Recent development in multizonal scaffolds for osteochondral regeneration

Abstract

Osteochondral (OC) repair is an extremely challenging topic due to the complex biphasic structure and poor intrinsic regenerative capability of natural osteochondral tissue. In contrast to the current surgical approaches which yield only short-term relief of symptoms, tissue engineering strategy has been shown more promising outcomes in treating OC defects since its emergence in the 1990s. In particular, the use of multizonal scaffolds (MZSs) that mimic the gradient transitions, from cartilage surface to the subchondral bone with either continuous or discontinuous compositions, structures, and properties of natural OC tissue, has been gaining momentum in recent years. Scrutinizing the latest developments in the field, this review offers a comprehensive summary of recent advances, current hurdles, and future perspectives of OC repair, particularly the use of MZSs including bilayered, trilayered, multilayered, and gradient scaffolds, by bringing together onerous demands of architecture designs, material selections, manufacturing techniques as well as the choices of growth factors and cells, each of which possesses its unique challenges and opportunities.

Keywords: Cells; Fabrication; Growth factors; Multizonal scaffolds; Osteochondral regeneration.

Graphical abstract

Fig. 1

Decision-making process in fabrication of a novel MZS. Choices of materials and architectures are crucial for the biological and mechanical performances of the MZS. Fabrication methods need to be adapted to increase the degree of control on the structural and composition parameters. Loading GFs and/or cells are add-on strategies to adjust the chondrogenic and osteogenic properties of corresponding regions by providing biological and environmental cues.

Fig. 2

Schematic illustration of the multilayered structure of osteochondral tissue and its main individual components, including collagen fibers, chondrocytes, and extracellular matrix composition.

Fig. 3

A. A bilayered scaffold consisting of a 3D printed gelatin-based matrix with HAP in the bone layer (BL), and growth factors (GFs) in the cartilage layer (CL). Adapted with permission from Ref. [76]. B. Bilayered scaffold with BL and CL separated by a thin electrospun PCL tidemark. Adapted with permission from Ref. [94]. C. A category of trilayered scaffolds used bioceramics in the middle layer (calcified cartilage layer, CCL) in addition to BL. Adapted with permission from Ref. [96]. D. In another version of trilayered scaffolds, a non-mineralized middle layer (ML) was used. Adapted with permission from Ref. [58]. E. A high degree of complexity can be achieved with multilayered scaffolds, with more than three layers, multiple polymeric materials, bioceramics, and GFs. Adapted with permission from Ref. [65]. F. A category of gradient scaffolds used gradient porosity to reproduce the structure features of the OC tissue (the pore size was in the range of 360–700 μm). Adapted with permission from Ref. [72]. G. Gradient composition scaffolds usually involve progressive HAP contents with a higher HAP content (30 wt%) in the BL. Adapted with permission from Ref. [97]. H. Gradient scaffolds fabricated from ECM take advantage of the gradient porosity and composition naturally present in the OC tissue but require a decellularization process and/or in combination with other materials. Adapted with permission from Ref. [66].

Fig. 4

Map of the materials and their combination used in recent MZSs. Polymeric materials, especially chitosan, PCL and PLGA, are the most popular materials used as a matrix. Bioceramics are used as mineral additions in the BL. A. Trilayered scaffolds with metal in the BL and polymeric materials in the CL. Adapted with permission from Ref. [81]. B. Trilayered scaffolds composed of chitosan, glycerophosphate, and gelatin at various ratios and gradient porosities (86–95%). Adapted with permission from Ref. [108]. C. Bilayered scaffolds with alginate and agarose hydrogel reinforced by PCL, PLA, or PLGA fibers in the bone layer. Adapted with permission from Ref. [91]. D. Pure PLGA scaffolds divided into two regions with different pore sizes (100–200 μm and 300–450 μm). Adapted with permission from Ref. [68]. E. Cell seeded trilayered scaffolds with alginate hydrogel and cartilage ECM in the CL, addition of PLGA microspheres in the ML, and PLGA microspheres in the bone layer. Adapted with permission from Ref. [23]. F. Bilayered scaffolds made of decellularized cartilage ECM (134 μm pores) in the CL and decalcified bone ECM (336 μm pores) in the BL. Adapted with permission from Ref. [80]. G. Trilayered scaffolds with varying volume fractions of SF and HAP inclusion in the BL. Adapted with permission from Ref. [98]. H. Trilayered scaffolds with different chitosan:collagen ratios and embedded OCP in the BL. Adapted with permission from Ref. [111]. I. 3D printed bilayered and trilayered scaffolds with alginate-methylcellulose (MC) and alginate-MC-CaP inks. Adapted with permission from Ref. [190]. J. Trilayered scaffolds with a top layer made of methacrylated hyaluronic acid (MeHA) hydrogel embedding Diclofenac to regulate inflammation, and 3D printed PCL-MeHA and PCL-TCP layers. Adapted with permission from Ref. [188]. K. Trilayered scaffolds with PEG-diacrylate and N-acryloyl 6-aminocaproic acid (A6ACA) cryogel matrix and CaP in the BL, PEGDA-CaP in the CCL and pure PEGDA in the CL. Adapted with permission from Ref. [21]. L. Trilayered scaffolds combining oriented cartilage ECM, compact PLGA-TCP, and porous PLGA-TCP. Adapted with permission from Ref. [64]. M. Decellularized porcine articular cartilage ECM scaffolds with gradient lattice-arranged conical micropores added by laser. Adapted with permission from Ref. [125].

Fig. 5

Representative fabrication processes of MZSs manufactured by multilayered lyophilization or single layered lyophilization in combination with other techniques. A. Schematic illustration of the preparation of scaffold with different pore structures through lyophilization. Adapted with permission from Ref. [51]. B/C. Schematic illustration of the three-step process of the iterative layering process and SEM images of the three-layered scaffold fabricated by multilayered lyophilization. Adapted with permission from Ref. [113]. D. PMMA mold used to unidirectionally freeze collagen suspensions to fabricate lamellar superficial layer. Adapted with permission from Ref. [22]. E/F. Schematical illustration of the lyophilization bonding process and SEM images of the monolithic MZS with distinct zonal specific fiber orientations. Adapted with permission from Ref. [259]. G/H. Schematic illustration of the process involved in the cryostructuring process for the fabrication of the three-layered osteochondral scaffold. Adapted with permission from Ref. [260]. I. Schematic image shows pore size gradient in different zones of the trilayered scaffold design prepared by lyophilization in combination with a porogen-leaching out method. Adapted with permission from Ref. [59]. J. Schematic illustration of the design and osteochondral strategy of the biphasic scaffold with dense and nanofibrous morphologies fabricated by a combination of lyophilization, leaching out and phase separation methods. Adapted with permission from Ref. [78]. K. Schematic illustration of the methodology used for the preparation of a bilayered OC scaffold using the combination of lyophilization and liquid phase synthesis method. Adapted with permission from Ref. [261].

Fig. 6

Representative fabrication processes of MZSs manufactured by electrospinning-based, 3D printing and other strategies. A/B. Schematic process and an SEM image of the chitosan/nHAP porous layer and zein/POSS fiber layer fabricated by a combination of ultrasonication, lyophilization, and electrospinning. Adapted with permission from Ref. [287]. C. Schematical illustration of the process for fabrication of a PCL-GO-collagen scaffold using repeated electrospinning. Adapted with permission from Ref. [288]. D. Schematics of representative 3D printing techniques: a) inkjet, b) extrusion, c) laser-assisted, and d) stereolithography printing. Adapted with permission from Ref. [289]. E. Schematic of advanced extrusion-based 3D pneumatic bioprinting system affiliated with a temperature controller. Adapted with permission from Ref. [290]. F. Schematic of a 3D multi-nozzle pneumatic printing system used to fabricate gelatin methacrylate (GelMA)/nHAP-based scaffold. Adapted with permission from Ref. [110]. G. Schematical diagram of the fabrication processes and SEM images of the bilayered integrated OC scaffold with inconsecutive channels obtained using SLS. Adapted with permission from Ref. [291]. H. Schematic drawing of the MEW setup and the fiber network for fabrication of a trilayered scaffold. Adapted with permission from Ref. [112]. I. schematic illustration of the fabrication process of a full-scale OC graft consisting of natural chondrocytes secreted ECM cartilage layer and sintered microsphere scaffold (SMS) subchondral bone layer. Adapted with permission from Ref. [23]. J. Schematic illustration of the gradient polarization process using a DC electric field to grant uniform scaffolds with gradient piezoelectricity for osteochondral regeneration. Adapted with permission from Ref. [209].

Fig. 7

A. Four continuous and overlapping stages involved with bone/cartilage tissue regeneration and important immune molecules and signaling during tissue regeneration. Adapted with permission from Ref. [324]. B. Anatomical illustration of major types of cells residing in osteochondral tissue. Adapted with permission from Ref. [325]. C. Multiple anatomical sites of long bone skeletal stem cells in mice. Adapted with permission from Ref. [332]. D. Different mechanisms of cartilage regeneration promoted by MSCs. Adapted with permission from Ref. [333].

Fig. 8

Illustration of typical loading routes and release patterns of natural and synthetic growth factors for osteochondral repair.