Ligament tissue engineering using synthetic biodegradable fiber scaffolds

The cellular responses of corneal fibroblasts to cyclic stretching loads

Mechanical signaling plays a crucial role in maintaining extracellular matrix (ECM) homeostasis in various structures. In this study, we investigated the responses of corneal fibroblasts to cyclic stretching loads using an in vitro cell culture system. Bovine corneal fibroblasts were cultured and subjected to equibiaxial cyclic strain of 15% for 72 h at a frequency of 0.25 Hz, with bovine skin fibroblasts used as a comparison. We explored various cellular behaviors, including morphological changes, cell proliferation, and metabolism in response to mechanical stretching loads. The expression of genes, protein secretion, and enzymatic activity for several major metalloproteinases was also determined through Q-PCR, Western blot, and gel zymography. Additionally, we investigated the involvement of mitogen-activated protein kinases (MAPKs) signaling pathways in the corneal fibroblasts when subjected to mechanical stimuli. Our findings revealed that, compared to skin fibroblasts, corneal fibroblasts were reluctant to morphological changes in response to a prolonged (72 h) and high-amplitude (15% of strain) cyclic stretching load. However, cyclic stretching loads stimulated the upregulation of MMP-2 expression in corneal fibroblasts via the MAPK signaling pathways involving extracellular signal-regulated kinase and p38. Together with a lack of upregulation in type I collagen expression, our results indicate the induction of the ECM degradation process in corneal fibroblasts in response to cyclic stretching. These findings emphasize the mechanoresponsive nature of corneal fibroblasts and shed light on the potential impact of intense mechanical stress on the cornea in both normal and pathological conditions such as keratoconus, providing valuable insights for understanding corneal mechanobiology.

Advanced Gene Therapy Strategies for the Repair of ACL Injuries

The anterior cruciate ligament (ACL), the principal ligament for stabilization of the knee, is highly predisposed to injury in the human population. As a result of its poor intrinsic healing capacities, surgical intervention is generally necessary to repair ACL lesions, yet the outcomes are never fully satisfactory in terms of long-lasting, complete, and safe repair. Gene therapy, based on the transfer of therapeutic genetic sequences via a gene vector, is a potent tool to durably and adeptly enhance the processes of ACL repair and has been reported for its workability in various experimental models relevant to ACL injuries in vitro, in situ, and in vivo. As critical hurdles to the effective and safe translation of gene therapy for clinical applications still remain, including physiological barriers and host immune responses, biomaterial-guided gene therapy inspired by drug delivery systems has been further developed to protect and improve the classical procedures of gene transfer in the future treatment of ACL injuries in patients, as critically presented here.

Second Generation of Tissue-Engineered Ligament Substitutes for Torn ACL Replacement: Adaptations for Clinical Applications

The anterior cruciate ligament (ACL) of the knee joint is one of the strongest ligaments of the body and is often the target of traumatic injuries. Unfortunately, its healing potential is limited, and the surgical options for its replacement are frequently associated with clinical issues. A bioengineered ACL (bACL) was developed using a collagen matrix, seeded with autologous cells and successfully grafted and integrated into goat knee joints. We hypothesize that, in order to reduce the cost and simplify the model, an acellular bACL can be used as a substitute for a torn ACL, and bone plugs can be replaced by endobuttons to fix the bACL in situ. First, acellular bACLs were successfully grafted in the goat model with 18% recovery of ultimate tensile strength 6 months after implantation (94 N/mm² vs. 520). Second, a bACL with endobuttons was produced and tested in an exvivo bovine knee model. The natural collagen scaffold of the bACL contributes to supporting host cell migration, growth and differentiation in situ post-implantation. Bone plugs were replaced by endobuttons to design a second generation of bACLs that offer more versatility as biocompatible grafts for torn ACL replacement in humans. A robust collagen bACL will allow solving therapeutic issues currently encountered by orthopedic surgeons such as donor-site morbidity, graft failure and post-traumatic osteoarthritis.

Incorporating regenerative medicine into rehabilitation programmes: a potential treatment for ankle sprain

Ankle sprain has a great effect on morbidity and complications of chronic diseases. Experts have come to a consensus where ankle sprain can be managed by rest, ice, compression and elevation, non-steroidal anti-inflammatory drugs, immobilisation, functional support such as the use of an ankle brace, exercise, surgery and other therapies that include physiotherapy modalities and acupuncture. However, the time required for healing is still relatively long in addition to post-operative complications. Because of the challenges and setbacks faced by interventions to manage ankle sprains and in view of the recent trend and development in the field of regenerative medicine, this article discusses future treatments focusing on a personalised and holistic approach for ankle sprain management. This narrative review provides a novel idea for incorporating regenerative medicine into conventional therapy as an intervention for ankle sprain based on theoretical concepts and available evidence on regenerative medicine involving ligament injuries.

In Vitro and In Vivo Performance of Tissue-Engineered Tendons for Anterior Cruciate Ligament Reconstruction

BACKGROUND

Anterior cruciate ligament (ACL) reconstruction is the current standard of care for ACL tears. However, the results are not consistently successful; autografts or allografts have certain disadvantages; and synthetic grafts have had poor clinical results.

PURPOSE

To determine if recellularization of decellularized tendons combined with mechanical stimulation in a bioreactor could replicate the mechanical properties of the native ACL and be successfully used for ACL reconstruction in vivo.

STUDY DESIGN

Controlled laboratory study.

METHODS

Porcine tibialis tendons were decellularized and then recellularized with human adult bone marrow-derived stem cells. Tendons were cultured in a tissue bioreactor that provided biaxial cyclic loading for up to 7 days. To reproduce mechanical stresses similar to those experienced by the ACL within the knee joint, the tendons were subjected to simultaneous tension and torsion in the bioreactor. Expression of tendon-specific genes and newly synthesized collagen and glycosaminoglycan were used to quantify the efficacy of recellularization and dynamic bioreactor culture. The ultimate tensile load to failure and stiffness of recellularized constructs were measured after dynamic stimulation. Finally, the tissue-engineered tendons were used to reconstruct the ACL in 24 pigs, and ultimate tensile load to failure and stiffness were assessed after 3 months.

RESULTS

Dynamic bioreactor culture significantly increased the expression of tendon-specific genes, the quantity of newly synthesized collagen and glycosaminoglycan, and the ultimate tensile load and stiffness of recellularized tendons. After in vivo reconstruction, the ultimate tensile load and stiffness of the tissue-engineered tendons increased significantly up to 3 months after surgery and were within 80% of the ultimate tensile load of the natural ACL.

CONCLUSION

This translational study indicates that recellularization and dynamic mechanical stimuli can significantly enhance matrix synthesis and ultimate tensile load of decellularized porcine tibialis tendons. This approach to tissue engineering can be very useful for ACL reconstruction and may overcome some of the disadvantages of autografts and allografts.

CLINICAL RELEVANCE

Dynamic bioreactor cultivation of tissue-engineered tendons may overcome the limitations of autografts and allografts.

Synthesis of Aligned TiO2 Nanofibers Using Electrospinning

In this work, aligned TiO2 nanofibers (NFs) were synthesized using an electrospinning technique with a two-piece Al collector. The effects of processing parameters, including the calcination temperature, applied voltage, and needle-to-Al-collector distance, were studied. The final phase of TiO2 was determined using the calcination temperature; calcination at 500 °C resulted in the formation of anatase phase, whereas calcination at 600 °C resulted in the formation of rutile phase. In addition, with the increase of calcination temperature, the diameter of the synthesized NFs decreased owing to the sintering and coalescence of nanograins existing in individual NFs. A specially designed collector configuration resulted in the parallel alignment of TiO2 NFs. The needle-to-collector distance and applied voltages caused the change in diameter and alignment of NFs. The diameter had an inverse relation with the needle-to-collector distance, and an optimal value of applied voltage was required to achieve TiO2 NFs with the smallest diameter. Furthermore, with the increase of applied voltage, the morphology of TiO2 NFs changed from an aligned to a disordered state.

Recent advances on biomedical applications of scaffolds in wound healing and dermal tissue engineering

The tissue engineering field has developed in response to the shortcomings related to the replacement of the tissues lost to disease or trauma: donor tissue rejection, chronic inflammation and donor tissue shortages. The driving force behind the tissue engineering is to avoid the mentioned issues by creating the biological substitutes capable of replacing the damaged tissue. This is done by combining the scaffolds, cells and signals in order to create the living, physiological, three-dimensional tissues. A wide variety of skin substitutes are used in the treatment of full-thickness injuries. Substitutes made from skin can harbour the latent viruses, and artificial skin grafts can heal with the extensive scarring, failing to regenerate structures such as glands, nerves and hair follicles. New and practical skin scaffold materials remain to be developed. The current article describes the important information about wound healing scaffolds. The scaffold types which were used in these fields were classified according to the accepted guideline of the biological medicine. Moreover, the present article gave the brief overview on the fundamentals of the tissue engineering, biodegradable polymer properties and their application in skin wound healing. Also, the present review discusses the type of the tissue engineered skin substitutes and modern wound dressings which promote the wound healing.

Hybrid polyurea elastomers with enzymatic degradation and tunable mechanical properties

Herein, we report on the synthesis and characterization of enzymatically labile polyureas for use as a tissue-engineered ligament scaffold. Polyureas were selected due to their excellent tensile properties, fatigue resistance, and highly tunable nature. Incorporation of a collagenase-sensitive peptide into the backbone of the polyurea provided a means to confer cell-responsive degradation to the synthetic polymer. Chemical, morphological, and mechanical testing were used to confirm incorporation of the peptide and characterize polyurea films. Notably, the incorporation of the peptide resulted in an increase in modulus, elongation, and tensile strength. This was attributed to an increase in phase mixing and an increase in hydrogen bonding between the hard and soft segments. Candidate polyureas with varying levels of collagen-mimetic peptide (0%, 10%, 20%) were then subjected to degradation in collagenase media or buffer at 37°C over 4 weeks. Statistically significant decreases in strength and elongation were observed in polyureas with 20% peptide content after collagenase treatment, whereas specimens in phosphate-buffered saline showed no statistically significant difference. These observations confirmed that enzyme-specific degradation was conferred to the polyurea. Overall, these polyureas hold great promise as a material for ligament reconstruction due to the promising mechanical properties and potential for cell-mediated degradation.

The past, present and future of ligament regenerative engineering

Regenerative engineering has been defined as the convergence of Advanced Materials Sciences, Stem Cell Sciences, Physics, Developmental Biology and Clinical Translation for the regeneration of complex tissues and organ systems. Anterior cruciate ligament (ACL) reconstruction necessitates the regeneration of bone, ligament and their interface to achieve superior clinical results. In the past, the ACL has been repaired with the use of autologous and allogeneic grafts, which have their respective drawbacks. Currently, investigations on the use of biodegradable matrices to achieve knee stability and permit tissue regeneration are making promising advancements. In the future, utilizing regenerative biology cues to induce an endogenous regenerative response may aid the enhancement of clinical ACL reconstruction outcomes.

Anterior cruciate ligament reconstruction in a rabbit model using silk-collagen scaffold and comparison with autograft

The objective of the present study was to perform an in vivo assessment of a novel silk-collagen scaffold for anterior cruciate ligament (ACL) reconstruction. First, a silk-collagen scaffold was fabricated by combining sericin-extracted knitted silk fibroin mesh and type I collagen to mimic the components of the ligament. Scaffolds were electron-beam sterilized and rolled up to replace the ACL in 20 rabbits in the scaffold group, and autologous semitendinosus tendons were used to reconstruct the ACL in the autograft control group. At 4 and 16 weeks after surgery, grafts were retrieved and analyzed for neoligament regeneration and tendon-bone healing. To evaluate neoligament regeneration, H&E and immunohistochemical staining was performed, and to assess tendon-bone healing, micro-CT, biomechanical test, H&E and Russell-Movat pentachrome staining were performed. Cell infiltration increased over time in the scaffold group, and abundant fibroblast-like cells were found in the core of the scaffold graft at 16 weeks postoperatively. Tenascin-C was strongly positive in newly regenerated tissue at 4 and 16 weeks postoperatively in the scaffold group, similar to observations in the autograft group. Compared with the autograft group, tendon-bone healing was better in the scaffold group with trabecular bone growth into the scaffold. The results indicate that the silk-collagen scaffold has considerable potential for clinical application.

Nano/Micro Hybrid Scaffold of PCL or P3HB Nanofibers Combined with Silk Fibroin for Tendon and Ligament Tissue Engineering

A novel biodegradable nano/micro hybrid structure was obtained by electrospinning P3HB or PCL nanofibers onto a twisted silk fibroin (SF) structure, with the aim of fabricating a suitable scaffold for tendon and ligament tissue engineering. The electrospinning (ES) processing parameters for P3HB and PCL were optimized on 2D samples, and applied to produce two different nano/micro hybrid constructs (SF/ES-PCL and SF/ES-P3HB).

Morphological, chemico-physical and mechanical properties of the novel hybrid scaffolds were evaluated by SEM, ATR FT-IR, DSC, tensile and thermodynamic mechanical tests. The results demonstrated that the nanofibers were tightly wrapped around the silk filaments, and the crystallinity of the SF twisted yarns was not influenced by the presence of the electrospun polymers. The slightly higher mechanical properties of the hybrid constructs confirmed an increase of internal forces due to the interaction between nano and micro components. Cell culture tests with L929 fibroblasts, in the presence of the sample eluates or in direct contact with the hybrid structures, showed no cytotoxic effects and a good level of cytocompatibility of the nano/micro hybrid structures in term of cell viability, particularly at day 1. Cell viability onto the nano/micro hybrid structures decreased from the first to the third day of culture when compared with the control culture plastic, but appeared to be higher when compared with the uncoated SF yarns. Although additional in vitro and in vivo tests are needed, the original fabrication method here described appears promising for scaffolds suitable for tendon and ligament tissue engineering.

Status and headway of the clinical application of artificial ligaments

The authors first reviewed the history of clinical application of artificial ligaments. Then, the status of clinical application of artificial ligaments was detailed. Some artificial ligaments possessed comparable efficacy to, and fewer postoperative complications than, allografts and autografts in ligament reconstruction, especially for the anterior cruciate ligament. At the end, the authors focused on the development of two types of artificial ligaments: polyethylene glycol terephthalate artificial ligaments and tissue-engineered ligaments. In conclusion, owing to the advancements in surgical techniques, materials processing, and weaving methods, clinical application of some artificial ligaments so far has demonstrated good outcomes and will become a trend in the future.

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

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

Combined effects of chemical priming and mechanical stimulation on mesenchymal stem cell differentiation on nanofiber scaffolds

Functional tissue engineering of connective tissues such as the anterior cruciate ligament (ACL) remains a significant clinical challenge, largely due to the need for mechanically competent scaffold systems for grafting, as well as a reliable cell source for tissue formation. We have designed an aligned, polylactide-co-glycolide (PLGA) nanofiber-based scaffold with physiologically relevant mechanical properties for ligament regeneration. The objective of this study is to identify optimal tissue engineering strategies for fibroblastic induction of human mesenchymal stem cells (hMSC), testing the hypothesis that basic fibroblast growth factor (bFGF) priming coupled with tensile loading will enhance hMSC-mediated ligament regeneration. It was observed that compared to the unloaded, as well as growth factor-primed but unloaded controls, bFGF stimulation followed by physiologically relevant tensile loading enhanced hMSC proliferation, collagen production and subsequent differentiation into ligament fibroblast-like cells, upregulating the expression of types I and III collagen, as well as tenasin-C and tenomodulin. The results of this study suggest that bFGF priming increases cell proliferation, while mechanical stimulation of the hMSCs on the aligned nanofiber scaffold promotes fibroblastic induction of these cells. In addition to demonstrating the potential of nanofiber scaffolds for hMSC-mediated functional ligament tissue engineering, this study yields new insights into the interactive effects of chemical and mechanical stimuli on stem cell differentiation.

Optimization of culture conditions for stem cells derived from human anterior cruciate ligament and bone marrow

Tissue engineering with stem cells is a fascinating approach for treating anterior cruciate ligament (ACL) injuries. In our previous study, stem cells isolated from the human anterior cruciate ligament were shown to possess extensive proliferation and differentiation capabilities when treated with specific growth factors. However, optimal culture conditions and the usefulness of fetal bovine serum (FBS) as a growth factor in in vitro culture systems are yet to be determined. In this study, we compared the effects of different culture media containing combinations of various concentrations of FBS and the growth factors basic fibroblastic growth factor (bFGF) and transforming growth factor-β1 (TGF-β1) on the proliferation and differentiation of ligament-derived stem cells (LSCs) and bone marrow mesenchymal stem cells (BMSCs). We found that α-MEM plus 10% FBS and bFGF was able to maintain both LSCs and BMSCs in a relatively undifferentiated state but with lower major extracellular matrix (ECM) component gene expression and protein production, which is beneficial for stem cell expansion. However, the differentiation and proliferation potentials of LSCs and BMSCs were increased when cultured in MesenPRO, a commercially available stem cell medium containing 2% FBS. MesenPRO in conjunction with TGF-β1 had the greatest ability to induce the differentiation of BMSCs and LSCs to ligament fibroblasts, which was evidenced by the highest ligamentous ECM gene expression and protein production. These results indicate that culture media and growth factors play a very important role in the success of tissue engineering. With α-MEM plus 10% FBS and bFGF, rapid proliferation of stem cells can be achieved. In this study, MesenPRO was able to promote differentiation of both LSCs and BMSCs to ligament fibroblasts. Differentiation was further increased by TGF-β1. With increasing understanding of the effects of different culture media and growth factors, manipulation of stem cells in the desired direction for ligament tissue engineering can be achieved.

Nanotopography-guided tissue engineering and regenerative medicine

Human tissues are intricate ensembles of multiple cell types embedded in complex and well-defined structures of the extracellular matrix (ECM). The organization of ECM is frequently hierarchical from nano to macro, with many proteins forming large scale structures with feature sizes up to several hundred microns. Inspired from these natural designs of ECM, nanotopography-guided approaches have been increasingly investigated for the last several decades. Results demonstrate that the nanotopography itself can activate tissue-specific function in vitro as well as promote tissue regeneration in vivo upon transplantation. In this review, we provide an extensive analysis of recent efforts to mimic functional nanostructures in vitro for improved tissue engineering and regeneration of injured and damaged tissues. We first characterize the role of various nanostructures in human tissues with respect to each tissue-specific function. Then, we describe various fabrication methods in terms of patterning principles and material characteristics. Finally, we summarize the applications of nanotopography to various tissues, which are classified into four types depending on their functions: protective, mechano-sensitive, electro-active, and shear stress-sensitive tissues. Some limitations and future challenges are briefly discussed at the end.

Biomimetic poly(lactide) based fibrous scaffolds for ligament tissue engineering

The aim of this study was to fabricate a fibrous scaffold that closely resembled the micro-structural architecture and mechanical properties of collagen fibres found in the anterior cruciate ligament (ACL). To achieve this aim, fibrous scaffolds were made by electrospinning L-lactide based polymers. L-Lactide was chosen primarily due to its demonstrated biocompatibility, biodegradability and high modulus. The electrospun fibres were collected in tension on a rotating wire mandrel. Upon treating these fibres in a heated aqueous environment, they possessed a crimp-like pattern having a wavelength and amplitude similar to that of native ACL collagen. Of the polymer fibre scaffolds studied, those made from poly(L-lactide-co-D,L-lactide) PLDLA exhibited the highest modulus and were also the most resilient to in vitro hydrolytic degradation, undergoing a slight decrease in modulus compared to the other polymeric fibres over a 6 month period. Bovine fibroblasts seeded on the wavy, crimp-like PLDLA fibres attached, proliferated and deposited extracellular matrix (ECM) molecules on the surface of the fibrous scaffold. In addition, the deposited ECM exhibited bundle formation that resembled the fascicles found in native ACL. These findings demonstrate the importance of replicating the geometric microenvironment in developing effective tissue engineering scaffolds.

Synthesis of collagenase-sensitive polyureas for ligament tissue engineering

Recently, poly(ester urethanes) were investigated for use as ligament grafts due to their exceptional mechanical properties and highly tunable structure; however, these grafts are susceptible to hydrolytic degradation that occurs independent of tissue regeneration. To address this limitation, polyureas containing collagen-derived peptides were synthesized which enable cellular release of proteases to dictate degradation rate. It is hypothesized that this cell-responsive design will facilitate load transfer from the biodegradable scaffold to neotissue at a rate that promotes proper tissue orientation and function while maintaining construct integrity.

Aggregation of bovine anterior cruciate ligament fibroblasts or marrow stromal cells promotes aggrecan production

The development of a tissue-engineered alternative for current ligament grafts requires the creation of a fibrocartilaginous interface between the engineered ligament midsubstance and bone tissue. Therefore, the focus of this study was to examine the potential for cartilaginous extracellular matrix (ECM) formation by altering culture parameters for bovine anterior cruciate ligament (ACL) fibroblasts and marrow stromal cells (MSCs). Specifically, cells were cultured without chondrogenic media supplements on aggrecan-coated surfaces, tissue culture-treated control surfaces, and nonadhesive surfaces that promoted cell aggregation, and examined over 14 days. Aggrecan-coated surfaces promoted the aggregation of ACL fibroblasts and MSCs within 24 h after seeding. Aggrecan gene expression was significantly upregulated in cell aggregates, regardless of how cell clustering was induced, with as much as 10.9 ± 1.2-fold upregulation in ACL fibroblasts and 9.7 ± 1.1-fold in MSCs after 3 days, compared to control surfaces. Dimethylmethylene blue (DMMB) results and immunostaining verified the presence of aggrecan in ACL fibroblast and MSC aggregates throughout the culture period. Results indicate that ACL fibroblasts retained the ability to alter their gene expression and produce aggrecan, though MSCs, in general, had a more consistent response to aggregation. These findings support the use of aggregate-inducing materials to encourage production of aggrecan and suggest that altering the degree of clustering could produce a range of phenotypes from a single cell source. As such, this represents a first step which may inform future approaches to producing tissue-engineered ligament grafts.

Novel strategies in tendon and ligament tissue engineering: Advanced biomaterials and regeneration motifs

Tendon and ligaments have poor healing capacity and when injured often require surgical intervention. Tissue replacement via autografts and allografts are non-ideal strategies that can lead to future problems. As an alternative, scaffold-based tissue engineering strategies are being pursued. In this review, we describe design considerations and major recent advancements of scaffolds for tendon/ligament engineering. Specifically, we outline native tendon/ligament characteristics critical for design parameters and outcome measures, and introduce synthetic and naturally-derived biomaterials used in tendon/ligament scaffolds. We will describe applications of these biomaterials in advanced tendon/ligament engineering strategies including the utility of scaffold functionalization, cyclic strain, growth factors, and interface considerations. The goal of this review is to compile and interpret the important findings of recent tendon/ligament engineering research in an effort towards the advancement of regenerative strategies.

Effect of cyclic three-dimensional strain on cell proliferation and collagen synthesis of fibroblast-seeded chitosan-hyaluronan hybrid polymer fiber

BACKGROUND

Tissue engineering techniques using biodegradable three-dimensional (3D) scaffolds with cultured cells offer more potential alternatives for the treatment of severe ligament and tendon injuries. In tissue engineering, one of the crucial roles of 3D scaffolds is to provide a temporary template with the biomechanical characteristics of the native extracellular matrix (ECM) until the regenerated tissue matures. The purpose of the present study was to assess the effect of various cyclic mechanical stresses on cell proliferation and ECM production in a 3D scaffold made from chitosan and hyaluronan for ligament and tendon tissue engineering.

METHODS

Three-dimensional scaffolds seeded with rabbit patella tendon fibroblasts were attached to a bioreactor under various conditions: static group, no strain; stretch group, tensile strain; rotational group, rotational strain; combined group, rotational and tensile strain. In the Static group, 3 weeks of stationary culture was performed. In the remaining three groups, a loading regimen of 0.5 Hz for 18 h and then 6 h rest was carried out for 2 weeks after 1 week of static culture. The DNA content was determined to quantify cell proliferation. Real-time reverse transcription polymerase chain reaction analysis was performed to assess the mRNA levels of the ECM products.

RESULTS

DNA content of the combined group was significantly higher than that of the static and stretch groups, and that of the rotational group was significant higher than that of the static and stretch groups at 21 days after cultivation. The mRNA level of types I and III collagen and fibromodulin in the combined group was significantly higher than that in the other three groups. The amount of collagen synthesis in the combined group was higher than that in the static group, but the difference was not significant.

CONCLUSIONS

Multidimensional cyclic mechanical strain to mimic the physiological condition in vivo has the potential to improve or accelerate tissue regeneration in ligament and tendon tissue engineering using 3D scaffolds in vitro.

A comparison of degradable synthetic polymer fibers for anterior cruciate ligament reconstruction

We compared mechanical properties, degradation rates, and cellular compatibilities of two synthetic polymer fibers potentially useful as ACL reconstruction scaffolds: poly(desaminotyrosyl-tyrosine dodecyl dodecanedioate)(12,10), p(DTD DD) and poly(L-lactic acid), PLLA. The yield stress of ethylene oxide (ETO) sterilized wet fibers was 150 +/- 22 MPa and 87 +/- 12 MPa for p(DTD DD) and PLLA, respectively, with moduli of 1.7 +/- 0.1 MPa and 4.4 +/- 0.43 MPa. Strength and molecular weight retention were determined after incubation under physiological conditions at varying times. After 64 weeks strength decreased to 20 and 37% of the initial sterile fiber values and MW decreased to 41% and 36% of the initial values for p(DTD DD) and PLLA, respectively. ETO sterilization had no significant effect on mechanical properties. Differences in mechanical behavior may be due to the semicrystalline nature of PLLA and the small degree of crystallinity induced by mesogenic ordering in p(DTD DD) suggested by DSC analysis. Fibroblast growth was similar on 50-fiber scaffolds of both polymers through 16 days in vitro. These data suggest that p(DTD DD) fibers, with higher strength, lower stiffness, favorable degradation rate and cellular compatibility, may be a superior alternative to PLLA fibers for development of ACL reconstruction scaffolds.

Tissue engineering of the anterior cruciate ligament-sodium dodecyl sulfate-acellularized and revitalized tendons are inferior to native tendons

The acellularization of tendons using detergents (sodium dodecyl sulfate, Triton-X, tri-nitro-butyl-phosphate) is a new source of scaffolds for tissue engineering in anterior cruciate ligament (ACL) repair. In vitro testing demonstrated that acellular tendon scaffolds are biocompatible and show good biomechanical properties, but in vivo confirmation of these results is not yet available. Therefore, the aim of this study was to see in vivo if an acellular allogenic construct colonized with autologous fibroblasts improves the quality of ACL reconstruction. ACL replacement was performed in 31 New Zealand White rabbits using a standardized model. Fifteen animals received autologous semitendinosus tendon, whereas 16 animals were treated with a tissue-engineered construct. This construct was made by acellularization of allogenic semitendinosus tendons using sodium dodecyl sulfate and subsequent in vitro colonization with autologous fibroblasts. Eight weeks postoperatively, macroscopic, biomechanical (ultimate load to failure, elongation, stiffness; n = 8/9), and histological (n = 5) examinations were performed. Biomechanical testing showed decreasing strength of the constructs at 8 weeks after implantation compared with the direct postsurgical strength. However, tissue-engineered constructs (F = 19.7 +/- 20.3 N) were significantly weaker than autologous tendons (F = 61.2 +/- 31.2 N). Histologically, the autologous tendons showed signs of partial necrosis and tissue remodeling. The tissue-engineered constructs exhibited an inflammatory reaction and showed both repopulated and acellular regions. In conclusion, in vivo results were much more unfavorable than in vitro results had suggested. Further studies have to be performed to test if modifications of the acellularization process yield better results in vivo.

Modulation of gene expression and collagen production of anterior cruciate ligament cells through cell shape changes on polycaprolactone/chitosan blends

Our previous study has illustrated that chitosan could enhance human anterior cruciate ligament (ACL) cells to exhibit a dramatic effect on increasing the gene expression of transforming growth factor beta1 (TGF-beta1), which is a specific gene for wound healing and collagen synthesis. However, human ACL cells could not adhere and proliferate well on chitosan. In order to overcome this drawback, we introduced polycaprolactone (PCL) into chitosan by the method of blending in this study. It was found that the morphology, viability and gene expression of human ACL cells on the chitosan/PCL blends could be effectively regulated. With the increase of PCL content in blends, human ACL cells presented more flatten shape, well-organized cytoskeleton, and higher proliferated ability. Compared to flatten shape, human ACL cells with round shape exhibited higher levels of mRNA expression of TGF-beta1 and collagen type III through 3-day culture period. Furthermore, these blended materials could upregulate protein synthesis of human ACL cells, which corresponded to their gene expressions. Therefore, it is possible to combine the advantages of chitosan and PCL to create a new blended material, which could control cellular morphologies specifically, and further to regulate the gene expression and protein production of cells for specific applications. We expected this concept, controlling the cell shape through biomaterial to modulate the behavior of cells, could provide a new vision for the material selection of ligament tissue engineering.

Tissue reactions to engineered cartilage based on poly-L-lactic acid scaffolds

Tissue reactions against poly-L-lactic acid (PLLA) in engineered cartilage may influence the size or maturity of regenerative tissue. To understand the biological events in these reactions, we subcutaneously transplanted engineered constructs of PLLA scaffolds with or without human chondrocytes or atelocollagen in nude mice and evaluated neovascularization and macrophage activation, which can be assessed even in nude mice. Although not showing cartilage regeneration, PLLA alone demonstrated dense localization of macrophages and blood vessels, as well as a high level of interleukin-1 beta and tissue hemoglobin at 2 and 8 weeks. Otherwise, constructs with PLLA and chondrocytes with or without atelocollagen (PLLA/cell/gel or PLLA/cell) formed mature cartilage by 8 weeks, which was more prominent in PLLA/cell/gel. Although accumulation of macrophages and blood vessels in PLLA/cell/gel and PLLA/cell was comparable with that in PLLA at 2 weeks, that in PLLA/cell/gel markedly decreased by 8 weeks, with blood vessels and macrophages excluded into non-cartilage areas. Macrophage migration inhibitory factor could be involved in these suppressed tissue reactions, because it was expressed in chondrocytes of engineered cartilage. Intense tissue reactions inevitably occurred in biopolymers alone, but it is possible that maturation of engineered cartilage suppressed these reactions, which may contribute to circumventing deformity or malformation of engineered tissues.

Cell-generated forces influence the viability, metabolism and mechanical properties of fibroblast-seeded collagen gel constructs

The aim of this study was to investigate the influence of the endogenous forces generated by fibroblast-mediated contraction, using four individual collagen gel models that differed with respect to the ability of the cells to contract the gel. Human neonatal dermal fibroblasts were seeded in type I collagen and the gels were cast in a racetrack-shaped mould containing a removable central island. Two of the models were mechanically stressed (20 mm and 10 mm), as complete contraction was prevented by the presence of a central island. The central island was removed in the third model (released) and the final model was cast in a Petri dish and detached, allowing full multi-axial contraction (SR). Cell viability was maintained in the 10 mm, released and SR models over a 6 day culture period but localized regions of cell death were evident in the 20 mm model. Cell and collagen alignment was developed in the 20 mm and 10 mm models and to a lesser extent in the released model, but was absent in the SR model. Cell proliferation and collagen synthesis was lower in the 20 mm model compared to the other systems and there was evidence of enhanced matrix metalloproteinase production. The mechanical properties of the 20 mm model system were inferior to the 10 mm and released systems. The 10 mm model system induced a high level of cell and matrix orientation and may, therefore, represent the best option for tissue-engineered ligament repair involving an orientated fibroblast-seeded collagen gel.

A biomimetic hierarchical scaffold: natural growth of nanotitanates on three-dimensional microporous Ti-based metals

Nanophase materials are promising alternative implant materials in tissue engineering. Here we report for the first time the large-scale direct growth of nanostructured bioactive titanates on three-dimensional (3D) microporous Ti-based metal (NiTi and Ti) scaffolds via a facile low temperature hydrothermal treatment. The nanostructured titanates show characteristics of 1D nanobelts/nanowires on a nanoskeleton layer. Besides resembling cancelous bone structure on the micro/macroscale, the 1D nanostructured titanate on the exposed surface is similar to the lowest level of hierarchical organization of collagen and hydroxyapatite. The resulting surface displays superhydrophilicity and favors deposition of hydroxyapatite and accelerates cell attachment and proliferation. The remarkable simplicity of this process makes it widely accessible as an enabling technique for applications from engineering materials treatment including energy-absorption materials and pollution-treatment materials to biotechnology.

The effects of GDF-5 and uniaxial strain on mesenchymal stem cells in 3-D culture

Recent endeavors in tissue engineering have attempted to identify the optimal parameters to create an artificial ligament. Both mechanical and biochemical stimulation have been used by others to independently modulate growth and differentiation, although few studies have explored their interactions. We applied previously described fabrication techniques to create a highly porous (90%-95% porosity, 212-300 microm), 3-D, bioabsorbable polymer scaffold (polycaprolactone). Scaffolds were coated with bovine collagen, and growth and differentiation factor 5 (GDF-5) was added to half of the scaffolds. Scaffolds were seeded with mesenchymal stem cells and cultured in a custom bioreactor under static or cyclic strain (10% strain, 0.33 Hz) conditions. After 48 hours, both mechanical stimulation and GDF-5 increased mRNA production of collagen I, II, and scleraxis compared to control; tenascin C production was not increased. Combining stimuli did not change gene expression; however, cellular metabolism was 1.7 times higher in scaffolds treated with both stimuli. We successfully grew a line of mesenchymal stem cells in 3-D culture, and our initial data indicate mechanical stimulation and GDF-5 influenced cellular activity and mRNA production; we did not, however, observe additive synergism with the mechanical and biological stimuli.

Regeneration of anterior cruciate ligament by biodegradable scaffold combined with local controlled release of basic fibroblast growth factor and collagen wrapping

The objective of this study was to increase the therapeutic efficacy of anterior cruciate ligament (ACL) surgery using an artificial ligament material developed through a combination of tissue engineering technologies. A poly-L-lactic acid (PLLA) scaffold of plain-woven braid was incorporated with a gelatin hydrogel for controlled release of basic fibroblast growth factor (bFGF) and wrapped with a collagen membrane to allow space for ligament regeneration. For the ACL reconstruction surgery, the PLLA braid scaffold combined with the gelatin hydrogel incorporating bFGF and the collagen wrapping was applied to a tunnel prepared in the femur and tibia of rabbits. The hydrogel was placed in the bone, whereas the portion of the braid inside the joint cavity was wrapped with the membrane. As controls, the PLLA scaffold was applied with the hydrogel or the membrane, or without either material. Bone regeneration in the tunnel and ACL tissue regeneration in the joint cavity were histologically evaluated, and the mechanical strength and collagen content of the regenerated ACL were assessed. When the PLLA scaffold was integrated with both the hydrogel and the membrane, bone and ACL tissues were regenerated in the corresponding sites, in marked contrast to the control groups. Combination of bFGF-controlled release resulted in enhanced mechanical strength of the regenerated ACL tissue. In the joint cavity, it is possible that the local bFGF release inside the membrane enhanced the cell migration and collagen production, and that the surrounding PLLA scaffold results in the biological regeneration of ligament-like tissue. Additionally, significant bone regeneration around the scaffold was observed in the bone tunnel. It is therefore possible that the local controlled release of bFGF near the PLLA braid induced both osseointegration and intrascaffold cell migration in the bone tunnel and joint cavity, respectively, resulting in an overall increase in the mechanical strength of the regenerated ACL.

Synthesis, characterization and chondroprotective properties of a hyaluronan thioethyl ether derivative

Hyaluronan (HA), a non-sulfated glycosaminoglycan, is widely used in the clinic for viscosurgery, viscosupplementation, and treatment of osteoarthritis. Four decades of chemical modifications of HA have generated derivatives in which the biophysical and biochemical properties, as well as the rates of enzymatic degradation in vivo have been manipulated and tailored for specific clinical needs. One earlier modification adds multiple thiol groups to HA through hydrazide linkages, leading to a readily crosslinkable material for adhesion prevention and wound healing. We now describe the synthesis and chemical characterization of a novel thioethyl ether derivative of HA, HA-sulfhydryl (HASH), with a minimal tether between the HA and the thiol group. Unlike earlier thiol-modified HA derivatives, HASH cannot be readily crosslinked to form a hydrogel using either oxidative or bivalent electrophilic conditions, thus offering a unique polymeric polythiol that remains soluble. Moreover, HASH showed no cytotoxicity towards primary human fibroblasts and reduced the apoptosis rates of primary chondrocytes exposed to hydrogen peroxide in vitro. These properties foreshadow the clinical potential of HASH to moderate inflammation and to act as a chondroprotective agent in vivo.

Cyclic mechanical compression increases mineralization of cell-seeded polymer scaffolds in vivo

Despite considerable documentation of the ability of normal bone to adapt to its mechanical environment, very little is known about the response of bone grafts or their substitutes to mechanical loading even though many bone defects are located in load-bearing sites. The goal of this research was to quantify the effects of controlled in vivo mechanical stimulation on the mineralization of a tissue-engineered bone replacement and identify the tissue level stresses and strains associated with the applied loading. A novel subcutaneous implant system was designed capable of intermittent cyclic compression of tissue-engineered constructs in vivo. Mesenchymal stem cell-seeded polymeric scaffolds with 8 weeks of in vitro preculture were placed within the loading system and implanted subcutaneously in male Fisher rats. Constructs were subjected to 2 weeks of loading (3 treatments per week for 30 min each, 13.3 N at 1 Hz) and harvested after 6 weeks of in vivo growth for histological examination and quantification of mineral content. Mineralization significantly increased by approximately threefold in the loaded constructs. The finite element method was used to predict tissue level stresses and strains within the construct resulting from the applied in vivo load. The largest principal strains in the polymer were distributed about a modal value of -0.24% with strains in the interstitial space being about five times greater. Von Mises stresses in the polymer were distributed about a modal value of 1.6 MPa, while stresses in the interstitial tissue were about three orders of magnitude smaller. This research demonstrates the ability of controlled in vivo mechanical stimulation to enhance mineralized matrix production on a polymeric scaffold seeded with osteogenic cells and suggests that interactions with the local mechanical environment should be considered in the design of constructs for functional bone repair.

ACL reconstruction in a rabbit model using irradiated Achilles allograft seeded with mesenchymal stem cells or PDGF-B gene-transfected mesenchymal stem cells

The present study was conducted to develop a new strategy to accelerate reconstruction of the anterior cruciate ligament (ACL) by modifying the Achilles allograft with autogenous mesenchymal stem cells (MSCs) or PDGF-B transfected MSCs in a rabbit model. The allografts were first irradiated with Co60, stored at -80 degrees C, and then seeded with cells for implantation. Bilateral ACL reconstructions were performed. On the left, the allograft was either seeded with MSCs or PDGF-B transfected MSCs and acted as the experimental group. On the right, the graft without any cells seeded acted as control. At 3, 6 and 12 weeks after surgery, histological observation found that implantation of MSCs or PDGF-B transfected MSCs accelerated cellular infiltration into the ACL and enhanced collagen deposition in the wound. PDGF-B transfected MSCs could also lead to an initial promotion of angiogenesis. This gene transfer technique or cell implantation may be a potentially useful tool for improving ligament remodeling.

Nanofibers and their applications in tissue engineering

Developing scaffolds that mimic the architecture of tissue at the nanoscale is one of the major challenges in the field of tissue engineering. The development of nanofibers has greatly enhanced the scope for fabricating scaffolds that can potentially meet this challenge. Currently, there are three techniques available for the synthesis of nanofibers: electrospinning, self-assembly, and phase separation. Of these techniques, electrospinning is the most widely studied technique and has also demonstrated the most promising results in terms of tissue engineering applications. The availability of a wide range of natural and synthetic biomaterials has broadened the scope for development of nanofibrous scaffolds, especially using the electrospinning technique. The three dimensional synthetic biodegradable scaffolds designed using nanofibers serve as an excellent framework for cell adhesion, proliferation, and differentiation. Therefore, nanofibers, irrespective of their method of synthesis, have been used as scaffolds for musculoskeletal tissue engineering (including bone, cartilage, ligament, and skeletal muscle), skin tissue engineering, vascular tissue engineering, neural tissue engineering, and as carriers for the controlled delivery of drugs, proteins, and DNA. This review summarizes the currently available techniques for nanofiber synthesis and discusses the use of nanofibers in tissue engineering and drug delivery applications.

Development of hybrid polymer scaffolds for potential applications in ligament and tendon tissue engineering

Fibre-based scaffolds have been widely used for tendon and ligament tissue engineering. Knitted scaffolds have been proved to favour collagenous matrix deposition which is crucial for tendon/ligament reconstruction. However, such scaffolds have the limitation of being dependent on a gel system for cell seeding, which is unstable in a dynamic environment such as the knee joint. This study developed three types of hybrid scaffolds, based on knitted biodegradable polyester scaffolds, aiming to improve mechanical properties and cell attachment and proliferation on the scaffolds. The hybrid scaffolds were created by coating the knitted scaffolds with a thin film of poly (epsilon-caprolactone) (group I), poly (D, L-lactide-co-glycolide) nanofibres (group II) and type 1 collagen (group III). Woven scaffolds were also fabricated and compared with the various hybrid scaffolds in terms of their mechanical properties during in vitro degradation and cell attachment and growth. This study demonstrated that the coating techniques could modulate the mechanical properties and facilitate cell attachment and proliferation in the hybrid scaffold, which could be applied with promise in tissue engineering of tendons/ligaments.

The Use of Ultrasonication to Aid Recellularization of Acellular Natural Tissue Scaffolds for Use in Anterior Cruciate Ligament Reconstruction

Tissue engineering offers a promising solution to the replacement of anterior cruciate ligament. A decellularized porcine patella tendon scaffold was produced by immersing whole tissues sequentially in hypotonic buffer, 0.1% (w/v) sodium dodecyl sulfate (SDS) in hypotonic buffer, and nuclease solution prior to sterilization with 0.1% (w/v) peracetic acid. Initial studies revealed that primary human tenocytes would attach to, but failed to penetrate into, the decellularized scaffold. A novel use of ultrasonication was therefore developed to allow extrinsic cells to migrate into the acellular scaffold. Various intensities of ultrasonication were tested in order to produce a microscopically more open porous matrix without damaging the overall architecture of the scaffold. Ultrasonication treatment with the intensity of 360 W and a pulse time of 1 s for a total of 1 min was found to be the optimal treatment. This process did not have a significant effect upon the biochemical constituents (collagen, glycosaminoglycans), nor did it denature the collagen. Moreover, the acellular sonicated scaffold retained the essential biomechanical characteristics of the native tissue. Primary human tenocytes penetrated into the center of whole acellular sonicated scaffolds over a 3-week period in static culture. The viability of the cells in the center of the scaffold (depth of circa 2.5 mm) was, however, compromised. To circumvent the problem of nutrient limitation, acellular sonicated scaffolds were split into fascicular scaffolds (500 mum thick). Cells seeded onto the fascicular scaffolds penetrated throughout the scaffold and remained viable after 3 weeks of culture. This study has shown that an acellular biocompatible tendon scaffold can be produced using 0.1% (w/v) SDS and that ultrasonication can provide a novel method to enhance the recellularization of decellularized natural tissues.

In vitro and in vivo degradability and cytocompatibility of poly(l-lactic acid) scaffold fabricated by a gelatin particle leaching method

Porous poly(l-lactic acid) (PLLA) scaffolds fabricated by a gelatin particle-leaching technique have good mechanical property and cytocompatibility, as demonstrated by a previous in vitro study. Here we investigate further the in vitro degradation of the scaffolds in terms of weight loss, water uptake, weight-average molecular weight, thermal behavior and morphology during a 39 week period in phosphate-buffered saline. The water uptake decreased dramatically during the initial stage due to release of the remaining gelatin, and then increased slightly with degradation time. The weight-average molecular weight decreased linearly as a function of time, while the crystallinity steadily increased with slightly decreased melting temperature. After degradation, many defects and big holes were seen in the scaffolds by scanning electron microscopy. Cartilage regeneration and scaffold disappearance in vivo were compared by implanting the construct into nude mice for 30-120 days. While the scaffolds maintained their intact pore structure after 23 weeks of degradation in vitro, they almost disappeared in vivo at the same time, implying a faster degradation rate in vivo. By 120 days after implantation, the scaffolds were hardly seen in the newly formed cartilage-like tissue. The regenerated cartilages could not maintain their predesigned shape after a long period of in vivo culture due to the weakening of the mechanical strength of the constructs as a result of PLLA degradation. The regions occupied initially by PLLA scaffold were filled later by collagen type II secreted by the chondrocytes, but with no evident basophilic proteoglycan.

Translation from Research to Applications

The article summarizes the collective views expressed at the fourth session of the workshop Tissue Engineering--the Next Generation, which was devoted to the translation of results of tissue engineering research into applications. Ernst Hunziker described the paradigm of a dual translational approach, and argued that tissue engineering should be guided by the dimensions and physiological setting of the bodily compartment to be repaired. Myron Spector discussed collagen-glycosaminoglycan (GAG) scaffolds for musculoskeletal tissue engineering. Jeanette Libera focused on the biological and clinical aspects of cartilage tissue engineering, and described a completely autologous procedure for engineering cartilage using the patient's own chondrocytes and blood serum. Arthur Gertzman reviewed the applications of allograft tissues in orthopedic surgery, and outlined the potential of allograft tissues as models for biological and medical studies. Savio Woo discussed a list of functional tissue engineering approaches designed to restore the biochemical and biomechanical properties of injured ligaments and tendons to be closer to that of the normal tissues. Specific examples of using biological scaffolds that have chemoattractants as well as growth factors with unique contact guidance properties to improve their healing process were shown. Anthony Ratcliffe discussed the translation of the results of research into products that are profitable and meet regulatory requirements. Michael Lysaght challenged the proposition that commercial and clinical failures of early tissue engineering products demonstrate a need for more focus on basic research. Arthur Coury described the evolution of tissue engineering products based on the example of Genzyme, and how various definitions of success and failure can affect perceptions and policies relative to the status and advancement of the field of tissue engineering.

The effects of short-term stimulation on fibroblast spreading in an in vitro 3D system

Although the short-term effects of mechanical and biochemical stimulation on cell protein and DNA production have recently begun to be evaluated using 3D models, the effects that such stimulation have on cell morphology and adaptation remains unclear. Using a custom-built bioreactor, we were able to create a systematic model to examine the short-term effects of stimulation on cell morphology in a 3D model, specifically by evaluating cell spreading as the short-term indicator of cell adaptation. Fibroblasts were seeded on a porous poly(L-lactic acid) scaffold and cultured in a computerized bioreactor for 24 h under various uniaxial strains (0, 0.6, 3, 6%) and frequencies (0.0125, 0.125 Hz). Also, the effects of transforming growth factor (TGF-beta1) (1, 10, 100 ng/mL) were examined on static, nonstimulated cells-scaffold constructs after 24 h. Fibroblasts that had been subjected to mechanical stretching were found to exhibit significantly more spreading than the static control group. Conversely, TGF-beta1 between 1 and 100 ng/mL did not produce any significant difference in fibroblast spreading from the control groups after 24 h. Collectively, the findings suggest that cell morphology and adaptation may be affected by short-term mechanical stimulation, as seen by increased cell spreading by the fibroblasts under these experimental conditions.

Tissue engineering for anterior cruciate ligament reconstruction: a review of current strategies

The anterior cruciate ligament (ACL) is one the most commonly injured ligaments of the knee. Chronic ACL insufficiency can result in episodic instability, chondral and meniscal injury, and early osteoarthritis. The intra-articular environment of the ligament precludes normal healing and surgical replacement of the injured ligament is often mandated to restore stability. Current surgical strategies include the use of local autograft or allograft tissues for ligament reconstruction. These procedures have yielded superior long-term clinical results yet have the potential for serious associated morbidities. Existing limitations have prompted ongoing research designed to engineer a replacement ligament that will parallel the native ACL in both its biologic properties and mechanical durability. Ligament engineering necessitates the use of appropriate source cells and a growth matrix to support cell proliferation and collagen synthesis. The identification of appropriate growth modulators including both biochemical factors and mechanical stimuli are requisites for successful tissue growth. The characterization of the elements essential for successful graft development represents a significant challenge for investigators. This review examines the current literature regarding the potential and limitations of ligament engineering and describes the development of a novel 3-dimensional scaffold and bioreactor system at our institution.

Development of Ligament-Like Structural Organization and Properties in Cell-Seeded Collagen Scaffolds in vitro

Acute anterior cruciate ligament (ACL) injuries lead to poor joint function, instability, and eventually osteoarthritis if left untreated. Current surgical treatment options are not ideal; however, tissue engineering may provide mechanically sound, biocompatible reconstructions. Collagen fiber scaffolds were combined with fibroblast-seeded collagen gels and maintained in culture for up to 20 days. The tensile and viscoelastic behavior of the constructs closely mimicked that of natural ligament. Constructs' mechanical and viscoelastic properties did not degrade over time in culture, and peak stress was significantly higher for constructs with embedded fibroblasts. Immunocytochemical and histological analyses demonstrated cell proliferation and ligament-like organization. We have created an engineered tissue that closely approaches key mechanical and viscoelastic properties of the ACL, does not degrade after 20 days in culture, and is histologically similar to the native tissue. This study should aid in developing effective treatments for ACL injury.