Functional tissue engineering: the role of biomechanics

Human adipose-derived stem cells: potential clinical applications in surgery

Regenerative medicine is emerging as a rapidly evolving field of research and therapeutics. Stem cells hold great promise for future translational research and clinical applications in many fields. Much research has focused on mesenchymal stem cells isolated from bone marrow in vitro and in vivo; however, bone marrow procurement causes considerable discomfort to the patient and yields a relatively small number of harvested cells. By contrast, adipose tissue represents an abundant and easily accessible source of adult stem cells, termed adipose-derived stem cells (ADSCs), with the ability to equally differentiate along multiple lineage pathways. These stem cells have angiogenic properties, possibly because of their secretion of cytokines. They may also play a role in healing acute and chronic tissue damage. Subsequently, they have a wide range of potential clinical implications. This article reviews the potential preclinical and clinical applications of mesenchymal stem cells, especially ADSCs, in surgery.

Directional fluid flow enhances in vitro periosteal tissue growth and chondrogenesis on poly-epsilon-caprolactone scaffolds

The purpose of this study was to investigate the effect of directional fluid flow on periosteal chondrogenesis. Periosteal explants were harvested from 2-month-old rabbits and sutured onto poly-epsilon-caprolactone (PCL) scaffolds with the cambium layer facing away from the scaffolds. The periosteum/PCL composites were cultured in suspension in spinner flask bioreactors and exposed to various fluid flow velocities: 0, 20, 60, and 150 rpm for 4 h each day for 6 weeks. The application of fluid flow significantly increased percent cartilage yield in periosteal explants from 17% in the static controls to 65-75% under fluid flow (there was no significant difference between 20, 60, or 150 rpm). The size of the neocartilage was also significantly greater in explants exposed to fluid flow compared with static culture. The development of zonal organization within the engineered cartilage was observed predominantly in the tissue exposed to flow conditions. The Young's modulus of the engineered cartilage exposed to 60 rpm was significantly greater than the samples exposed to 150 and 20 rpm. These results demonstrate that application of directional fluid flow to periosteal explants secured onto PCL scaffolds enhances cell proliferation, chondrogenic differentiation, and cell organization and alters the biomechanical properties of the engineered cartilage.

A novel bioreactor for the dynamic stimulation and mechanical evaluation of multiple tissue-engineered constructs

Systematic advancements in the field of musculoskeletal tissue engineering require clear communication about the mechanical environments that promote functional tissue growth. To support the rapid discovery of effective mechanostimulation protocols, this study developed and validated a mechanoactive transduction and evaluation bioreactor (MATE). The MATE provides independent and consistent mechanical loading of six specimens with minimal hardware. The six individual chambers accurately applied static and dynamic loads (1 and 10 Hz) in unconfined compression from 0.1 to 10 N. The material properties of poly(ethylene glycol) diacrylate hydrogels and bovine cartilage were measured by the bioreactor, and these values were within 10% of the values obtained from a standard single-chamber material testing system. The bioreactor was able to detect a 1-day 12% reduction (2 kPa) in equilibrium modulus after collagenase was added to six collagenase sensitive poly(ethylene glycol) diacrylate hydrogels (p = 0.03). By integrating dynamic stimulation and mechanical evaluation into a single batch-testing research platform, the MATE can efficiently map the biomechanical development of tissue-engineered constructs during long-term culture.

Multifunctional hybrid three-dimensionally woven scaffolds for cartilage tissue engineering

The successful replacement of large-scale cartilage defects or osteoarthritic lesions using tissue-engineering approaches will likely require composite biomaterial scaffolds that have biomimetic mechanical properties and can provide cell-instructive cues to control the growth and differentiation of embedded stem or progenitor cells. This study describes a novel method of constructing multifunctional scaffolds for cartilage tissue engineering that can provide both mechanical support and biological stimulation to seeded progenitor cells. 3-D woven PCL scaffolds were infiltrated with a slurry of homogenized CDM of porcine origin, seeded with human ASCs, and cultured for up to 42 d under standard growth conditions. These constructs were compared to scaffolds derived solely from CDM as well as 3-D woven PCL fabric without CDM. While all scaffolds promoted a chondrogenic phenotype of the ASCs, CDM scaffolds showed low compressive and shear moduli and contracted significantly during culture. Fiber-reinforced CDM scaffolds and 3-D woven PCL scaffolds maintained their mechanical properties throughout the culture period, while supporting the accumulation of a cartilaginous extracellular matrix. These findings show that fiber-reinforced hybrid scaffolds can be produced with biomimetic mechanical properties as well as the ability to promote ASC differentiation and chondrogenesis in vitro.

Initial fiber alignment pattern alters extracellular matrix synthesis in fibroblast-populated fibrin gel cruciforms and correlates with predicted tension

Human dermal fibroblasts entrapped in fibrin gels cast in cross-shaped (cruciform) geometries with 1:1 and 1:0.5 ratios of arm widths were studied to assess whether tension and alignment of the cells and fibrils affected ECM deposition. The cruciforms of contrasting geometry (symmetric vs. asymmetric), which developed different fiber alignment patterns, were harvested at 2, 5, and 10 weeks of culture. Cruciforms were subjected to planar biaxial testing, polarimetric imaging, DNA and biochemical analyses, histological staining, and SEM imaging. As the cruciforms compacted and developed fiber alignment, fibrin was degraded, and elastin and collagen were produced in a geometry-dependent manner. Using a continuum mechanical model that accounts for direction-dependent stress due to cell traction forces and cell contact guidance with aligned fibers that occurs in the cruciforms, the mechanical stress environment was concluded to influence collagen deposition, with deposition being the greatest in the narrow arms of the asymmetric cruciform where stress was predicted to be the largest.

Supermacroprous chitosan-agarose-gelatin cryogels: in vitro characterization and in vivo assessment for cartilage tissue engineering

The study focuses on the synthesis of a novel polymeric scaffold having good porosity and mechanical characteristics synthesized by using natural polymers and their optimization for application in cartilage tissue engineering. The scaffolds were synthesized via cryogelation technology using an optimized ratio of the polymer solutions (chitosan, agarose and gelatin) and cross-linker followed by the incubation at sub-zero temperature (-12°C). Microstructure examination of the chitosan-agarose-gelatine (CAG) cryogels was done using scanning electron microscopy (SEM) and fluorescent microscopy. Mechanical analysis, such as the unconfined compression test, demonstrated that cryogels with varying chitosan concentrations, i.e. 0.5-1% have a high compression modulus. In addition, fatigue tests revealed that scaffolds are suitable for bioreactor studies where gels are subjected to continuous cyclic strain. In order to confirm the stability, cryogels were subjected to high frequency (5 Hz) with 30 per cent compression of their original length up to 1 × 10(5) cycles, gels did not show any significant changes in their mass and dimensions during the experiment. These cryogels have exhibited degradation capacity under aseptic conditions. CAG cryogels showed good cell adhesion of primary goat chondrocytes examined by SEM. Cytotoxicity of the material was checked by MTT assay and results confirmed the biocompatibility of the material. In vivo biocompatibility of the scaffolds was checked by the implantation of the scaffolds in laboratory animals. These results suggest the potential of CAG cryogels as a good three-dimensional scaffold for cartilage tissue engineering.

Cell-scaffold transplant of hydrogel seeded with rat bone marrow progenitors for bone regeneration

Bone is the second most frequently transplanted tissue in humans and efforts are focused on developing cell-scaffold constructs which can be employed for autologous implantation in place of allogenic transplants. The objective of the present study was to examine the efficacy of a gelatin-based hydrogel scaffold to support osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells (MSCs) and its application in a cranial defect model. MSCs which were cultured on hydrogel under osteogenic conditions demonstrated typical osteogenic differentiation which included cluster formation with positive Alizarin Red S staining, sedimentation of calcium phosphate as defined by SEM and EDS spectroscopy and expression of mRNA osteogenic markers. Empty scaffolds or those containing either differentiated cells or naïve cells were implanted into cranial defects of athymic nude mice and the healing process was followed by μCT. Substantial bone formation (65%) was observed with osteogenic cell-scaffold constructs when compared to the naïve cell construct (25%) and the cell free scaffold (10%). Results demonstrated the potential of hydrogel scaffolds to serve as a supportive carrier for bone marrow-derived MSCs.

Ligament-derived matrix stimulates a ligamentous phenotype in human adipose-derived stem cells

Human adipose stem cells (hASCs) can differentiate into a variety of phenotypes. Native extracellular matrix (e.g., demineralized bone matrix or small intestinal submucosa) can influence the growth and differentiation of stem cells. The hypothesis of this study was that a novel ligament-derived matrix (LDM) would enhance expression of a ligamentous phenotype in hASCs compared to collagen gel alone. LDM prepared using phosphate-buffered saline or 0.1% peracetic acid was mixed with collagen gel (COL) and was evaluated for its ability to induce proliferation, differentiation, and extracellular matrix synthesis in hASCs over 28 days in culture at different seeding densities (0, 0.25 x 10(6), 1 x 10(6), or 2 x 10(6) hASC/mL). Biochemical and gene expression data were analyzed using analysis of variance. Fisher's least significant difference test was used to determine differences between treatments following analysis of variance. hASCs in either LDM or COL demonstrated changes in gene expression consistent with ligament development. hASCs cultured with LDM demonstrated more dsDNA content, sulfated-glycosaminoglycan accumulation, and type I and III collagen synthesis, and released more sulfated-glycosaminoglycan and collagen into the medium compared to hASCs in COL (p <or= 0.05). Increased seeding density increased DNA content incrementally over 28 days in culture for LDM but not COL constructs (p <or= 0.05). These findings suggest that LDM can stimulate a ligament phenotype by hASCs, and may provide a novel scaffold material for ligament engineering applications.

Triggered drug release from dynamic microspheres via a protein conformational change

In this study we formed and characterized dynamic hydrogel microspheres in which a protein conformational change was used to control microsphere volume changes and the release of an encapsulated drug. In particular, a specific biochemical ligand, trifluoperazine, induced calmodulin's nanometer scale conformation change, which translated to a 48.7% microsphere volume decrease. This specific, ligand-induced volume change triggered the release of a model drug, vascular endothelial growth factor (VEGF), at pre-determined times. After release from the microspheres, 85.6 +/- 10.5% of VEGF was in its native conformation. Taken together, these results suggest that protein conformational change could serve as a useful mechanism to control drug release from dynamic hydrogels.

2010 Nicolas Andry Award: Multipotent adult stem cells from adipose tissue for musculoskeletal tissue engineering

BACKGROUND

Cell-based therapies such as tissue engineering provide promising therapeutic possibilities to enhance the repair or regeneration of damaged or diseased tissues but are dependent on the availability and controlled manipulation of appropriate cell sources.

QUESTIONS/PURPOSES

The goal of this study was to test the hypothesis that adult subcutaneous fat contains stem cells with multilineage potential and to determine the influence of specific soluble mediators and biomaterial scaffolds on their differentiation into musculoskeletal phenotypes.

METHODS

We reviewed recent studies showing the stem-like characteristics and multipotency of adipose-derived stem cells (ASCs), and their potential application in cell-based therapies in orthopaedics.

RESULTS

Under controlled conditions, ASCs show phenotypic characteristics of various cell types, including chondrocytes, osteoblasts, adipocytes, neuronal cells, or muscle cells. In particular, the chondrogenic differentiation of ASCs can be induced by low oxygen tension, growth factors such as bone morphogenetic protein-6 (BMP-6), or biomaterial scaffolds consisting of native tissue matrices derived from cartilage. Finally, focus is given to the development of a functional biomaterial scaffold that can provide ASC-based constructs with mechanical properties similar to native cartilage.

CONCLUSIONS

Adipose tissue contains an abundant source of multipotent progenitor cells. These cells show cell surface marker profiles and differentiation characteristics that are similar to but distinct from other adult stem cells, such as bone marrow mesenchymal stem cells (MSCs).

CLINICAL RELEVANCE

The availability of an easily accessible and reproducible cell source may greatly facilitate the development of new cell-based therapies for regenerative medicine applications in the musculoskeletal system.

Geometry and force control of cell function

Tissue engineering is becoming increasingly ambitious in its efforts to create functional human tissues, and to provide stem cell scientists with culture systems of high biological fidelity. Novel engineering designs are being guided by biological principles, in an attempt to recapitulate more faithfully the complexities of native cellular milieu. Three-dimensional (3D) scaffolds are being designed to mimic native-like cell environments and thereby elicit native-like cell responses. Also, the traditional focus on molecular regulatory factors is shifting towards the combined application of molecular and physical factors. Finally, methods are becoming available for the coordinated presentation of molecular and physical factors in the form of controllable spatial and temporal gradients. Taken together, these recent developments enable the interrogation of cellular behavior within dynamic culture settings designed to mimic some aspects of native tissue development, disease, or regeneration. We discuss here these advanced cell culture environments, with emphasis on the derivation of design principles from the development (the biomimetic paradigm) and the geometry-force control of cell function (the biophysical regulation paradigm).

A model for the stretch-mediated enzymatic degradation of silk fibers

To restore physiological function through regenerative medicine, biomaterials introduced into the body must degrade at a rate that matches new tissue formation. For effective therapies, it is essential that we understand the interaction between physiological factors, such as routine mechanical loading specific to sites of implantation, and the resultant rate of material degradation. These relationships are poorly characterized at this time. We hypothesize that mechanical forces alter the rates of remodeling of biomaterials, and this impact is modulated by the concentration of enzymes and the duration of the mechanical loads encountered in situ. To test this hypothesis we subjected silk fibroin fibers to repeated cyclic loading in the presence of enzymatic degradation (either alpha-chymotrypsin or Protease XIV) and recorded the stress-strain response. Data were collected daily for a duration of 2 weeks and compared to the control cases of stretched fibers in the presence of phosphate buffered saline or non-stretched samples in the presence of enzyme alone. We observed that incubation with proteases in the absence of mechanical loads causes a reduction of the ultimate tensile strength but no change in stiffness. However, cyclic loading caused the accumulation of residual strain and softening in the material's properties. We utilize these data to formulate a mathematical model to account for residual strain and reduction of mechanical properties during silk fiber degradation. Numerical predictions are in fair agreement with experimental data. The improved understanding of the degradation phenomenon will be significant in many clinical repair cases and may be synergistic to decrease silk's mechanical properties after in vivo implantation.

Functional properties of cell-seeded three-dimensionally woven poly(epsilon-caprolactone) scaffolds for cartilage tissue engineering

Articular cartilage possesses complex mechanical properties that provide healthy joints the ability to bear repeated loads and maintain smooth articulating surfaces over an entire lifetime. In this study, we utilized a fiber-reinforced composite scaffold designed to mimic the anisotropic, nonlinear, and viscoelastic biomechanical characteristics of native cartilage as the basis for developing functional tissue-engineered constructs. Three-dimensionally woven poly(epsilon-caprolactone) (PCL) scaffolds were encapsulated with a fibrin hydrogel, seeded with human adipose-derived stem cells, and cultured for 28 days in chondrogenic culture conditions. Biomechanical testing showed that PCL-based constructs exhibited baseline compressive and shear properties similar to those of native cartilage and maintained these properties throughout the culture period, while supporting the synthesis of a collagen-rich extracellular matrix. Further, constructs displayed an equilibrium coefficient of friction similar to that of native articular cartilage (mu(eq) approximately 0.1-0.3) over the prescribed culture period. Our findings show that three-dimensionally woven PCL-fibrin composite scaffolds can be produced with cartilage-like mechanical properties, and that these engineered properties can be maintained in culture while seeded stem cells regenerate a new, functional tissue construct.

Effects of tensile strain and fluid flow on osteoarthritic human chondrocyte metabolism in vitro

This study examined the hypothesis that tensile strain and fluid flow differentially influence osteoarthritic human chondrocyte metabolism. Primary high-density monolayer chondrocytes cultures were exposed to varying magnitudes of tensile strain and fluid-flow using a four-point bending system. Metabolic changes were quantified by real-time PCR measurement of aggrecan, IL-6, SOX-9, and type II collagen gene expression, and by determination of nitric oxide levels in the culture medium. A linear regression model was used to investigate the roles of strain, fluid flow, and their interaction on metabolic activity. Aggrecan, type II collagen, and SOX9 mRNA expression were negatively correlated to increases in applied strain and fluid flow. An effect of the strain on the induction of nitric oxide release and IL-6 gene expression varied by level of fluid flow (and visa versa). This interaction between strain and fluid flow was negative for nitric oxide and positive for IL-6. These results confirm that articular chondrocyte metabolism is responsive to tensile strain and fluid flow under in vitro loading conditions. Although the articular chondrocytes reacted to the mechanically applied stress, it was notable that there was a differential effect of tensile strain and fluid flow on anabolic and catabolic markers.

Passaged adult chondrocytes can form engineered cartilage with functional mechanical properties: a canine model

It was hypothesized that previously optimized serum-free culture conditions for juvenile bovine chondrocytes could be adapted to generate engineered cartilage with physiologic mechanical properties in a preclinical, adult canine model. Primary or passaged (using growth factors) adult chondrocytes from three adult dogs were encapsulated in agarose, and cultured in serum-free media with transforming growth factor-beta3. After 28 days in culture, engineered cartilage formed by primary chondrocytes exhibited only small increases in glycosaminoglycan content. However, all passaged chondrocytes on day 28 elaborated a cartilage matrix with compressive properties and glycosaminoglycan content in the range of native adult canine cartilage values. A preliminary biocompatibility study utilizing chondral and osteochondral constructs showed no gross or histological signs of rejection, with all implanted constructs showing excellent integration with surrounding cartilage and subchondral bone. This study demonstrates that adult canine chondrocytes can form a mechanically functional, biocompatible engineered cartilage tissue under optimized culture conditions. The encouraging findings of this work highlight the potential for tissue engineering strategies using adult chondrocytes in the clinical treatment of cartilage defects.

The impact of biomechanics in tissue engineering and regenerative medicine

Biomechanical factors profoundly influence the processes of tissue growth, development, maintenance, degeneration, and repair. Regenerative strategies to restore damaged or diseased tissues in vivo and create living tissue replacements in vitro have recently begun to harness advances in understanding of how cells and tissues sense and adapt to their mechanical environment. It is clear that biomechanical considerations will be fundamental to the successful development of clinical therapies based on principles of tissue engineering and regenerative medicine for a broad range of musculoskeletal, cardiovascular, craniofacial, skin, urinary, and neural tissues. Biomechanical stimuli may in fact hold the key to producing regenerated tissues with high strength and endurance. However, many challenges remain, particularly for tissues that function within complex and demanding mechanical environments in vivo. This paper reviews the present role and potential impact of experimental and computational biomechanics in engineering functional tissues using several illustrative examples of past successes and future grand challenges.

Tissue engineering strategies for the regeneration of orthopedic interfaces

A major focus in the field of orthopedic tissue engineering is the development of tissue engineered bone and soft tissue grafts with biomimetic functionality to allow for their translation to the clinical setting. One of the most significant challenges of this endeavor is promoting the biological fixation of these grafts with each other as well as the implant site. Such fixation requires strategic biomimicry to be incorporated into the scaffold design in order to re-establish the critical structure-function relationship of the native soft tissue-to-bone interface. The integration of distinct tissue types (e.g. bone and soft tissues such as cartilage, ligaments, or tendons), necessitates a multi-phased or stratified scaffold with distinct yet continuous tissue regions accompanied by a gradient of mechanical properties. This review discusses tissue engineering strategies for regenerating common tissue-to-tissue interfaces (ligament-to-bone, tendon-to-bone, or cartilage-to-bone), and the strategic biomimicry implemented in stratified scaffold design for multi-tissue regeneration. Potential challenges and future directions in this emerging field will also be presented. It is anticipated that interface tissue engineering will enable integrative soft tissue repair, and will be instrumental for the development of complex musculoskeletal tissue systems with biomimetic complexity and functionality.

Mechanical design criteria for intervertebral disc tissue engineering

Due to the inability of current clinical practices to restore function to degenerated intervertebral discs, the arena of disc tissue engineering has received substantial attention in recent years. Despite tremendous growth and progress in this field, translation to clinical implementation has been hindered by a lack of well-defined functional benchmarks. Because successful replacement of the disc is contingent upon replication of some or all of its complex mechanical behaviors, it is critically important that disc mechanics be well characterized in order to establish discrete functional goals for tissue engineering. In this review, the key functional signatures of the intervertebral disc are discussed and used to propose a series of native tissue benchmarks to guide the development of engineered replacement tissues. These benchmarks include measures of mechanical function under tensile, compressive, and shear deformations for the disc and its substructures. In some cases, important functional measures are identified that have yet to be measured in the native tissue. Ultimately, native tissue benchmark values are compared to measurements that have been made on engineered disc tissues, identifying where functional equivalence was achieved, and where there remain opportunities for advancement. Several excellent reviews exist regarding disc composition and structure, as well as recent tissue engineering strategies; therefore this review will remain focused on the functional aspects of disc tissue engineering.

The use of mesenchymal stem cells in collagen-based scaffolds for tissue-engineered repair of tendons

Tendon and ligament injuries are significant contributors to musculoskeletal injuries. Unfortunately, traditional methods of repair are not uniformly successful and can require revision surgery. Our research is focused on identifying appropriate animal injury models and using tissue-engineered constructs (TECs) from bone-marrow-derived mesenchymal stem cells and collagen scaffolds. Critical to this effort has been the development of functional tissue engineering (FTE). We first determine the in vivo mechanical environment acting on the tissue and then precondition the TECs in culture with aspects of these mechanical signals to improve repair outcome significantly. We describe here a detailed protocol for conducting several complete iterations around our FTE 'road map.' The in vitro portion, from bone marrow harvest to TEC collection, takes 54 d. The in vivo portion, from TEC implantation to limb harvest, takes 84 d. One complete loop around the tissue engineering road map, as presented here, takes 138 d to complete.

Mesenchymal Stem or Stromal Cells: Toward a Better Understanding of Their Biology?

The adult bone marrow has been generally considered to be composed of hematopoietic tissue and the associated supporting stroma. Within the latter compartment, a subset of cells with multipotent differentiation capacity exists, usually referred to as mesenchymal stem cells. Mesenchymal stem cells can easily be expanded ex vivo and induced to differentiate into several cell types, including osteoblasts, adipocytes and chondrocytes. Up to now, mesenchymal stem cells have gained wide popularity. Despite the rapid growth in this field, irritations remain with respect to the defining characteristics of these cells, including their differentiation potency, self-renewal and in vivo properties. As a consequence, there is a growing tendency to challenge the term mesenchymal stem cell, especially with respect to the stem cell characteristics. Here, we revisit the experimental origins of mesenchymal stem cells, their classical differentiation capacity into mesodermal lineages and their immunophenotype in order to assess their stemness and function. Based on these essentials, it has to be revisited if the designation as a stem cell remains an appropriate term.

Use of an insulating mask for controlling anisotropy in multilayer electrospun scaffolds for tissue engineering

Tissue engineering of various musculoskeletal or cardiovascular tissues requires scaffolds with controllable mechanical anisotropy. However, native tissues also exhibit significant inhomogeneity in their mechanical properties, and the principal axes of anisotropy may vary with site or depth from the tissue surface. Thus, techniques to produce multilayered biomaterial scaffolds with controllable anisotropy may provide improved biomimetic properties for functional tissue replacements. In this study, poly(ε-caprolactone) scaffolds were electrospun onto a collecting electrode that was partially covered by rectangular or square shaped insulating masks. The use of a rectangular mask resulted in aligned scaffolds that were significantly stiffer in tension in the axial direction than the transverse direction at 0 strain (22.9 ± 1.3 MPa axial, 16.1 ± 0.9 MPa transverse), and at 0.1 strain (4.8 ± 0.3 MPa axial, 3.5 ± 0.2 MPa transverse). The unaligned scaffolds, produced using a square mask, did not show this anisotropy, with similar stiffness in the axial and transverse directions at 0 strain (19.7 ± 1.4 MPa axial, 20.8 ± 1.3 MPa transverse) and 0.1 strain (4.4 ± 0.2 MPa axial, 4.6 ± 0.3 MPa, transverse). Aligned scaffolds also induced alignment of adipose stem cells near the expected axis on aligned scaffolds (0.015 ± 0.056 rad), while on the unaligned scaffolds, their orientation showed more variation and was not along the expected axis (1.005 ± 0.225 rad). This method provides a novel means of creating multilayered electrospun scaffolds with controlled anisotropy for each layer, potentially providing a means to mimic the complex mechanical properties of various native tissues.

Three-dimensional in vitro effects of compression and time in culture on aggregate modulus and on gene expression and protein content of collagen type II in murine chondrocytes

The objectives of this study were to determine how culture time and dynamic compression, applied to murine chondrocyte-agarose constructs, influence construct stiffness, expression of col2 and type II collagen. Chondrocytes were harvested from the ribs of six newborn double transgenic mice carrying transgenes that use enhanced cyan fluorescent protein (ECFP) and green fluorescent protein (GFP-T) as reporters for expression from the col2a1 and col1a1 promoters, respectively. Sixty-three constructs (8 mm diameter x 3 mm thick) per animal were created by seeding chondrocytes (10 x 10(6) per mL) in agarose gel (2% w/v). Twenty-eight constructs from each animal were stimulated for 7, 14, 21, or 28 days in a custom bioreactor housed in an electromagnetic system. Twenty-eight constructs exposed to identical culture conditions but without mechanical stimulation served as nonstimulated controls for 7, 14, 21, and 28 days. The remaining seven constructs served as day 0 controls. Fluorescing cells with rounded morphology were present in all constructs at all five time points. Seven, 14, 21, and 28 days of stimulation significantly increased col2 expression according to ECFP fluorescence and messenger RNA expression according to quantitative reverse transcriptase polymerase chain reaction. Col2 gene expression in stimulated and nonstimulated constructs showed initial increases up to day 14 and then showed decreases by day 28. Stimulation significantly increased type II collagen content at 21 and 28 days and aggregate modulus only at 28 days. There was a significant increase in aggregate modulus in stimulated constructs between day 0 and 7 and between day 21 and day 28. This study reveals that compressive mechanical stimulation is a potent stimulator of col2 gene expression that leads to measurable but delayed increases in protein (type II collagen) and then biomechanical stiffness. Future studies will examine the effects of components of the mechanical signal in culture and address the question of whether such in vitro improvements in tissue-engineered constructs enhance repair outcomes after surgery.

Chondroitinase ABC treatment results in greater tensile properties of self-assembled tissue-engineered articular cartilage

Collagen content and tensile properties of engineered articular cartilage have remained inferior to glycosaminoglycan (GAG) content and compressive properties. Based on a cartilage explant study showing greater tensile properties after chondroitinase ABC (C-ABC) treatment, C-ABC as a strategy for cartilage tissue engineering was investigated. A scaffold-less approach was employed, wherein chondrocytes were seeded into non-adherent agarose molds. C-ABC was used to deplete GAG from constructs 2 weeks after initiating culture, followed by 2 weeks culture post-treatment. Staining for GAG and type I, II, and VI collagen and transmission electron microscopy were performed. Additionally, quantitative total collagen, type I and II collagen, and sulfated GAG content were measured, and compressive and tensile mechanical properties were evaluated. At 4 wks, C-ABC treated construct ultimate tensile strength and tensile modulus increased 121% and 80% compared to untreated controls, reaching 0.5 and 1.3 MPa, respectively. These increases were accompanied by increased type II collagen concentration, without type I collagen. As GAG returned, compressive stiffness of C-ABC treated constructs recovered to be greater than 2 wk controls. C-ABC represents a novel method for engineering functional articular cartilage by departing from conventional anabolic approaches. These results may be applicable to other GAG-producing tissues functioning in a tensile capacity, such as the musculoskeletal fibrocartilages.

In situ deformation of cartilage in cyclically loaded tibiofemoral joints by displacement-encoded MRI

OBJECTIVES

Cartilage displacement and strain patterns were documented noninvasively in intact tibiofemoral joints in situ by magnetic resonance imaging (MRI). This study determined the number of compressive loading cycles required to precondition intact joints prior to imaging, the spatial distribution of displacements and strains in cartilage using displacement-encoded MRI, and the depth-dependency of these measures across specimens.

DESIGN

Juvenile porcine tibiofemoral joints were cyclically compressed at one and two times body weight at 0.1 Hz to achieve a quasi-steady state load-displacement response. A 7.0 T MRI scanner was used for displacement-encoded imaging with stimulated echoes and a fast spin echo acquisition (DENSE-FSE) in eight intact joints. Two-dimensional displacements and strains were determined throughout the thickness of the tibial and femoral cartilage and then normalized over the tissue thickness.

RESULTS

Two-dimensional displacements and strains were heterogeneous through the depth of femoral and tibial cartilage under cyclic compression. Strains in the loading direction were compressive and were maximal in the middle zone of femoral and tibial cartilage, and tensile strains were observed in the direction transverse to loading.

CONCLUSIONS

This study determined the depth-dependent displacements and strains in intact juvenile porcine tibiofemoral joints using displacement-encoded imaging. Displacement and strain distributions reflect the heterogeneous biochemistry of cartilage and the biomechanical response of the tissue to compression in the loading environment of an intact joint. This unique information about the biomechanics of cartilage has potential for comparisons of healthy and degenerated tissue and in the design of engineered replacement tissues.

Tensile stimulation of murine stem cell-collagen sponge constructs increases collagen type I gene expression and linear stiffness

The objectives of this study were to determine how tensile stimulation delivered up to 14 days in culture influenced type I collagen gene expression in stem cells cultured in collagen sponges, and to establish if gene expression, measured using a fluorescence method, correlates with an established method, real-time quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). Using a novel model system, mesenchymal stem cells were harvested from six double transgenic mice in which the type I and type II collagen promoters were linked to green fluorescent protein-topaz and enhanced cyan fluorescent protein, respectively. Tissue-engineered constructs were created by seeding 0.5 x 10(6) mesenchymal stem cells onto type I collagen sponge scaffolds in a silicone dish. Constructs were then transferred to a custom pneumatic mechanical stimulation system housed in a standard incubator and stimulated for 5 h=day in tension for either 7 or 14 days using a repeated profile (2.4% peak strain for 20 s at 1 Hz followed by a rest period at 0% strain for 100 s). Control specimens were exposed to identical culture conditions but without mechanical stimulation. At three time points (0, 7, and 14 days), constructs were then prepared for evaluation of gene expression using fluorescence analysis and qRT-PCR, and the remaining constructs were failed in tension. Both analytical methods showed that constructs stimulated for 7 and 14 days showed significantly higher collagen type I gene expression than nonstimulated controls at the same time interval. Gene expression measured using qRT-PCR and fluorescence analysis was positively correlated (r = 0.9). Linear stiffness of stimulated constructs was significantly higher at both 7 and 14 days than that of nonstimulated controls at the same time intervals. Linear stiffness of the stimulated constructs at day 14 was significantly different from that of day 7. Future studies will vary the mechanical signal to optimize type I collagen gene expression to improve construct biomechanics and in vivo tendon repair.

Bioengineering challenges for heart valve tissue engineering

Surgical replacement of diseased heart valves by mechanical and tissue valve substitutes is now commonplace and enhances survival and quality of life for many patients. However, repairs of congenital deformities require very small valve sizes not commercially available. Further, a fundamental problem inherent to the use of existing mechanical and biological prostheses in the pediatric population is their failure to grow, repair, and remodel. It is believed that a tissue engineered heart valve can accommodate many of these requirements, especially those pertaining to somatic growth. This review provides an overview of the field of heart valve tissue engineering, including recent trends, with a focus on the bioengineering challenges unique to heart valves. We believe that, currently, the key bioengineering challenge is to determine how biological, structural, and mechanical factors affect extracellular matrix (ECM) formation and in vivo functionality. These factors are fundamental to any approach toward developing a clinically viable tissue engineered heart valve (TEHV), regardless of the particular approach. Critical to the current approaches to TEHVs is scaffold design, which must simultaneously provide function (valves must function from the time of implant) as well as stress transfer to the new ECM. From a bioengineering point of view, a hierarchy of approaches will be necessary to connect the organ-tissue relationships with underpinning cell and sub-cellular events. Overall, such approaches need to be structured to address these fundamental issues to lay the basis for TEHVs that can be developed and designed according to truly sound scientific and engineering principles.

Advanced material strategies for tissue engineering scaffolds

Tissue engineering seeks to restore the function of diseased or damaged tissues through the use of cells and biomaterial scaffolds. It is now apparent that the next generation of functional tissue replacements will require advanced material strategies to achieve many of the important requirements for long-term success. Here we provide representative examples of engineered skeletal and myocardial tissue constructs in which scaffolds were explicitly designed to match native tissue mechanical properties as well as to promote cell alignment. We discuss recent progress in microfluidic devices that can potentially serve as tissue engineering scaffolds, since mass transport via microvascular-like structures will be essential in the development of tissue engineered constructs on the length scale of native tissues. Given the rapid evolution of the field of tissue engineering, it is important to consider the use of advanced materials in light of the emerging role of genetics, growth factors, bioreactors, and other technologies.

Bioreactor-based roadmap for the translation of tissue engineering strategies into clinical products

Despite the compelling clinical need to regenerate damaged tissues/organs, impressive advances in the field of tissue engineering have yet to result in viable engineered tissue products with widespread therapeutic adoption. Although bioreactor systems have been proposed as a key factor in the manufacture of standardized and cost-effective engineered products, this concept appears slow to be embraced and implemented. Here we address scientific, regulatory and commercial challenges intrinsic to the bioreactor-based translation of tissue engineering models into clinical products, proposing a roadmap for the implementation of a new paradigm. The roadmap highlights that bioreactors must be implemented throughout product development, allowing scientific, medical, industrial and regulatory parties to address basic research questions, conduct sound pre-clinical studies and ultimately facilitating effective commercialization of engineered clinical products.

The treatment of collagen fibrils by tissue transglutaminase to promote vascular smooth muscle cell contractile signaling

The enzyme tissue transglutaminase 2 (TG2) appears to play an important role in several physiological processes such as wound healing, the progression of cancer and of vascular disease. Additionally, TG2 has been proposed as a means of stabilizing collagen extracellular matrix (ECM) scaffolds for tissue engineering applications. In this report, we examined the effect of TG2 treatment on the mechanical properties of the ECM, and associated cell responses. Using a model ECM of fibrillar collagen, we quantitatively examined vascular smooth muscle cell (vSMC) response to untreated, or TG2 treated collagen. We show that cells respond to TG2 treated collagen with increased spreading, an increase in contractile response as indicated by elevated F-actin polymerization and myosin light chain phosphorylation, and increased proliferation, without apparent changes in integrin specificity or matrix topography. Comparative atomic force microscopy loading studies indicate that TG2 treated fibrils are 3 times more resistant to shearing force from an AFM tip than untreated fibrils. The data suggest that TG2 treatment of collagen increases matrix mechanical stiffness, which apparently alters the contractile and proliferative response of vSMC.

Articular cartilage deformation determined in an intact tibiofemoral joint by displacement-encoded imaging

This study demonstrates the in vitro displacement and strain of articular cartilage in a cyclically-compressed and intact joint using displacement-encoded imaging with stimulated echoes (DENSE) and fast spin echo (FSE). Deformation and strain fields exhibited complex and heterogeneous patterns. The displacements in the loading direction ranged from -1688 to -1481 microm in the tibial cartilage and from -1601 to -764 microm in the femoral cartilage. Corresponding strains ranged from -9.8% to 0.7% and from -4.3% to 0.0%. The displacement and strain precision were determined to be 65 microm and less than 0.2%, respectively. Displacement-encoded magnetic resonance imaging is capable of determining the nonuniform displacements and strains in the articular cartilage of an intact joint to a high precision. Knowledge of these nonuniform strains is critical for the in situ characterization of normal and diseased tissue, as well as the comprehensive evaluation of repair constructs designed using regenerative medicine.

On the biomechanics of heart valve function

Heart valves (HVs) are fluidic control components of the heart that ensure unidirectional blood flow during the cardiac cycle. However, this description does not adequately describe the biomechanical ramifications of their function in that their mechanics are multi-modal. Moreover, they must replicate their cyclic function over an entire lifetime, with an estimated total functional demand of least 3x10(9) cycles. The focus of the present review is on the functional biomechanics of heart valves. Thus, the focus of the present review is on functional biomechanics, referring primarily to biosolid as well as several key biofluid mechanical aspects underlying heart valve physiological function. Specifically, we refer to the mechanical behaviors of the extracellular matrix structural proteins, underlying cellular function, and their integrated relation to the major aspects of valvular hemodynamic function. While we focus on the work from the author's laboratories, relevant works of other investigators have been included whenever appropriate. We conclude with a summary of important future trends.

Anterior cruciate ligament regeneration using mesenchymal stem cells and silk scaffold in large animal model

Although in vivo studies in small animal model show the ligament regeneration by implanting mesenchymal stem cells (MSCs) and silk scaffold, large animal studies are still needed to evaluate the silk scaffold before starting a clinical trial. The aim of this study is to regenerate anterior cruciate ligament (ACL) in pig model. The micro-porous silk mesh was fabricated by incorporating silk sponges into knitted silk mesh with lyophilization. Then the scaffold was prepared by rolling the micro-porous silk mesh around a braided silk cord to produce a tightly wound shaft. In vitro study indicated that MSCs proliferated profusely on scaffold and differentiated into fibroblast-like cells by expressing collagen I, collagen III and tenascin-C genes in mRNA level. Then the MSCs-seeded scaffold was implanted in pig model to regenerate ACL. At 24 weeks postoperatively, the MSCs in regenerated ligament exhibited fibroblast morphology. The key ligament-specific extracellular matrix components were produced prominently and indirect ligament-bone insertion with three zones (bone, Sharpey's fibers and ligament) was observed. Although there was remarkable scaffold degradation, the maximum tensile load of regenerated ligament could be maintained after 24 weeks of implantation. In conclusion, the results imply that silk-based material has great potentials for clinical applications.

Using functional tissue engineering and bioreactors to mechanically stimulate tissue-engineered constructs

Bioreactors precondition tissue-engineered constructs (TECs) to improve integrity and hopefully repair. In this paper, we use functional tissue engineering to suggest criteria for preconditioning TECs. Bioreactors should (1) control environment during mechanical stimulation; (2) stimulate multiple constructs with identical or individual waveforms; (3) deliver precise displacements, including those that mimic in vivo activities of daily living (ADLs); and (4) adjust displacement patterns based on reaction loads and biological activity. We apply these criteria to three bioreactors. We have placed a pneumatic stimulator in a conventional incubator and stretched four constructs in each of five silicone dishes. We have also programmed displacement-limited stimuli that replicate frequencies and peak in vivo patellar tendon (PT) strains. Cellular activity can be monitored from spent media. However, our design prevents direct TEC force measurement. We have improved TEC stiffness as well as PT repair stiffness and shown correlations between the two. We have also designed an incubator to fit within each of two electromagnetic stimulators. Each incubator provides cell viability like a commercial incubator. Multiple constructs are stimulated with precise displacements that can mimic ADL strain patterns and record individual forces. Future bioreactors could be further improved by controlling and measuring TEC displacements and forces to create more functional tissues for surgeons and their patients.

Fibril microstructure affects strain transmission within collagen extracellular matrices

The next generation of medical devices and engineered tissues will require development of scaffolds that mimic the structural and functional properties of the extracellular matrix (ECM) component of tissues. Unfortunately, little is known regarding how ECM microstructure participates in the transmission of mechanical load information from a global (tissue or construct) level to a level local to the resident cells ultimately initiating relevant mechanotransduction pathways. In this study, the transmission of mechanical strains at various functional levels was determined for three-dimensional (3D) collagen ECMs that differed in fibril microstructure. Microstructural properties of collagen ECMs (e.g., fibril density, fibril length, and fibril diameter) were systematically varied by altering in vitro polymerization conditions. Multiscale images of the 3D ECM macro- and microstructure were acquired during uniaxial tensile loading. These images provided the basis for quantification and correlation of strains at global and local levels. Results showed that collagen fibril microstructure was a critical determinant of the 3D global and local strain behaviors. Specifically, an increase in collagen fibril density reduced transverse strains in both width and thickness directions at both global and local levels. Similarly, collagen ECMs characterized by increased fibril length and decreased fibril diameter exhibited increased strain in width and thickness directions in response to loading. While extensional strains measured globally were equivalent to applied strains, extensional strains measured locally consistently underpredicted applied strain levels. These studies demonstrate that regulation of collagen fibril microstructure provides a means to control the 3D strain response and strain transfer properties of collagen-based ECMs.

Catabolic responses of chondrocyte-seeded peptide hydrogel to dynamic compression

This study investigated the role of matrix metalloproteases and aggrecanases during dynamic compression-induced aggrecan catabolism in chondrocyte-seeded self-assembling peptide hydrogel. One- to two-week-old bovine chondrocytes were encapsulated into peptide hydrogel and cultured for 14 days prior to the application of an alternate day loading protocol. Dynamic compression-induced aggrecan catabolism was explored by evaluating GAG loss to the culture medium, zymography for matrix metalloproteases (MMPs), gene expression of MMPs and ADAMTS proteases, and Western blot analysis for aggrecan fragments. The application of loading over 4 days increased GAG loss to the medium three- to four-fold relative to free-swelling controls. Zymogram analysis detected increased concentrations of latent MMP-9 and MMP-3 in the culture medium relative to free-swelling culture. Real-time PCR showed expression levels of MMPs and ADAMTS proteases in loaded samples that ranged from 2.5- to 95-fold higher than free-swelling culture. Aggrecan fragment analysis did not detect small (50-80 kDa) molecular weight fragments in free-swelling culture; however, dynamic compression samples contained 60-80 kDa fragments that were detected by both anti-G1 and NITEGE probes, demonstrating ADAMTS but not MMP degradation. These data suggest that partially mature cartilage tissue engineering constructs may be susceptible to catabolic degradation.

Biomechanics: cell research and applications for the next decade

With the recent revolution in Molecular Biology and the deciphering of the Human Genome, understanding of the building blocks that comprise living systems has advanced rapidly. We have yet to understand, however, how the physical forces that animate life affect the synthesis, folding, assembly, and function of these molecular building blocks. We are equally uncertain as to how these building blocks interact dynamically to create coupled regulatory networks from which integrative biological behaviors emerge. Here we review recent advances in the field of biomechanics at the cellular and molecular levels, and set forth challenges confronting the field. Living systems work and move as multi-molecular collectives, and in order to understand key aspects of health and disease we must first be able to explain how physical forces and mechanical structures contribute to the active material properties of living cells and tissues, as well as how these forces impact information processing and cellular decision making. Such insights will no doubt inform basic biology and rational engineering of effective new approaches to clinical therapy.

One breath closer to making engineered tissues a clinical reality

Reported recently in Lancet, Macchiarini and colleagues (2008) implanted a living tissue-engineered airway in a female patient. The restoration of the patient's quality of life testifies to this successful translation of benchtop to bedside studies and provides promise for the application of regenerative medicine strategies to other clinical disorders.