced collagen III and alpha-SMA, indicating a faster and less fibrotic tendon healing.

Supporting Cell-Based Tendon Therapy: Effect of PDGF-BB and Ascorbic Acid on Rabbit Achilles Tenocytes in Vitro

Cell-based tendon therapies with tenocytes as a cell source need effective tenocyte in vitro expansion before application for tendinopathies and tendon injuries. Supplementation of tenocyte culture with biomolecules that can boost proliferation and matrix synthesis is one viable option for supporting cell expansion. In this in vitro study, the impacts of ascorbic acid or PDGF-BB supplementation on rabbit Achilles tenocyte culture were studied. Namely, cell proliferation, changes in gene expression of several ECM and tendon markers (collagen I, collagen III, fibronectin, aggrecan, biglycan, decorin, ki67, tenascin-C, tenomodulin, Mohawk, α-SMA, MMP-2, MMP-9, TIMP1, and TIMP2) and ECM deposition (collagen I and fibronectin) were assessed. Ascorbic acid and PDGF-BB enhanced tenocyte proliferation, while ascorbic acid significantly accelerated the deposition of collagen I. Both biomolecules led to different changes in the gene expression profile of the cultured tenocytes, where upregulation of collagen I, Mohawk, decorin, MMP-2, and TIMP-2 was observed with ascorbic acid, while these markers were downregulated by PDGF-BB supplementation. Vice versa, there was an upregulation of fibronectin, biglycan and tenascin-C by PDGF-BB supplementation, while ascorbic acid led to a downregulation of these markers. However, both biomolecules are promising candidates for improving and accelerating the in vitro expansion of tenocytes, which is vital for various tendon tissue engineering approaches or cell-based tendon therapy.

Impact of UV sterilization and short term storage on the in vitro release kinetics and bioactivity of biomolecules from electrospun scaffolds

To effectively translate bioactive scaffolds into a preclinical setting, proper sterilization techniques and storage conditions need to be carefully considered, as the chosen sterilization technique and storage condition might affect the structural and mechanical properties of the scaffolds, as well as the bioactivity and release kinetics of the incorporated biomolecules. Since rarely tested or quantified, we show here in a proof-of-concept study how these parameters are affected by UV sterilization and one week storage at different temperatures using bioactive electrospun DegraPol scaffolds that were specifically designed for application in the field of tendon rupture repair. Even though UV sterilization and the different storage conditions did not impact the morphology or the physicochemical properties of the bioactive scaffolds, UV sterilization caused significant attenuation of the growth factor release kinetics, here platelet derived growth factor (PDGF-BB) release (by approx. 85%) and slight decrease in ascorbic acid release (by approx. 20%). In contrast, 4 °C and −20 °C storage did not have a major effect on the release kinetics of PDGF-BB, while storage at room temperature caused increase in PDGF-BB released. All storage conditions had little effect on ascorbic acid release. Equally important, neither UV sterilization nor storage affected the bioactivity of the released PDGF-BB, suggesting stability of the bioactive scaffolds for at least one week and showing potential for bioactive DegraPol scaffolds to be translated into an off-the-shelf available product. These parameters are expected to be scaffold and protein-dependent.

Electrospinning and Electrospun Nanofibers: Methods, Materials, and Applications

Electrospinning is a versatile and viable technique for generating ultrathin fibers. Remarkable progress has been made with regard to the development of electrospinning methods and engineering of electrospun nanofibers to suit or enable various applications. We aim to provide a comprehensive overview of electrospinning, including the principle, methods, materials, and applications. We begin with a brief introduction to the early history of electrospinning, followed by discussion of its principle and typical apparatus. We then discuss its renaissance over the past two decades as a powerful technology for the production of nanofibers with diversified compositions, structures, and properties. Afterward, we discuss the applications of electrospun nanofibers, including their use as "smart" mats, filtration membranes, catalytic supports, energy harvesting/conversion/storage components, and photonic and electronic devices, as well as biomedical scaffolds. We highlight the most relevant and recent advances related to the applications of electrospun nanofibers by focusing on the most representative examples. We also offer perspectives on the challenges, opportunities, and new directions for future development. At the end, we discuss approaches to the scale-up production of electrospun nanofibers and briefly discuss various types of commercial products based on electrospun nanofibers that have found widespread use in our everyday life.

Vascular Endothelial Growth Factor Enhances Proliferation of Human Tenocytes and Promotes Tenogenic Gene Expression

BACKGROUND

In obtaining human tenocytes for tendon tissue engineering, a low proliferation rate and phenotype loss during passaging is a problem. It was the authors' aim to evaluate the influence of vascular endothelial growth factor (VEGF) on human tenocyte growth and gene expression.

METHODS

Human tenocytes were exposed to human VEGF in various concentrations (5, 10, and 20 ng/ml) for 5 days. Cell proliferation was counted and expression of tendon-related genes was analyzed.

RESULTS

Tenocyte count was 1.4 × 10(5)/ml, 2.7 × 10(5)/ml, 2.3 × 10(5)/ml, and 3.7 × 10(5)/ml for 0, 5, 10, and 20 ng/ml VEGF, respectively. Expression of Col1 was up-regulated 6.4 ± 4.2-fold, 60.1 ± 21.6-fold, and 15.8 ± 10.2-fold for 5, 10, and 20 ng/ml VEGF; all differences were significant with p < 0.05. Col3 was down-regulated to 0.2 ± 0.1-fold, 0.3 ± 0.1-fold, and 0.1 ± 0.03-fold for 5, 10, and 20 ng/ml VEGF; all differences were significant. Eln was up-regulated 2.3 ± 1.7-fold, 25.5 ± 10.9-fold, and 16.6 ± 9.0-fold for 5, 10, and 20 ng/ml VEGF; differences were significant for 10 and 20 ng/ml VEGF. TSC was down-regulated to 0.3 ± 0.1-fold and 0.3 ± 0.1-fold for 5 and 20 ng/ml VEGF; differences were significant for 5 and 20 ng/ml. SCX was up-regulated to 31.3 ± 8.5-fold, 49.1 ± 23.4-fold, and 20.9 ± 9.5-fold for 5, 10, and 20 ng/ml VEGF; all changes were significant.

CONCLUSIONS

VEGF enhances proliferation and expression of tendon-related genes in human tenocytes. It could therefore be a useful addition for tenocyte cultivation.

Biofabrication of Electrospun Scaffolds for the Regeneration of Tendons and Ligaments

Tendon and ligament tissue regeneration and replacement are complex since scaffolds need to guarantee an adequate hierarchical structured morphology, and non-linear mechanical properties. Moreover, to guide the cells' proliferation and tissue re-growth, scaffolds must provide a fibrous texture mimicking the typical of the arrangement of the collagen in the extracellular matrix of these tissues. Among the different techniques to produce scaffolds, electrospinning is one of the most promising, thanks to its ability to produce fibers of nanometric size. This manuscript aims to provide an overview to researchers approaching the field of repair and regeneration of tendons and ligaments. To clarify the general requirements of electrospun scaffolds, the first part of this manuscript presents a general overview concerning tendons' and ligaments' structure and mechanical properties. The different types of polymers, blends and particles most frequently used for tendon and ligament tissue engineering are summarized. Furthermore, the focus of the review is on describing the different possible electrospinning setups and processes to obtain different nanofibrous structures, such as mats, bundles, yarns and more complex hierarchical assemblies. Finally, an overview concerning how these technologies are exploited to produce electrospun scaffolds for tendon and ligament tissue applications is reported together with the main findings and outcomes.

Current trends in tendinopathy: consensus of the ESSKA basic science committee. Part II: treatment options

The treatment of painful chronic tendinopathy is challenging. Multiple non-invasive and tendon-invasive methods are used. When traditional non-invasive treatments fail, the injections of platelet-rich plasma autologous blood or cortisone have become increasingly favored. However, there is little scientific evidence from human studies supporting injection treatment. As the last resort, intra- or peritendinous open or endoscopic surgery are employed even though these also show varying results. This ESSKA basic science committee current concepts review follows the first part on the biology, biomechanics and anatomy of tendinopathies, to provide a comprehensive overview of the latest treatment options for tendinopathy as reported in the literature.

Harnessing biochemical and structural cues for tenogenic differentiation of adipose derived stem cells (ADSCs) and development of an in vitro tissue interface mimicking tendon-bone insertion graft

Tendon-bone interface tissue is extremely challenging to engineer because it exhibits complex gradients of structure, composition, biologics, and cellular phenotypes. As a step toward engineering these transitional zones, we initially analyzed how different (topographical or biological) cues affect tenogenic differentiation of adipose-derived stem cells (ADSCs). We immobilized platelet-derived growth factor - BB (PDGF-BB) using polydopamine (PD) chemistry on random and aligned nanofibers and investigated ADSC proliferation and tenogenic differentiation. Immobilized PDGF greatly enhanced the proliferation and tenogenic differentiation of ADSCs; however, nanofiber alignment had no effect. Interestingly, the PDGF immobilized aligned nanofiber group showed a synergistic effect with maximum expression of tenogenic markers for 14 days. We also generated a nanofiber surface with spatially controlled presentation of immobilized PDGF on an aligned architecture, mimicking native tendon tissue. A gradient of immobilized PDGF was able to control the phenotypic differentiation of ADSCs into tenocytes in a spatially controlled manner, as confirmed by analysis of the expression of tenogenic markers and immunofluorescence staining. We further explored the gradient formation strategy by generation of a symmetrical gradient on the nanofiber surface for the generation of a structure mimicking bone-patellar-tendon-bone with provision for gradient immobilization of PDGF and controlled mineralization. Our study reveals that, together with biochemical cues, favorable topographical cues are important for tenogenic differentiation of ADSCs, and gradient presentation of PDGF can be used as a tool for engineering stem cell-based bone-tendon interface tissues.

A Simple Modification Method to Obtain Anisotropic and Porous 3D Microfibrillar Scaffolds for Surgical and Biomedical Applications

In native tissues, cellular organization is predominantly anisotropic. Yet, it remains a challenge to engineer anisotropic scaffolds that promote anisotropic cellular organization at macroscopic length scales. To overcome this challenge, an innovative, cheap and easy method to align clinically approved non-woven surgical microfibrillar scaffolds is presented. The method involves a three-step process of coating, unidirectional stretching of scaffolds after heating them above glass transition temperature, and cooling back to room temperature. Briefly, a polymer coating is applied to a non-woven mesh that results in a partial welding of randomly oriented microfibers at their intersection points. The coated scaffold is then heated above the glass transition temperature of the coating and the scaffold polymer. Subsequently, the coated scaffold is stretched to produce aligned and three dimentional (3D) porous fibrillar scaffolds. In a proof of concept study, a polyglycolic acid (PGA) micro-fibrillar scaffold was coated with poly(4-hydroxybutirate) (P4HB) acid and subsequently aligned. Fibroblasts were cultured in vitro within the scaffold and results showed an increase in cellular alignment along the direction of the PGA fibers. This method can be scaled up easily for industrial production of polymeric meshes or directly applied to small pieces of scaffolds at the point of care.

Viscoelastic shear lag model to predict the micromechanical behavior of tendon under dynamic tensile loading

Owing to its viscoelastic nature, tendon exhibits stress rate-dependent breaking and stiffness function. A Kelvin-Voigt viscoelastic shear lag model is proposed to illustrate the micromechanical behavior of the tendon under dynamic tensile conditions. Theoretical closed-form expressions are derived to predict the deformation and stress transfer between fibrils and interfibrillar matrix while tendon is dynamically stretched. The results from the analytical solutions demonstrate that how the fibril overlap length and fibril volume fraction affect the stress transfer and mechanical properties of tendon. We find that the viscoelastic property of interfibrillar matrix mainly results in collagen fibril failure under fast loading rate or creep rupture of tendon. However, discontinuous fibril model and hierarchical structure of tendon ensure relative sliding under slow loading rate, helping dissipate energy and protecting fibril from damage, which may be a key reason why regularly staggering alignment microstructure is widely selected in nature. According to the growth, injury, healing and healed process of tendon observed by many researchers, the conclusions presented in this paper agrees well with the experimental findings. Additionally, the emphasis of this paper is on micromechanical behavior of tendon, whereas this analytical viscoelastic shear lag model can be equally applicable to other soft or hard tissues, owning the similar microstructure.