Delivering Mechanical Stimulation to Cells: State of the Art in Materials and Devices Design

Biochemical signals, such as growth factors, cytokines, and transcription factors are known to play a crucial role in regulating a variety of cellular activities as well as maintaining the normal function of different tissues and organs. If the biochemical signals are assumed to be one side of the coin, the other side comprises biophysical cues. There is growing evidence showing that biophysical signals, and in particular mechanical cues, also play an important role in different stages of human life ranging from morphogenesis during embryonic development to maturation and maintenance of tissue and organ function throughout life. In order to investigate how mechanical signals influence cell and tissue function, tremendous efforts have been devoted to fabricating various materials and devices for delivering mechanical stimuli to cells and tissues. Here, an overview of the current state of the art in the design and development of such materials and devices is provided, with a focus on their design principles, and challenges and perspectives for future research directions are highlighted.

Additive manufacturing of cartilage-mimetic scaffolds as off-the-shelf implants for joint regeneration

Biomimetic scaffolds that provide a tissue-specific environment to cells are particularly promising for tissue engineering and regenerative medicine applications. The goal of this study was to integrate emerging additive manufacturing and biomaterial design strategies to produce articular cartilage (AC) mimetic scaffolds that could be used as 'off-the-shelf' implants for joint regeneration. To this end alginate sulfate, a sulfated glycosaminoglycan (sGAG) mimic, was used to functionalize porous alginate-based scaffolds and to support the sustained release of transforming growth factor-β3 (TGF-β3). Covalent crosslinking dramatically improved the elasticity of the alginate/alginate sulfate scaffolds, while scaffold architecture could be tailored using a directional freezing technique. Introducing such an anisotropic architecture was found to promote mesenchymal stem cell (MSC) infiltration into the scaffold and to direct the orientation of the deposited extracellular matrix, leading to the development of cartilage tissue with a biomimetic zonal architecture. In vitro experiments also demonstrated the capacity of the sulfated scaffolds to both enhance chondrogenesis of MSCs and to control the release of TGF-β3, leading to the development of a tissue rich in sGAG and type II collagen. The scaffolds were further reinforced with a 3D printed PLCL framework, leading to composite implants that were more elastic than those reinforced with PCL, and which better mimicked the bulk mechanical properties of native cartilage tissue. The ability of this composite scaffold to support chondrogenesis was then confirmed within a dynamic culture system. Altogether, these findings demonstrate the potential of such biomimetic scaffolds as putative 'single-stage' or 'off-the-shelf' strategies for articular cartilage regeneration.

Functional testing on engineered cartilage to identify the role played by shearing

Compressive loading is crucial for tissue regeneration in cartilage; however, the role played by shearing induced from translational or rotational motion of the knee joint has yet to be identified. This study aims at investigating the effects of in vivo like dynamic load-compression integrated with shearing on tissue regeneration, particularly to identify the role played by shearing induced from rotational motion. Tissue samples fabricated from a calcium alginate hydrogel embedded with chondrocytes were subjected to a dynamic tissue culture. Three culturing regimes were included: a static culture control (CON), compression combined with shearing induced from translational motion (CS), and compression combined with shearing induced from both translational and rotational motion (CSR). The results indicate that the CS group has a significantly larger chondrocyte proliferation rate (p < .01), and that the CSR group has no advantages over the CS group. However, the CSR group was found to have a marked influence on the matrix synthesis compared to that of the CS group (p < .01). It can be concluded that shearing from individual joint motions offers a different contribution to the chondrocyte proliferation, matrix synthesis, and phenotype maintenance, and better insight into these individual roles will be necessary for determining the efficacy of in vivo/vitro cartilageous tissue functionalization.

Enhanced Cartilaginous Tissue Formation with a Cell Aggregate-Fibrin-Polymer Scaffold Complex

Cell density is one of the factors required in the preparation of engineered cartilage from mesenchymal stem cells (MSCs). Additionally, it is well known for having a significant role in chemical and physical stimulations when stem cells undergo chondrogenic differentiation. Here, we developed an engineered cartilage with a cell aggregate-hydrogel-polymer scaffold complex capable of inducing the effective regeneration of cartilage tissue similar to natural cartilage while retaining a high mechanical strength, flexibility, and morphology. Cell aggregates were generated by the hanging drop method with rabbit bone marrow stromal cells (BMSCs), and poly (lactide-co-caprolactone) (PLCL) scaffolds were fabricated with 78.3 ± 5.3% porosity and a 300⁻500 μm pore size with a gel-pressing method. We prepared the cell aggregate-fibrin-poly (lactide-co-caprolactone) (PLCL) scaffold complex, in which the cell aggregates were evenly dispersed in the fibrin, and they were immobilized onto the surface of the polymer scaffold while filling up the pores. To examine the chondrogenic differentiation of seeded BMSCs and the formation of chondral extracellular matrix onto the complexes, they were cultured in vitro or subcutaneously implanted into nude mice for up to eight weeks. The results of the in vitro and in vivo studies revealed that the accumulation of the chondral extracellular matrices was increased on the cell aggregate-fibrin-PLCL scaffold complexes (CAPs) compared to the single cell-fibrin-PLCL scaffold complexes (SCPs). Additionally, we examined whether the mature and well-developed cartilaginous tissues and lacunae structures typical of mature cartilage were evenly distributed in the CAPs. Consequently, the cell aggregates in the hybrid scaffolds of fibrin gels and elastic PLCL scaffolds can induce themselves to differentiate into chondrocytes, maintain their phenotypes, enhance glycosaminoglycan (GAG) production, and improve the quality of cartilaginous tissue formed in vitro and in vivo.

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

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

Bi-layered PLCL/(PLGA/β-TCP) composite scaffold for osteochondral tissue engineering

There have been many studies published on bi-layered osteochondral scaffolds that can induce tissue ingrowth for cartilage and bone. However, delivering different signals to each tissue effectively and enabling maturation into complex composite tissues remain challenging. In this study, an osteochondral composite scaffold was fabricated by combining a sintering method and a gel pressing method with poly(lactide-co-glycolide), beta-tricalcium phosphate, and poly(lactide-co-caprolactone). The composite scaffold constituted a poly(lactide-co-glycolide)/beta-tricalcium phosphate scaffold, which has osteoconduction activity for bone regeneration, and an elastic poly(lactide-co-caprolactone) scaffold, which has mechano-active properties for cartilage regeneration. To examine mechanical and biological properties, tensile tests, porosimetry, electron microscopy, and in vitro and in vivo experiments were performed. From the results, we confirmed that the scaffold had a homogeneously interconnected porous structure without a skin layer that exhibited exposed bioceramics onto the scaffold surface. Furthermore, it was shown that poly(lactide-co-glycolide)/beta-tricalcium phosphate of the osteochondral composite scaffold improved osteogenic differentiation and bone tissue formation. Also, chondrogenic differentiation of seeded cells was sustained on the highly elastic poly(lactide-co-caprolactone) of the osteochondral composite scaffold, and the amount of chondral extracellular matrix was increased significantly. Consequently, our osteochondral composite scaffolds may have the appropriate physiological and biological properties for regeneration of osteochondral composites.

Blends and Nanocomposite Biomaterials for Articular Cartilage Tissue Engineering

This review provides a comprehensive assessment on polymer blends and nanocomposite systems for articular cartilage tissue engineering applications. Classification of various types of blends including natural/natural, synthetic/synthetic systems, their combination and nanocomposite biomaterials are studied. Additionally, an inclusive study on their characteristics, cell responses ability to mimic tissue and regenerate damaged articular cartilage with respect to have functionality and composition needed for native tissue, are also provided.

Synthesis and characterization of the biodegradable and elastic terpolymer poly(glycolide-co-L-lactide-co-ϵ-caprolactone) for mechano-active tissue engineering

We synthesized a series of tri-component biodegradable copolymers with elastic characteristics by ring-opening copolymerization of cyclic lactones, that is, glycolide, L-lactide, and ϵ-caprolactone, in the presence of stannous octoate as a catalyst. We evaluated the physical and chemical characteristics of poly(glycolide-co-L-lactide-co-ϵ-caprolactone) (PGLCL) copolymers. The synthesized PGLCL had a high molecular weight of about 100 kD and an amorphous structure. It was confirmed that the physical and chemical properties of these terpolymers could be modulated by adjusting copolymer composition. PGLCL films exhibited rubber-like elasticity and showed almost complete recovery when subjected to 50% of the tensile strain. To examine the biodegradability of the PGLCL copolymers, we performed in vitro degradation tests for 12 weeks and observed changes in molecular weight, gross weight, and composition. These results showed that the glycolide was degraded most quickly and that ϵ-caprolactone was the slowest to degrade. Additionally, cytotoxicity tests revealed that none of the polymers were toxic. In summary, the mechanical properties and biodegradability of PGLCL terpolymers could be controlled by changing the monomer content, which may be useful for a wide range of tissue engineering applications based on mechanical property requirements.

Functional biomaterials for cartilage regeneration

The injury and degeneration of articular cartilage and associated arthritis are leading causes of disability worldwide. Cartilage tissue engineering as a treatment modality for cartilage defects has been investigated for over 20 years. Various scaffold materials have been developed for this purpose, but has yet to achieve feasibility and effectiveness for widespread clinical use. Currently, the regeneration of articular cartilage remains a formidable challenge, due to the complex physiology of cartilage tissue and its poor healing capacity. Although intensive research has been focused on the developmental biology and regeneration of cartilage tissue and a diverse plethora of biomaterials have been developed for this purpose, cartilage regeneration is still suboptimal, such as lacking a layered structure, mechanical mismatch with native cartilage and inadequate integration between native tissue and implanted scaffold. The ideal scaffold material should have versatile properties that actively contribute to cartilage regeneration. Functional scaffold materials may overcome the various challenges faced in cartilage tissue engineering by providing essential biological, mechanical, and physical/chemical signaling cues through innovative design. This review thus focuses on the complex structure of native articular cartilage, the critical properties of scaffolds required for cartilage regeneration, present strategies for scaffold design, and future directions for cartilage regeneration with functional scaffold materials.

Dynamic compression of rabbit adipose-derived stem cells transfected with insulin-like growth factor 1 in chitosan/gelatin scaffolds induces chondrogenesis and matrix biosynthesis

Articular cartilage is routinely subjected to mechanical forces and growth factors. Adipose-derived stem cells (ASCs) are multi-potent adult stem cells and capable of chondrogenesis. In the present study, we investigated the comparative and interactive effects of dynamic compression and insulin-like growth factor-I (IGF-I) on the chondrogenesis of rabbit ASCs in chitosan/gelatin scaffolds. Rabbit ASCs with or without a plasmid overexpressing of human IGF-1 were cultured in chitosan/gelatin scaffolds for 2 days, then subjected to cyclic compression with 5% strain and 1 Hz for 4 h per day for seven consecutive days. Dynamic compression induced chondrogenesis of rabbit ASCs by activating calcium signaling pathways and up-regulating the expression of Sox-9. Dynamic compression plus IGF-1 overexpression up-regulated expression of chondrocyte-specific extracellular matrix genes including type II collagen, Sox-9, and aggrecan with no effect on type X collagen expression. Furthermore, dynamic compression and IGF-1 expression promoted cellular proliferation and the deposition of proteoglycan and collagen. Intracellular calcium ion concentration and peak currents of Ca(2+) ion channels were consistent with chondrocytes. The tissue-engineered cartilage from this process had excellent mechanical properties. When applied together, the effects achieved by the two stimuli (dynamic compression and IGF-1) were greater than those achieved by either stimulus alone. Our results suggest that dynamic compression combined with IGF-1 overexpression might benefit articular cartilage tissue engineering in cartilage regeneration.

Improved mesenchymal stem cells attachment and in vitro cartilage tissue formation on chitosan-modified poly(L-lactide-co-epsilon-caprolactone) scaffold

Considering the load-bearing physiological requirement of articular cartilage, scaffold for cartilage tissue engineering should exhibit appropriate mechanical responses as natural cartilage undergoing temporary deformation on loading with little structural collapse, and recovering to the original geometry on unloading. A porous elastomeric poly l-lactide-co-ɛ-caprolactone (PLCL) was generated and crosslinked at the surface to chitosan to improve its wettability. Human bone marrow derived mesenchymal stem cells (MSC) attachment, morphological change, proliferation and in vitro cartilage tissue formation on the chitosan-modified PLCL scaffold were compared with the unmodified PLCL scaffold. Chitosan surface promoted more consistent and even distribution of the seeded MSC within the scaffold. MSC rapidly adopted a distinct spread-up morphology on attachment on the chitosan-modified PLCL scaffold with the formation of F-actin stress fiber which proceeded to cell aggregation; an event much delayed in the unmodified PLCL. Enhanced cartilage formation on the chitosan-modified PLCL was shown by real-time PCR analysis, histological and immunochemistry staining and biochemical assays of the cartilage extracellular matrix components. The Young's modulus of the derived cartilage tissues on the chitosan-modified PLCL scaffold was significantly increased and doubled that of the unmodified PLCL. Our results show that chitosan modification of the PLCL scaffold improved the cell compatibility of the PLCL scaffold without significant alteration of the physical elastomeric properties of PLCL and resulted in the formation of cartilage tissue of better quality.

Physical stimulation of chondrogenic cells in vitro: a review

BACKGROUND

Mechanical stimuli are of crucial importance for the development and maintenance of articular cartilage. For conditioning of cartilaginous tissues, various bioreactor systems have been developed that have mainly aimed to produce cartilaginous grafts for tissue engineering applications. Emphasis has been on in vitro preconditioning, whereas the same devices could be used to attempt to predict the response of the cells in vivo or as a prescreening method before animal studies. As a result of the complexity of the load and motion patterns within an articulating joint, no bioreactor can completely recreate the in vivo situation.

QUESTIONS/PURPOSES

This article aims to classify the various loading bioreactors into logical categories, highlight the response of mesenchymal stem cells and chondrocytes to the various stimuli applied, and determine which data could be used within a clinical setting.

METHODS

We performed a Medline search using specific search terms, then selectively reviewed relevant research relating to physical stimulation of chondrogenic cells in vitro, focusing on cellular responses to the specific load applied.

RESULTS

There is much data pertaining to increases in chondrogenic gene expression as a result of controlled loading protocols. Uniaxial loading leads to selective upregulation of genes normally associated with a chondrogenic phenotype, whereas multiaxial loading results in a broader pattern of chondrogenic gene upregulation. The potential for the body to be used as an in vivo bioreactor is being increasingly explored.

CONCLUSIONS

Bioreactors are important tools for understanding the potential response of chondrogenic cells within the joint environment. However, to replicate the natural in vivo situation, more complex motion patterns are required to induce more physiological chondrogenic gene upregulation.

Three-dimensional culture and bioreactors for cellular therapies

A bioreactor is defined as a specifically designed vessel to facilitate the growth of organisms and cells through application of physical and/or electrical stimulus. When cells with therapeutic potential were first discovered, they were initially cultured and expanded in two-dimensional (2-D) culture vessels such as plates or T-flasks. However, it was soon discovered that bioreactors could be used to expand and maintain cultures more easily and efficiently. Since then, bioreactors have come to be accepted as an indispensable tool to advance cell and tissue culture further. A wide array of bioreactors has been developed to date, and in recent years businesses have started supplying bioreactors commercially. Bioreactors in the research arena range from stirred tank bioreactors for suspension culture to those with various mechanical actuators that can apply different fluidic and mechanical stresses to tissues and three-dimensional (3-D) scaffolds. As regenerative medicine gains more traction in the clinic, bioreactors for use with cellular therapies are being developed and marketed. While many of the simpler bioreactors are fit for purpose, others fail to satisfy the complex requirements of tissues in culture. We have examined the use of different types of bioreactors in regenerative medicine and evaluated the application of bioreactors in the realization of emerging cellular therapies.

A viscoelastic chitosan-modified three-dimensional porous poly(L-lactide-co-ε-caprolactone) scaffold for cartilage tissue engineering

Biomaterials have been playing important roles in cartilage regeneration. Although many scaffolds have been reported to enhance cartilage regeneration, none of the scaffolds available are optimal regarding mechanical properties, integration with host cartilage and providing proper micro-environment for chondrocyte attachment, proliferation and differentiation. In the current study, chitosan-modified poly(L-lactide-co-ε-caprolactone) (PLCL) scaffolds were fabricated to simulate the main biochemical components of cartilage, as well as their interaction with the aim to endow them with viscoelasticity similar to native cartilage. Porous PLCL scaffolds were fabricated with porogen-leaching, freeze-extraction and freeze-gelation before chitosan was cross-linked. The acquired porous scaffolds had pore sizes ranging from 200 to 500 μm and about 85% porosity with good interconnection between individual pores. Chitosan was successfully cross-linked to PLCL scaffolds, as validated by ninhydrin staining and X-ray photoelectron spectroscopy (XPS). The viscoelasticity of the scaffolds was similar to that of bovine cartilage and they had a relatively good recovery ratio from compression deformation, while the Young's modulus was one order of magnitude less than cartilage. Not only could the chitosan-modified PLCL scaffolds promote cell adhesion and proliferation, but also they could significantly enhance excretion of aggrecan and type-II collagen, as testified by both histology and quantitative PCR, compared with PLCL scaffolds. With the fabrication of biomimetic scaffolds, it is possible to make scaffolds for cartilage tissue engineering, which are not only biocompatible, but also have mechanical properties similar to native cartilage.

Cartilage tissue engineering using electrospun PCL nanofiber meshes and MSCs

Mesenchymal stem cells (MSCs) have been recognized for their ability to differentiate into cells of different tissues such as bone, cartilage, or adipose tissue, and therefore are of great interest for potential therapeutic strategies. Adherent, colony-forming, fibroblastic cells were isolated from human bone marrow aspirates, from patients undergoing knee arthroplasties, and the MSCs phenotype characterized by flow cytometry. Afterward, cells were seeded onto electrospun polycaprolactone nanofiber meshes and cultured in a multichamber flow perfusion bioreactor to determine their ability to produce cartilagineous extracellular matrix. Results indicate that the flow perfusion bioreactor increased the chondrogenic differentiation of hBM-MSCs, as confirmed either by morphological and RT-PCR analysis. Cartilage-related genes such as aggrecan, collagen type II, and Sox9 were expressed. ECM deposition was also detected by histological procedures. Collagen type II was present in the samples, as well as collagen type I. Despite no statistically significant values being obtained for gene expression, the other results support the choice of the bioreactor for this type of culture.

Magnetic resonance studies of macromolecular content in engineered cartilage treated with pulsed low-intensity ultrasound

Noninvasive monitoring of matrix development in tissue-engineered cartilage constructs would permit ongoing assessment with the ability to modify culture conditions during development to optimize tissue characteristics. In this study, chondrocytes seeded in a collagen hydrogel were exposed for 20 min/day to pulsed low-intensity ultrasound (PLIUS) at 30 mWcm(-2) and cultured for up to 5 weeks. Biochemical assays, histology, immunohistochemistry, Fourier transform infrared spectroscopy, and magnetic resonance imaging (MRI) were performed at weeks 3 and 5 after initiation of growth. The noninvasive MRI measurements were correlated with those from the invasive studies. In particular, MRI transverse relaxation time (T2) and magnetization transfer rate (k(m)) correlated with macromolecular content, which was increased by application of PLIUS. This indicates the sensitivity of MR techniques to PLIUS-induced changes in matrix development, and highlights the potential for noninvasive assessment of the efficacy of anabolic interventions for engineered tissue.

The role of tissue engineering in articular cartilage repair and regeneration

Articular cartilage repair and regeneration continue to be largely intractable because of the poor regenerative properties of this tissue. The field of articular cartilage tissue engineering, which aims to repair, regenerate, and/or improve injured or diseased articular cartilage functionality, has evoked intense interest and holds great potential for improving articular cartilage therapy. This review provides an overall description of the current state of and progress in articular cartilage repair and regeneration. Traditional therapies and related problems are introduced. More importantly, a variety of promising cell sources, biocompatible tissue engineered scaffolds, scaffoldless techniques, growth factors, and mechanical stimuli used in current articular cartilage tissue engineering are reviewed. Finally, the technical and regulatory challenges of articular cartilage tissue engineering and possible future directions are also discussed.

Cartilage regeneration with highly-elastic three-dimensional scaffolds prepared from biodegradable poly(L-lactide-co-epsilon-caprolactone)

Compressive mechanical stimuli are crucial in regenerating cartilage with tissue engineering, which creates a need for scaffolds that can maintain their mechanical integrity while delivering mechanical signals to adherent cells during strain applications. With these goals in mind, the aim of this study was to develop a mechano-active scaffold that facilitated effective cartilaginous tissue formation under dynamic physiological environments. Using a gel-pressing method, we fabricated a biodegradable and highly-elastic scaffold from poly(L-lactide-co-epsilon-caprolactone) (PLCL; 5:5), with 85% porosity and a 300-500-microm pore size, and we compared it to control scaffolds made of rigid polylactide (PLA) or poly(lactide-co-glycolide) (PLGA). After tensile mechanical tests and recovery tests confirmed the elasticity of the PLCL scaffolds, we seeded them with rabbit chondrocytes, cultured them in vitro, and subcutaneously implanted them into nude mice for up to eight weeks. The PLCL scaffolds possessed a completely rubber-like elasticity, were easily twisted and bent, and exhibited an almost complete (over 97%) recovery from applied strain (up to 500%); the control PLA scaffolds showed little recovery. In vitro and in vivo accumulations of extracellular matrix on the cell-PLCL constructs demonstrated that they could not only sustain but also significantly enhance chondrogenic differentiation. Moreover, the mechanical stimulation of the dynamic in vivo environment promoted deposition of the chondral extracellular matrix onto the PLCL. In contrast, on the PLA scaffolds, most of the chondrocytes had de-differentiated and formed fibrous tissues. In a rabbit defect model, the groups treated with PLCL scaffolds exhibited significantly enhanced cartilage regeneration compared to groups harboring an empty control or PLGA scaffolds. These results indicated that the mechano-active PLCL scaffolds effectively delivered mechanical signals associated with biological environments to adherent chondrocytes, suggesting that these elastic PLCL scaffolds could successfully be used for cartilage regeneration.