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J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S 4 (2011) 922–932 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jmbbm Review article Applications of knitted mesh fabrication techniques to scaffolds for tissue engineering and regenerative medicine Xingang Wang a , Chunmao Han a,∗ , Xinlei Hu a , Huafeng Sun a , Chuangang You a , Changyou Gao b , Yang Haiyang a,1 a Department of Burns, Second Affiliated Hospital of Zhejiang University, College of Medicine, Hangzhou, 310009, China b Key Laboratory of Macromolecular Synthesis and Functionalization, Ministry of Education, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, China A R T I C L E I N F O A B S T R A C T Article history: Knitting is an ancient and yet, a fresh technique. It has a history of no less than 1,000 Received 13 January 2011 years. The development of tissue engineering and regenerative medicine provides a new Received in revised form role for knitting. Several meshes knitted from synthetic or biological materials have been 7 April 2011 designed and applied, either alone, to strengthen materials for the patching of soft tissues, Accepted 11 April 2011 or in combination with other kinds of biomaterials, such as collagen and fibroin, to Published online 19 April 2011 repair or replace damaged tissues/organs. In the latter case, studies have demonstrated that knitted mesh scaffolds (KMSs) possess excellent mechanical properties and can Keywords: promote more effective tissue repair, ligament/tendon/cartilage regeneration, pipe-like- Knitted mesh organ reconstruction, etc. In the process of tissue regeneration induced by scaffolds, an Mechanical properties important synergic relationship emerges between the three-dimensional microstructure Microstructure and the mechanical properties of scaffolds. This paper presents a comprehensive overview Regeneration of the status and future prospects of knitted meshes and its KMSs for tissue engineering and regenerative medicine. c 2011 Elsevier Ltd. All rights reserved. ⃝ Contents 1. Introduction ................................................................................................................................................................................. 923 2. Fundamentals of knitting ............................................................................................................................................................. 923 3. Ways to fabricate knitted mesh scaffolds ..................................................................................................................................... 925 3.1. One-step moulding ............................................................................................................................................................ 925 3.2. Assembly ........................................................................................................................................................................... 925 4. Properties of knitted mesh scaffolds ............................................................................................................................................ 926 5. Clinical applications of knitted mesh scaffolds ............................................................................................................................ 926 5.1. Patches for soft tissues ...................................................................................................................................................... 926 5.2. Ligaments and tendons ..................................................................................................................................................... 926 ∗ Corresponding author. Tel.: +86 571 87767187; fax: +86 571 87784585. E-mail address: hanchunmao1@126.com (C. Han). in Dulwich College in UK. 1 Studying c 2011 Elsevier Ltd. All rights reserved. 1751-6161/$ - see front matter ⃝ doi:10.1016/j.jmbbm.2011.04.009 J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S 6. 4 (2011) 922–932 923 5.3. Cartilage ............................................................................................................................................................................ 927 5.4. Skin ................................................................................................................................................................................... 927 5.5. Blood vessels ..................................................................................................................................................................... 928 Conclusion and perspectives ........................................................................................................................................................ 929 Acknowledgements ...................................................................................................................................................................... 929 References .................................................................................................................................................................................... 929 1. Introduction Tissue-engineered scaffolds can be the artificial equivalents of natural extracellular matrices (ECMs) that are used to induce tissue regeneration or replace damaged tissues/organs (Langer and Vacanti, 1993; Griffith and Naughton, 2002). Although the requirements of scaffolds for tissue engineering are multifaceted and specific to the structure and function of the tissue of interest (Yang et al., 2001), ideal scaffolds should have good biocompatibility and biodegradability, highly porous and interconnected microstructures, and suitable mechanical support (Hutmacher, 2000; Yang et al., 2002; Wang et al., 2007). Comparatively, the processing and bioactivity of scaffolds have been paid more attention than others (Harley et al., 2007). Naturally derived materials such as collagen have been used extensively to prepare scaffolds due to their good biocompatibility, hydrophilicity and cell affinity. However, scaffolds constructed entirely of collagen sponge or gel possess poor strength to resist mechanical forces (Bell et al., 1979; Young et al., 1998; Awad et al., 1999; Ng et al., 2004; Nirmalanandhan et al., 2007; Mao et al., 2009). Nowadays, for tissue-engineered scaffolds, the importance of three-dimensional (3D) porous structures has been confirmed to allow in vitro cell adhesion, ingrowth and reorganisation, and provide the necessary space for neovascularisation in vivo (Schmidt and Baier, 2000; O’Brien et al., 2005; Puppi et al., 2010). The role of mechanical properties is being increasingly recognised to provide temporary mechanical support and proper mechanical cues (Leong et al., 2008), and to maintain space for cell ingrowth and matrix formation (Puppi et al., 2010), and rapidly restore tissue biomechanical function (Gloria et al., 2010). Some researchers state that constructing a scaffold that simultaneously possesses optimal mechanical properties, a porous structure and a biocompatible microenvironment, is a more intriguing orientation (Chen et al., 2002, 2008). A knitted mesh possesses highly ordered loop structures (Wintermantel et al., 1996) and versatile mechanical properties (Quaglini et al., 2008; Yeoman et al., 2010) that can provide sufficient internal connective space for tissue ingrowth (Ouyang et al., 2003). As a unique method of material processing, knitting has shown the potential to provide tissue engineering with many kinds of knitted meshes, or participate in the construction of tissue-engineered scaffolds (Chen et al., 2002). Nowadays, well-designed knitted meshes manufactured from synthetic or biological materials such as polylactide-co-glycolide (PLGA) (Ouyang et al., 2003) and silk (Chen et al., 2008) commonly have unique mechanical properties and have been used to provide improved physical support, either alone to strengthen materials for the patching of soft tissues Clave et al. (2010), or in combination with other types of biomaterials, for the construction of knitted mesh scaffolds (KMSs) (Tatekawa et al., 2010). Given increasing reports about the applications of a knitted mesh, this review presents a comprehensive overview of the status and future prospects of knitted meshes and KMSs for tissue engineering and regenerative medicine. The knitted mesh mentioned in the different papers cited may be associated with different names, e.g. knitted mesh, mesh, network or fabric. Meanwhile, KMSs may also have different terms used by the researchers, e.g. hybrid scaffold (Munirah et al., 2008; Urita et al., 2008; Dai et al., 2010), hybrid construct (Ananta et al., 2009), combined scaffold (Liu et al., 2008; Fan et al., 2009), composite web scaffold (Chen et al., 2003a), composite vascular graft (Xu et al., 2010), composite or 3D scaffold (Pu et al., 2010), etc. The inclusion criteria are based on the knitted structure described in detail in the cited articles. 2. Fundamentals of knitting Knitting as an ancient and yet, a fresh technique, has a history of at least 1000 years. The basics of knitting, upon which most of this section is based, have been well documented (Spencer, 1983; Hatch, 1993; Leong et al., 2000). The structure of a knitted mesh is often defined as a highly ordered arrangement of interlocking loops (Wintermantel et al., 1996). The general categories and characteristics of textiles according to the structures and processing are summarised in Table 1. Knitting and weaving, herein, are traditional techniques used for making fabrics from yarns or threads (Spencer, 1983). The yarns in knitted fabrics follow a meandering path to form symmetric loops, which is different from the criss-cross structure of straight threads in woven fabrics (Hatch, 1993). The knitting process can be used to produce fabrics by interlooping stitches using knitting needles. A continuous series of interlooped stitches is formed by the needle catching the yarn and drawing it through a previously formed loop to form a new loop Leong et al. (2000). In the knitted structures, rows running across the width of the knitted fabric are called courses, and columns running along the length of the fabric are known as wales. The loops in the courses and wales are supported by and interconnected with one another to form the final fabric (Leong et al., 2000). According to the direction of formed loops, knitting can be categorised into two major classifications: weft knitting and warp knitting. In weft knitting, the wales are perpendicular to the course of the yarn (Fig. 1(a)), where as in warp knitting, the wales and courses run roughly parallel (Fig. 1(b)) (Spencer, 1983). On this basis, an infinite variety of knitted structures has been successfully manufactured in the textile industry (Lau and Dias, 1994; Leong et al., 2000). 924 J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S 4 (2011) 922–932 Table 1 – Categories of textile structures.a . Type Woven Knitted Weft Warp Braided Nonwoven Source Dimension Extension Porosity Yarns Stable Low Low To unravel easily at the edges when cut or trimmed Merits and demerits Yarns Yarns Yarns Fibres Unstable Stable Stable Varying High Versatile Middle high High High High Varying Additional yarns are utilised to interlock the loops Having flexibility and inherent ability to resist unravelling The yarns criss-cross each other Determined by those of the constituent polymer or fibre and by the bonding process a Data collected mainly from Spencer (1983), Hatch (1993) and Wintermantel et al. (1996). c d cartilage, skin a e f g h ligament and tendon blood vessel, bladder, ureter, hernia,trachea, oesophagus, etc. b blood vessel, skin, etc. Fig. 1 – Schematic applications of a knitted mesh and KMSs in tissue engineering. (a, b): Schematic diagrams of the wale and course components of a knitted mesh, the principles of warp (a) and weft (b) knitting. In warp knitting, the wales and courses run roughly parallel (a), where as in weft knitting, the wales are perpendicular to the course of the yarn (b). (c, d): SEM images of PLGA knitted mesh (c) with typical warp-knit structures, and PLGA/collagen hybrid scaffold (d), prepared by forming collagen microsponges in the loops. (e, f): Gross image of knitted silk mesh (e) and SEM image of the combined scaffold (f) showing the integration of collagen sponge with the knitted silk mesh. (g, h): SEM of the knitted silk mesh (g) and phase-contrast image of the combined silk scaffold (h), showing web-like microporous silk sponges formed in the openings of the knitted silk mesh. Source: Reprinted with permission from Leong et al. (2000), Chen et al. (2005, 2008) and Liu et al. (2008). The weft-knit structures distort and stretch more easily, whereas the warp-knit ones are more stable and less formable. Generally, the weft-knit structures include three primary classifications: jersey structure and its derivatives, rib fabric and its derivatives, and purl fabric and its derivatives (Bruer and Smith, 2005), and the entire fabric can be fabricated from one yarn. In contrast, in warp knitting, one yarn is required for each wale. A typical piece of a knitted mesh may incorporate hundreds of wales. Therefore, the production rate of warp knitting is significantly higher than that of weft knitting, and warp knitting is more suitable for large-scale production (Leong et al., 2000). To obtain particular macroscopic properties and more complicated fabrics, float and tuck stitches are often used to modify the structure of the knitted fabric. Nowadays, knitting has reached a high level of mechanical automation, which has been well documented (Spencer, 1983; Leong et al., 2000). Warp knitting is performed by Tricot or Raschel knitting machines, and weft knitting may be finished by circular and flat-bed knitting machines. The development of tissue engineering and regenerative medicine provides a new role for knitting. Fig. 1 shows the schematic applications of warp- and weft-knit structures in the field of tissue engineering, which are further discussed in Sections 5 and 6 by providing more details. The materials used for the fabrication of the knitted mesh are commonly synthetic materials, such as poly(lactic-coglycolic acid) (PLGA), which are hydrophobic and lack the cellrecognition sites necessary for cell adhesion and function (Gunatillake and Adhikari, 2003). Besides, naturally derived materials with good biocompatibility are selected to fabricate J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S a d b 4 (2011) 922–932 925 c e f Fig. 2 – Schematic illustration of various designs of hybrid scaffolds from PLGA knitted mesh and collagen-based sponge. The position (a–c), quantity (d) and shape (e, f), and of the mesh in the KMSs can be regulated as needed. Pink: PLGA knitted mesh; yellow: collagen-based sponge. a knitted mesh, e.g. silk fibres, but knitted silk scaffolds with good internal communicating spaces are unsuitable for cell seeding (Ouyang et al., 2003). Hence, in the practical applications, a strategy of combining the knitted mesh and other biomaterials can be used to obtain optimised scaffolds for tissue engineering, such as KMSs. 3. Ways to fabricate knitted mesh scaffolds Theoretically, a knitted mesh can be designed and knitted into many specific shapes to suit the target tissues/organs (Chen et al., 2003a; Dai et al., 2010). To date, many methods have been developed to integrate a mesh into a scaffold; chief among these are one-step moulding and assembly. 3.1. One-step moulding One-step moulding entails simultaneous incorporation of the knitted mesh and fabrication of the KMSs. Lyophilisation is one of the most common methods for preparing such scaffolds. The quantity, shape, and position of the mesh in the KMSs can be regulated as needed, and the thickness of the KMSs can be adjusted by laminating or rolling the web sheets (Fig. 2(a), (d)–(f)) (Chen et al., 2003a). Dai et al. (2010)systemically investigated the effect of PLGA mesh/collagen hybrid scaffolds on cartilage regeneration; they used lyophilisation to design three kinds of hybrid scaffolds, i.e., thin, semi, and sandwich scaffolds (Fig. 2(a)–(c)), which differed based on the location of the PLGA knitted mesh. Besides, several novel practical scaffolds have been developed for ligamenttissue engineering by forming a collagen microstructure in the pores of a silk-based knitted mesh using a freeze-drying process (Chen et al., 2008; Fan et al., 2009). Xu et al. (2010) introduced another method to prepare a composite vascular graft reinforced by a tubular weft-knitted fabric. A tubular polyester mesh was dressed onto a glass mandrel and then uniformly coated with polyurethane/N, N-Dimethyl formamide (PU/DMF); after soaking in distilled water to remove the DMF, the hybrid vascular graft was obtained (Xu et al., 2010). Fig. 3 – Diagram showing a model of assembly for plastic compression of hyperhydrated hybrid scaffolds. By the faster and easier method, a multilayered scaffold can be obtained by compressing a hyperhydrated collagen gel onto a flat warp-knitted poly(lactic acid-co-caprolactone) (PLACL) mesh. Green arrow indicates the direction of loading. Black arrows indicate the direction of the expulsed liquid. 3.2. Assembly During assembly, the fabrication of KMSs is finished by assembling several units, even including cell elements, with the knitted mesh constituting one auxiliary part. Ananta et al. (2009) introduced a faster and easier technique for preparing a multilayered scaffold by compressing a hyperhydrated collagen gel onto a flat warp-knitted poly(lactic acid-cocaprolactone) (PLACL) mesh (Fig. 3). Ng and Hutmacher (2006) developed an assembly method for preparing bilayer skin equivalents by first peeling off fibroblast sheets and folding them over a PLGA knitted mesh to form a 3D dermal matrix and then seeding keratinocytes to form a skin substitute in the air–liquid culture. A specific example of assembly is DermagraftTM , a bioengineered dermal equivalent, which is prepared by culturing allogeneic fibroblasts in PLGA knitted mesh for 14–17 days to obtain a mixture of cells, ECM, and degradable polymers (Cooper et al., 1991; Bello et al., 2001; Marston, 2004). Some researchers have tried to fabricate more complicated scaffolds by combining one-step moulding and assembly. Tatekawa et al. (2010) utilised PLGA mesh/collagen 926 J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S hybrid scaffolds, tube stents, and basic fibroblast growth factor (bFGF)-impregnated gelatin-hydrogel sheets to repair tracheal defects. Though the scaffold combining bFGF and reinforcement induced incomplete regeneration of tracheal cartilage, it represented an improved method for fabricating better scaffolds (Tatekawa et al., 2010). 4. Properties of knitted mesh scaffolds Proper mechanical support has been under essential consideration for the construction of scaffolds (Ng et al., 2005; Leong et al., 2008). Especially for naturally derived materials, various methods have been developed to improve the mechanical performance of scaffolds (Ruijgrok et al., 1994; Ulubayram et al., 2002; Powell and Boyce, 2006; Rezwan et al., 2006; Hillberg et al., 2009). Universally, the mechanical strength should maintain enough spaces for cell ingrowth and functionalisation in vitro, and temporarily bear in vivo stresses and loading (Puppi et al., 2010). Mechanical strength seems not to be an independent contributing factor, but a synergetic one with other characteristics of scaffolds, such as porous structures (Ng et al., 2004). Nowadays, many fabrication techniques have been developed to design different scaffolds characterised by porous structures, good biocompatibility and excellent mechanical properties, e.g., incorporating poly(glycolic acid) fibre into collagen sponge (Hiraoka et al., 2003; Nagato et al., 2006), combining collagen and poly(L-lactic acid) (PLLA) braid (Idea et al., 2001), introducing collagen microsponges into the pores of synthetic polymer sponges (Chen et al., 2004b) or the interstices of knitted meshes (Chen et al., 2005), forming microporous silk sponges in the openings of a knitted silk scaffold (Liu et al., 2008), etc. The synthetic polymer sponge, braid, fibres and knitted meshes serve as a “skeleton” to reinforce the entire scaffolds, while the collagen or silk sponges provide the scaffolds with porous structures (Chen et al., 2002). However, porous synthetic materials prepared for engineering applications may show excellent elasticity but low shear resistance to suturing (Xu et al., 2010). Twisted or braided scaffolds can possess remarkable mechanical properties that are comparable to native tissue (Altman et al., 2002), but their limited internal space often hinders the ingrowth of neotissue (Chen et al., 2008). Scaffolds shaped by fibres provide a large surface for cell attachment and a rapid diffusion of nutrients, but a lack of structural stability (Chen et al., 2002). By contrast, a knitted mesh possesses ordered and stable loop structures, and internal connective spaces. By changing the geometric parameters of the fabric (yarn spacing and thickness) or the fibre material, the required mechanical properties can be achieved (Quaglini et al., 2008). Synthetic or biological meshes have been used to fabricate KMSs (Zdrahala, 1996; Chen et al., 2004b; Cui et al., 2009; He et al., 2010). How does the knitted mesh improve the mechanical strength of a combined or hybrid scaffold? Weftknitted structures in which loops are formed across the width of the fabric are inelastic but can provide high levels of compression and support, whereas warp-knitted meshes can offer better flexibility and elasticity due to the formation of loops along the length of the fabric (Anand, 2003). Knitted 4 (2011) 922–932 meshes incorporated into porous scaffolds can serve as a support structure to reinforce combined or hybrid scaffolds (Chen et al., 2005), and afford mechanical support to resist a physical load (Chen et al., 2008). Meanwhile, the mechanical properties of a knitted mesh can be influenced by the addition of a polymeric matrix that bonds the filaments in the mesh together (Wintermantel et al., 1996). In addition, although a knitted mesh can promote homogeneous cell distribution and tissue formation (Ng et al., 2004), the application of a knitted mesh has been reported rarely as a scaffold and has occasionally been regarded as a control (Ouyang et al., 2003; Chen et al., 2005; Liu et al., 2008). It has been used as an auxiliary tool in the construction of scaffolds for tissue engineering. For example, Lu et al. (2011) used a knitted PLGA mesh as a selectively removable template to prepare an autologous extracellular matrix (aECM) scaffold. The preparation of an aECM scaffold by combining culture of autologous cells in the knitted mesh, decellularisation, and template removal is depicted in Fig. 4; the in vivo experiment indicated that the aECM scaffold had excellent biocompatibility. 5. Clinical applications of knitted mesh scaffolds 5.1. Patches for soft tissues Various knitted meshes made from synthetic materials, commonly regarded as prosthetic materials such as polypropylene, poly(ethylene terephthalate), and polylactic acid have been designed for the treatment of hernia (Boukerrou et al., 2007), pelvic organ prolapse (Ganj et al., 2009), pelvic floor dysfunctions (Clave et al., 2010), body wall defects (Lamb et al., 1983; Tyrell et al., 1989), and so on. In addition to good mechanical properties, for this kind of application, all of the materials used for the knitted mesh should also have the ability to resist erosion and defect recurrence (Boukerrou et al., 2007). These materials are often inert or have very slow degradation rates, and the implanted meshes are replaced gradually by newly formed or surrounding tissues. However, such implants were reported to cause many complications, e.g. inflammation (Pu et al., 2010), infection (Urita et al., 2008), rejection and recurrence (Boukerrou et al., 2007), etc. In view of the above, some researchers considered tissue engineering as an alternative treatment strategy. For example, Urita et al. (2008) used a PLGA knitted mesh/collagen scaffold as an alternative prosthetic material to repair a diaphragmatic hernia and observed that the hybrid scaffold was better than a simple PLGA mesh for promoting in situ tissue regeneration. Pu et al. (2010) developed a PLLA–collagen scaffold by forming collagen sponge within the pores of a poly(L-lactic acid) (PLLA) knitted mesh, and cultured dermal fibroblasts on this scaffold for 7 days by using a flow perfusion bioreactor for abdominal wall repair. 5.2. Ligaments and tendons Once damaged, ligaments and tendons have no ability to heal through a regenerative process, but instead form J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S Cell seeding in a template Cells 4 (2011) 922–932 927 Cell-ECM-template construct Template ECM deposition Cell culture Decellularisation Template removal ECM scald ECM-template construct Fig. 4 – Fabrication scheme for an autologous extracellular matrix (aECM) scaffold. The aECM was prepared by combining culture of autologous cells in a template, decellularisation and template removal. The template as an auxiliary tool indicates a knitted PLGA mesh, which can be selectively removed. The cells can be fibroblasts, chondrocytes or mesenchymal stem cells (MSCs) (Lu et al., 2011). fibrotic scars (Favata et al., 2006). The development of tissue engineering and regenerative medicine may provide a promising method to treat ligament and tendon injuries. Of the many biomaterial grafts fabricated as ligament and tendon scaffolds (Torres et al., 2000; Gentleman et al., 2006; Sahoo et al., 2006; Cooper et al., 2007; Fini et al., 2007), knitted scaffolds have mechanical properties similar to those of biological tissues and are therefore a good prospect for ligament and tendon reconstruction. Chen et al. (2008) developed a new practical ligament scaffold based on the synergistic incorporation of a plain knitted silk structure and a collagen matrix. This scaffold, with better mechanical properties and a more native microstructure, induced ligament regeneration with greater collagen deposition in a rabbit model (Fig. 1(e), (f)), suggesting that the knitted mesh improved not only the mechanical strength of the scaffold but also the level of regeneration of the target tissue (Chen et al., 2008). Liu et al. (2008) fabricated a combined scaffold with web-like microporous silk sponges formed in the openings of a knitted silk mesh (Fig. 1(g), (h)) and subsequently seeded with adult human bone marrowderived mesenchymal stem cells (MSCs) for in vitro ligamenttissue engineering. Compared with a simple knitted scaffold, the combined scaffold promoted greater cell activity, with increased expression of type I and III collagen and tenascinC genes (Liu et al., 2008). Subsequently, Fan et al. (2009) rolled combined silk scaffold around a braided silk cord to regenerate the anterior cruciate ligament using MSCs in a pig model; the regenerated ligament showed obvious ligament–bone junction formation, and the MSCs in the regenerated ligament exhibited fibroblast-like morphology 24 weeks post the operation. With regard to tendon tissue engineering, Ouyang et al. (2003) used a knitted PLGA scaffold to repair a 10 mm gap in the Achilles tendon in a rabbit model. They showed that the knitted mesh induced tendon regeneration, and when loaded with bone marrow stromal cells, the scaffold repaired the tendon defects more effectively. 5.3. Cartilage In cartilage tissue engineering, the fundamental challenge is to develop a suitable scaffold that provides sufficient structural support to withstand the large forces applied to the new tissue. Chen and colleagues have conducted many in vitro and in vivo studies in cartilage reconstruction with the use of knitted PLGA mesh/collagen hybrid scaffolds (Chen et al., 2003a,b, 2004a,b; Dai et al., 2010; Kawazoe et al., 2010). The hybrid scaffolds, taking advantage of naturally derived and synthetic materials, were prepared by integrating collagen sponges with the knitted PLGA mesh, and with adjustable thickness (Chen et al., 2003a,b; Dai et al., 2010). Herein, the polymer mesh served as a “skeleton” and reinforced the hybrid scaffold, that facilitated cell seeding and cell distribution (Chen et al., 2003a,b). These studies demonstrated that the PLGA mesh/collagen scaffolds improved chondrocyte adhesion, proliferation (Chen et al., 2004b), facilitated the redifferentiation of the dedifferentiated multiplied chondrocytes (Chen et al., 2003b), and promoted chondrogenic differentiation of MSCs (Chen et al., 2004a). The engineered tissue obtained by this approach matched the native cartilage both histologically and mechanically (Chen et al., 2003a). A recent study has focused on a novel leak-proof PLGA mesh/collagen cell scaffold to transdifferentiate MSCs for cartilage tissue (Kawazoe et al., 2010). 5.4. Skin Collagen-based porous sponges have been developed to guide tissue regeneration and to provide a substrate for epidermalisation (Ruszczak, 2003). The support of collagenbased scaffolds is compromised by their low mechanical strength. Therefore, improved mechanical properties are important for collagen-based scaffolds to induce skin-tissue regeneration and reduce wound contraction. Chen et al. (2005) developed a novel porous scaffold by hybridising a knitted PLGA mesh and naturally derived bovine collagen (Fig. 1(c),(d)), and the hybrid scaffold exhibited good 928 J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S a 4 (2011) 922–932 b c Fig. 5 – SEM image of the cross section of the surface of PLGAm/CCS (a), prepared by incorporating PLGA knitted mesh into collagen–chitosan scaffold (CCS), and histological images of PLGAm/CCS (b) and CCS (c) implanted subcutaneously for one week. The letter S indicates the collagen–chitosan sponge side, and M indicates the mesh side. Yellow arrows indicate the direction of tissue infiltration (unpublished data). biocompatibility and high mechanical properties. Ananta et al. (2009) spatially distributed neonatal fibroblasts in a PLACL mesh/collagen gel composite that mimicked interstitial or epithelial tissue; this multilayered tissue substitute had homogeneous cell distribution and good biocompatibility, and it showed no macroscopic signs of contraction. Ng et al. (2005) developed a hybrid matrix by lyophilising collagen within a weft-knitted PLGA mesh, which simulated the mechanical properties of native dermis, supported cell attachment and proliferation in vitro, and evoked minimal host tissue response in vivo. Ng and Hutmacher (2006) seeded dermal fibroblasts and keratinocytes into a 3D matrix composed of a weft-knitted PLGA mesh and collagen–hyaluronic acid (CHA) sponge to construct a bilayer skin substitute. When transplanted in full-thickness onto wounds in nude rats for four weeks, the substitute inhibited wound contraction similarly to autografts (Ng and Hutmacher, 2006). Studies have revealed that collagen–chitosan scaffold possesses good biocompatibility and suitable porous structures for skin tissue engineering (Ma et al., 2003a,b, 2007). However, its poor mechanical strength compromises the resistance to mechanical forces (Mao et al., 2009). In order to develop a well-supported scaffold, we incorporated a PLGA knitted mesh into a collagen–chitosan sponge to construct a hybrid scaffold, abbreviated as PLGAm/CCS (Fig. 5(a)). The incorporation of PLGA mesh into CCS did little influence on the microstructure of the collagen–chitosan sponge, yet improved the mechanical strength of the hybrid scaffold similar to that of native dermis. Furthermore, when embedded subcutaneously in rats for one week, the hybrid scaffold promoted tissue infiltration more rapidly than the collagen–chitosan scaffold (Fig. 5(b), (c)). Meanwhile, in the same PLGAm/CCS, more abundant tissue formation was observed near to the mesh side than the collagen–chitosan sponge side (Fig. 5(b)). These results indicate that mechanical properties, cooperating with 3D microstructures, play an important part in tissue regeneration. 5.5. Blood vessels Tissue engineering techniques have been developed to construct vascular grafts, but the clinical use of bloodvessel alternatives remains limited. The primary obstacles include shortage of vascular endothelium, thrombosis, lumen collapse, low seeding-cell viability, infection risk, and insufficient mechanical strength to afford long-term patency (Baguneid et al., 2006; Chlupac et al., 2009; Ravi and Chaikof, 2010). For an ideal blood-vessel substitute, the fundamental requirements with respect to mechanical properties include sufficient burst pressure and viscoelasticity. Burst pressure, i.e., the strength required to rupture the vessel, is a critical characteristic of blood-vessel substitutes. For native arteries, the burst pressure is in excess of 2000 mm Hg; given the normal arterial pressure range of up to several hundred mm Hg, this indicates that native blood vessels are designed with a safety factor of approximately ten fold (Nerem, 2000). Another important characteristic is the viscoelasticity, which should match that of native blood vessels. Although it is difficult to construct an ideal substitute, incorporating a weft-knitted fabric into a polyurethane vascular graft can improve elasticity and strength (Xu et al., 2010). Among the polymeric materials that provide appropriate mechanical properties for vascular scaffolds, the knitted mesh has the potential to be used J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S in routine clinical practice (Ravi et al., 2009; Ju et al., 2010). Sawa et al. (Iwai et al., 2005; Torikai et al., 2008; Takahashi et al., 2009) have fabricated a tissue-engineered patch consisting of collagen microsponges, an interior layer of PGA knitted mesh, and an exterior layer of woven PLA fibres for reconstructing vascular walls. The results of animal experiments revealed that this patch enabled in situ regeneration in venous or arterial walls, with no development of aneurysm or thrombus within the patch area in any group. Further, the architecture of the neotissue was similar to that of the native tissue at 6 months after implantation in dogs. In addition to the above applications, a knitted mesh can also be used to construct scaffolds for the bladder, ureter, hernia, trachea, and oesophagus (Miki et al., 1999; Sha’ban et al., 2008; Ananta et al., 2009; Kawazoe et al., 2010; Roosa et al., 2010; Tatekawa et al., 2010). 6. Conclusion and perspectives For decades, knitted meshes have been applied surgically to reinforce frail tissues such as hernia (Boukerrou et al., 2007; Clave et al., 2010). Initially, the biomaterials constituting the knitted mesh were regarded as inert (Clave et al., 2010). Gradually, the biocompatibility and tissue-inducing regeneration of biomaterials have become recognised as important characteristics (Silva and Mooney, 2004). The mechanical properties of scaffolds have been shown to significantly affect cell behaviours and scaffold bioactivity (Grinnell et al., 1999; Freyman et al., 2001; Yeung et al., 2005; Harley et al., 2007). Therefore, improving the mechanical properties of scaffolds is not only a hot topic in the laboratory, but also a great challenge in the translation to clinical research. Knitted fabrics with unique structures and mechanical properties are an important element of the technical textile field. Well-designed knitted meshes, whether weft or warp, used alone or in combination, have been shown to be promising for tissue engineering and regenerative medicine (Wintermantel et al., 1996; Chen et al., 2002). A knitted mesh serves not only as a basic method for improving the mechanical strength of scaffolds but also as the ‘skeleton’ of grafts to maintain a porous microstructure, promote cell alignment, and induce tissue regeneration. The use of a knitted mesh has been examined in a variety of tissueengineering applications such as repair of ligaments (Chen et al., 2008), tendons (Ouyang et al., 2003), cartilages (Chen et al., 2004b; Dai et al., 2010; Kawazoe et al., 2010), skin (Ng et al., 2004), and pipe-like organs (Tatekawa et al., 2010; Xu et al., 2010). Some of these applications are still in preliminary, exploratory stages. Many issues relevant to knitted meshes and KMSs should be further investigated. Although knitted meshes have relatively homogeneous structures (Wintermantel et al., 1996), versatile mechanical properties (Quaglini et al., 2008; Yeoman et al., 2010), and controllable biological properties, the ideal knitted mesh for tissue regeneration is unknown. A knitted mesh, as an artificial fabric, possesses many alterable factors, e.g., knitting materials, knitting techniques, and fabric characteristics. With regard to the current status of mesh application 4 (2011) 922–932 929 for tissue engineering, more researches are needed to optimise biomaterials for knitting, develop suitable knitting methods to prepare biomimetic meshes for various target tissues/organs, match the rates of mesh degradation and neotissue formation. Moreover, although knitted structures exist in various forms in the textile industry, the knitted structures applied to tissue engineering are in the infant period. The possible reason may be that knitted structures have been selected to obtain better functions instead of for their appearances. KMSs, designed explicitly to match the mechanical properties of the native tissue or give cells proper mechanical cues, provide a good platform for studying the roles of mechanical properties in tissue regeneration. Many biomaterials such as silk, PLGA, and PLACL have been made into knitted fabrics with various biophysical properties (Chen et al., 2008; Ananta et al., 2009; Fan et al., 2009; Dai et al., 2010). So, how to understand the effects and mechanisms of KMSs on cell activity and tissue regeneration? The sound deduction may be: (1) knitted meshes with excellent mechanical properties change the distribution of tension and pressure on KMSs (Xu et al., 2010) and further maintain the 3D porous structures in vitro and in vivo; (2) well-maintained porous structures facilitate the diffusion of nutrients and the removal of waste products, and they provide enough space for cell migration and vascular ingrowth (Schmidt and Baier, 2000; Puppi et al., 2010); and (3) because specific surface areas (SSAs) to which cells adhere exist around the pores (O’Brien et al., 2005), mechanical properties can affect the distribution of SSAs by maintaining porous structures and can thereby regulate cell behaviours and tissue formation. Because the 3D microstructure of scaffolds plays a vital role in the regulation of cell activity and tissue regeneration (O’Brien et al., 2005; Ng and Hutmacher, 2006; Powell et al., 2008; Lin et al., 2009; Murphy et al., 2010; Murphy and O’Brien, 2010), an important synergic relationship may exist between the 3D microstructure and the mechanical properties of scaffolds in the process of scaffold-induced tissue regeneration (Ng et al., 2004). Additionally, criteria for knitted meshes should be established to assess their mechanical properties for use in tissue engineering and routine clinical practice. Acknowledgements The authors sincerely acknowledge Dr. Fergal J. O’Brien, Department of Anatomy, Royal College of Surgeons in Ireland & Trinity Centre for Bioengineering, Trinity College Dublin, for his constructive suggestions. This work was financially supported by the Major State Basic Program of China (2005CB623902) and the Major Science and Technology Project of Zhejiang, China (2007C13040). REFERENCES Altman, G.H., Horan, R.L., Lu, H.H., Moreau, J., Martin, I., Richmond, J.C., Kaplan, D.L., 2002. Silk matrix for tissue engineered anterior cruciate ligaments. Biomaterials 23 (20), 4131–4141. 930 J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S Anand, S., 2003. Spacers-at the technical frontiers. Knit. Int. 110 (1305), 38–41. Ananta, M., Aulin, C.E., Hilborn, J., Aibibu, D., Houis, S., Brown, R.A., Mudera, V., 2009. A poly(lactic acid-co-caprolactone)collagen hybrid for tissue engineering applications. Tissue Eng. Part A 15 (7), 1667–1675. Awad, H.A., Butler, D.L., Boivin, G.P., Smith, F.N., Malaviya, P., Huibregtse, B., Caplan, A.I., 1999. Autologous mesenchymal stem cell-mediated repair of tendon. Tissue Eng. 5 (3), 267–277. Baguneid, M.S., Seifalian, A.M., Salacinski, H.J., Murray, D., Hamilton, G., Walker, M.G., 2006. Tissue engineering of blood vessels. Brit. J. Surg. 93 (3), 282–290. Bell, E., Ivarsson, B., Merrill, C., 1979. Production of a tissuelike structure by contraction of collagen lattices by human fibroblasts of different proliferative potential in vitro. Proc. Natl. Acad. Sci. USA 76 (3), 1274–1278. Bello, Y.M., Falabella, A.F., Eaglstein, W.H., 2001. Tissue-engineered skin. Current status in wound healing. Am. J. Clin. Dermatol. 2 (5), 305–313. Boukerrou, M., Boulanger, L., Rubod, C., Lambaudie, E., Dubois, P., Cosson, M., 2007. Study of the biomechanical properties of synthetic mesh implanted in vivo. Eur. J. Obstet. Gynecol. Reprod. Biol. 134 (2), 262–267. Bruer, S.M., Smith, G., 2005. Three-dimensionally knit space fabrics: a review of production techniques and applications. JTATM 4 (4), 1–31. Chen, G., Liu, D., Tadokoro, M., Hirochika, R., Ohgushi, H., Tanaka, J., Tateishi, T., 2004a. Chondrogenic differentiation of human mesenchymal stem cells cultured in a cobweb-like biodegradable scaffold. Biochem. Biophys. Res. Commun. 322 (1), 50–55. Chen, X., Qi, Y.Y., Wang, L.L., Yin, Z., Yin, G.L., Zou, X.H., Ouyang, H.W., 2008. Ligament regeneration using a knitted silk scaffold combined with collagen matrix. Biomaterials 29 (27), 3683–3692. Chen, G., Sato, T., Ohgushi, H., Ushida, T., Tateishi, T., Tanaka, J., 2005. Culturing of skin fibroblasts in a thin PLGA-collagen hybrid mesh. Biomaterials 26 (15), 2559–2566. Chen, G., Sato, T., Ushida, T., Hirochika, R., Shirasaki, Y., Ochiai, N., Tateishi, T., 2003a. The use of a novel PLGA fiber/collagen composite web as a scaffold for engineering of articular cartilage tissue with adjustable thickness. J. Biomed. Mater. Res. A 67 (4), 1170–1180. Chen, G., Sato, T., Ushida, T., Hirochika, R., Tateishi, T., 2003b. Redifferentiation of dedifferentiated bovine chondrocytes when cultured in vitro in a PLGA-collagen hybrid mesh. FEBS Lett. 542 (1), 95–99. Chen, G., Sato, T., Ushida, T., Ochiai, N., Tateishi, T., 2004b. Tissue engineering of cartilage using a hybrid scaffold of synthetic polymer and collagen. Tissue Eng. 10 (3–4), 323–330. Chen, G., Ushida, T., Tateishi, T., 2002. Scaffold design for tissue engineering. Macromol. Biosci. 2 (2), 67–77. Chlupac, J., Filova, E., Bacakova, L., 2009. Blood vessel replacement: 50 years of development and tissue engineering paradigms in vascular surgery. Phys. Res. 58 (Suppl. 2S), 119–139. Clave, A., Yahi, H., Hammou, J.C., Montanari, S., Gounon, P., Clave, H., 2010. Polypropylene as a reinforcement in pelvic surgery is not inert: comparative analysis of 100 explants. Int. Urogynecol. J. Pel. 21 (3), 261–270. Cooper, M.L., Hansbrough, J.F., Spielvogel, R.L., Cohen, R., Bartel, R.L., Naughton, G., 1991. In vivo optimization of a living dermal substitute employing cultured human fibroblasts on a biodegradable polyglycolic acid or polyglactin mesh. Biomaterials 12 (2), 243–248. Cooper Jr, J.A., Sahota, J.S., Gorum 2nd, W.J., Carter, J., Doty, S.B., Laurencin, C.T., 2007. Biomimetic tissue-engineered anterior cruciate ligament replacement. Proc. Natl. Acad. Sci. USA 104 (9), 3049–3054. 4 (2011) 922–932 Cui, L., Wu, Y., Cen, L., Zhou, H., Yin, S., Liu, G., Liu, W., Cao, Y., 2009. Repair of articular cartilage defect in non-weight bearing areas using adipose derived stem cells loaded polyglycolic acid mesh. Biomaterials 30 (14), 2683–2693. Dai, W., Kawazoe, N., Lin, X., Dong, J., Chen, G., 2010. The influence of structural design of PLGA/collagen hybrid scaffolds in cartilage tissue engineering. Biomaterials 31 (8), 2141–2152. Fan, H., Liu, H., Toh, S.L., Goh, J.C., 2009. Anterior cruciate ligament regeneration using mesenchymal stem cells and silk scaffold in large animal model. Biomaterials 30 (28), 4967–4977. Favata, M., Beredjiklian, P.K., Zgonis, M.H., Beason, D.P., Crombleholme, T.M., Jawad, A.F., Soslowsky, L.J., 2006. Regenerative properties of fetal sheep tendon are not adversely affected by transplantation into an adult environment. J. Orthop. Res. 24 (11), 2124–2132. Fini, M., Torricelli, P., Giavaresi, G., Rotini, R., Castagna, A., Giardino, R., 2007. In vitro study comparing two collageneous membranes in view of their clinical application for rotator cuff tendon regeneration. J. Orthop. Res. 25 (1), 98–107. Freyman, T.M., Yannas, I.V., Yokoo, R., Gibson, L.J., 2001. Fibroblast contraction of a collagen-GAG matrix. Biomaterials 22 (21), 2883–2891. Ganj, F.A., Ibeanu, O.A., Bedestani, A., Nolan, T.E., Chesson, R.R., 2009. Complications of transvaginal monofilament polypropylene mesh in pelvic organ prolapse repair. Int. Urogynecol. J. Pel. 20 (8), 919–925. Gentleman, E., Livesay, G.A., Dee, K.C., Nauman, E.A., 2006. Development of ligament-like structural organization and properties in cell-seeded collagen scaffolds in vitro. Ann. Biomed. Eng. 34 (5), 726–736. Gloria, A., De Santis, R., Ambrosio, L., 2010. Polymer-based composite scaffolds for tissue engineering. J. Appl. Biomater. Biomech. 8 (2), 57–67. Griffith, L.G., Naughton, G., 2002. Tissue engineering-current challenges and expanding opportunities. Science 295 (5557), 1009–1014. Grinnell, F., Ho, C.H., Lin, Y.C., Skuta, G., 1999. Differences in the regulation of fibroblast contraction of floating versus stressed collagen matrices. J. Biol. Chem. 274 (2), 918–923. Gunatillake, P.A., Adhikari, R., 2003. Biodegradable synthetic polymers for tissue engineering. Eur. Cells Mater. 5, 1–16. Harley, B.A., Leung, J.H., Silva, E.C., Gibson, L.J., 2007. Mechanical characterization of collagen-glycosaminoglycan scaffolds. Acta Biomater. 3 (4), 463–474. Hatch, K.L., 1993. Textile Science. West Publishing Company, New York. He, X., Lu, H., Kawazoe, N., Tateishi, T., Chen, G., 2010. A novel cylinder-type poly(L-lactic acid)-collagen hybrid sponge for cartilage tissue engineering. Tissue. Eng. Part. C Methods 16 (3), 329–338. Hillberg, A.L., Holmes, C.A., Tabrizian, M., 2009. Effect of genipin cross-linking on the cellular adhesion properties of layer-bylayer assembled polyelectrolyte films. Biomaterials 30 (27), 4463–4470. Hiraoka, Y., Kimura, Y., Ueda, H., Tabata, Y., 2003. Fabrication and biocompatibility of collagen sponge reinforced with poly(glycolic acid) fiber. Tissue Eng. 9 (6), 1101–1112. Hutmacher, D.W., 2000. Scaffolds in tissue engineering bone and cartilage. Biomaterials 21 (24), 2529–2543. Idea, A., Sakane, M., Chen, G., Shimojo, H., Ushida, T., Tateishi, T., Wadano, Y., Miyanaga, Y., 2001. Collagen hybridization with poly(L-lactic acid) braid promotes ligament cell migration. Mater. Sci. Eng., C 17 (1–2), 95–99. Iwai, S., Sawa, Y., Taketani, S., Torikai, K., Hirakawa, K., Matsuda, H., 2005. Novel tissue-engineered biodegradable material for reconstruction of vascular wall. Ann. Thorac. Surg. 80 (5), 1821–1827. J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M AT E R I A L S Ju, Y.M., Choi, J.S., Atala, A., Yoo, J.J., Lee, S.J., 2010. Bilayered scaffold for engineering cellularized blood vessels. Biomaterials 31 (15), 4313–4321. Kawazoe, N., Inoue, C., Tateishi, T., Chen, G., 2010. A cell leakproof PLGA-collagen hybrid scaffold for cartilage tissue engineering. Biotechnol Prog. 26 (3), 819–826. Lamb, J.P., Vitale, T., Kaminski, D.L., 1983. Comparative evaluation of synthetic meshes used for abdominal wall replacement. Surgery 93 (5), 643–648. Langer, R., Vacanti, J.P., 1993. Tissue engineering. Science 260 (5110), 920–926. Lau, K.W., Dias, T., 1994. Knittability of high-modulus yarns. J. Text. Inst. 85 (2), 173–190. Leong, K.F., Chua, C.K., Sudarmadji, N., Yeong, W.Y., 2008. Engineering functionally graded tissue engineering scaffolds. J. Mech. Behav. Biomed. Mater. 1 (2), 140–152. Leong, K.H., Ramakrishna, S., Bibo, G.A., Huang, Z.M., 2000. The potential of knitting for engineering composites. Composites Part A 31 (3), 197–220. Lin, D., Li, Q., Li, W., Swain, M., 2009. Dental implant induced bone remodeling and associated algorithms. J. Mech. Behav. Biomed. Mater. 2 (5), 410–432. Liu, H., Fan, H., Wang, Y., Toh, S.L., Goh, J.C., 2008. The interaction between a combined knitted silk scaffold and microporous silk sponge with human mesenchymal stem cells for ligament tissue engineering. Biomaterials 29 (6), 662–674. Lu, H., Hoshiba, T., Kawazoe, N., Chen, G., 2011. Autologous extracellular matrix scaffolds for tissue engineering. Biomaterials 32 (10), 2489–2499. Ma, L., Gao, C., Mao, Z., Shen, J., Hu, X., Han, C., 2003a. Thermal dehydration treatment and glutaraldehyde cross-linking to increase the biostability of collagen-chitosan porous scaffolds used as dermal equivalent. J. Biomater. Sci. Polym. Ed. 14 (8), 861–874. Ma, L., Gao, C., Mao, Z., Zhou, J., Shen, J., Hu, X., Han, C., 2003b. Collagen/chitosan porous scaffolds with improved biostability for skin tissue engineering. Biomaterials 24 (26), 4833–4841. Mao, Z., Shi, H., Guo, R., Ma, L., Gao, C., Han, C., Shen, J., 2009. Enhanced angiogenesis of porous collagen scaffolds by incorporation of TMC/DNA complexes encoding vascular endothelial growth factor. Acta Biomater. 5 (8), 2983–2994. Marston, W.A., 2004. Dermagraft, a bioengineered human dermal equivalent for the treatment of chronic nonhealing diabetic foot ulcer. Expert. Rev. Med. Devic. 1 (1), 21–31. Ma, L., Shi, Y., Chen, Y., Zhao, H., Gao, C., Han, C., 2007. In vitro and in vivo biological performance of collagen-chitosan/silicone membrane bilayer dermal equivalent. J. Mater. Sci., Mater. Med. 18 (11), 2185–2191. Miki, H., Ando, N., Ozawa, S., Sato, M., Hayashi, K., Kitajima, M., 1999. An artificial esophagus constructed of cultured human esophageal epithelial cells, fibroblasts, polyglycolic acid mesh, and collagen. ASAIO J. 45 (5), 502–508. Munirah, S., Kim, S.H., Ruszymah, B.H., Khang, G., 2008. The use of fibrin and poly(lactic-co-glycolic acid) hybrid scaffold for articular cartilage tissue engineering: an in vivo analysis. Eur. Cells Mater. 15, 41–52. Murphy, C.M., Haugh, M.G., O’Brien, F.J., 2010. The effect of mean pore size on cell attachment, proliferation and migration in collagen-glycosaminoglycan scaffolds for bone tissue engineering. Biomaterials 31 (3), 461–466. Murphy, C.M., O’Brien, F.J., 2010. Understanding the effect of mean pore size on cell activity in collagen-glycosaminoglycan scaffolds. Cell Adh. Migr. 4 (3), 377–381. Nagato, H., Umebayashi, Y., Wako, M., Tabata, Y., Manabe, M., 2006. Collagen-poly glycolic acid hybrid matrix with basic fibroblast growth factor accelerated angiogenesis and granulation tissue formation in diabetic mice. J. Dermatol. 33 (10), 670–675. 4 (2011) 922–932 931 Nerem, R.M., 2000. Tissue engineering a blood vessel substitute: the role of biomechanics. Yonsei Med. J. 41 (6), 735–739. Ng, K.W., Hutmacher, D.W., 2006. Reduced contraction of skin equivalent engineered using cell sheets cultured in 3D matrices. Biomaterials 27 (26), 4591–4598. Ng, K.W., Khor, H.L., Hutmacher, D.W., 2004. In vitro characterization of natural and synthetic dermal matrices cultured with human dermal fibroblasts. Biomaterials 25 (14), 2807–2818. Ng, K.W., Louis, J., Ho, B.S.T., Achuth, H.N., Lu, J., Moochhala, S., Lim, T.C., Hutmacher, D.W., 2005. Characterization of a novel bioactive poly[(lactic acid)-co-(glycolic acid)] and collagen hybrid matrix for dermal regeneration. Polym. Int. 54 (10), 1449–1457. Nirmalanandhan, V.S., Rao, M., Sacks, M.S., Haridas, B., Butler, D.L., 2007. Effect of length of the engineered tendon construct on its structure–function relationships in culture. J. Biomech. 40 (11), 2523–2529. O’Brien, F.J., Harley, B.A., Yannas, I.V., Gibson, L.J., 2005. The effect of pore size on cell adhesion in collagen-GAG scaffolds. Biomaterials 26 (4), 433–441. Ouyang, H.W., Goh, J.C., Thambyah, A., Teoh, S.H., Lee, E.H., 2003. Knitted poly-lactide-co-glycolide scaffold loaded with bone marrow stromal cells in repair and regeneration of rabbit Achilles tendon. Tissue Eng. 9 (3), 431–439. Powell, H.M., Boyce, S.T., 2006. EDC cross-linking improves skin substitute strength and stability. Biomaterials 27 (34), 5821–5827. Powell, H.M., Supp, D.M., Boyce, S.T., 2008. Influence of electrospun collagen on wound contraction of engineered skin substitutes. Biomaterials 29 (7), 834–843. Puppi, D., Chiellini, F., Piras, A.M., Chiellini, E., 2010. Polymeric materials for bone and cartilage repair. Prog. Polym. Sci. 35 (4), 403–440. Pu, F., Rhodes, N.P., Bayon, Y., Chen, R., Brans, G., Benne, R., Hunt, J.A., 2010. The use of flow perfusion culture and subcutaneous implantation with fibroblast-seeded PLLAcollagen 3D scaffolds for abdominal wall repair. Biomaterials 31 (15), 4330–4340. Quaglini, V., Corazza, C., Poggi, C., 2008. Experimental characterization of orthotropic technical textiles under uniaxial and biaxial loading. Composites Part A 39 (8), 1331–1342. Ravi, S., Chaikof, E.L., 2010. Biomaterials for vascular tissue engineering. Regen. Med. 5 (1), 107–120. Ravi, S., Qu, Z., Chaikof, E.L., 2009. Polymeric materials for tissue engineering of arterial substitutes. Vascular 17 (Suppl. 1S), 45–54. Rezwan, K., Chen, Q.Z., Blaker, J.J., Boccaccini, A.R., 2006. Biodegradable and bioactive porous polymer/inorganic composite scaffolds for bone tissue engineering. Biomaterials 27 (18), 3413–3431. Roosa, S.M., Kemppainen, J.M., Moffitt, E.N., Krebsbach, P.H., Hollister, S.J., 2010. The pore size of polycaprolactone scaffolds has limited influence on bone regeneration in an in vivo model. J. Biomed. Mater. Res. A 92 (1), 359–368. Ruijgrok, J.M., de Wijn, J.R., Boon, M.E., 1994. Glutaraldehyde crosslinking of collagen: effects of time, temperature, concentration and presoaking as measured by shrinkage temperature. Clin. Mater. 17 (1), 23–27. Ruszczak, Z., 2003. Effect of collagen matrices on dermal wound healing. Adv. Drug Delivery Rev. 55 (12), 1595–1611. Sahoo, S., Ouyang, H., Goh, J.C., Tay, T.E., Toh, S.L., 2006. Characterization of a novel polymeric scaffold for potential application in tendon/ligament tissue engineering. Tissue Eng. 12 (1), 91–99. Schmidt, C.E., Baier, J.M., 2000. Acellular vascular tissues: natural biomaterials for tissue repair and tissue engineering. Biomaterials 21 (22), 2215–2231.