**4.5 Cartilage**

 Because of its avascular nature and the presence of low cell densities, cartilage defects will not completely self-regenerate [64]. It is therefore important that different treatment possibilities are investigated to initiate its' regeneration.

Xu et al. fabricated a loadbearing cartilage scaffold using an inkjet printer to bioprint rabbit articular chondrocytes suspended in a fibrin–collagen hydrogel onto an electrospun PCL matrix [65]. It was found that by including a PCL fibrous network within the scaffold, mechanical properties were significantly increased. Scaffolds implanted subcutaneously in mice developed a dense, well organized

#### **Figure 4.**

*Stereolithographic process to fabricate a cell-laden bone model to study breast cancer metastasis. (1) Homogenous suspension of MSCs and osteoblasts in a photocurable hydrogel; (2) cell-laden 3D structure that is seeded with breast cancer cells; (3) a close-up of homogeneously adhering MSCs/osteoblasts and a suspension of breast cancer cells.* 

collagen, not present in the control group (unseeded scaffolds). The cartilage tissue formation within the bioprinted scaffolds appeared histologically similar to normal elastic cartilage.

The use of PCL to provide structural support was also employed by Schuurman et al., who extruded bioprinted GelMA embedded with chondrocytes [66]. For improved cartilage development, scaffolds were also fabricated with a GelMA/hyaluronic acid hydrogel. The inclusion of hyaluronic acid improved cell viability *in vitro*, however when implanted in mice, both scaffolds types achieved similar levels of cartilage tissue formation. Other extruded PCL-alginate gel scaffolds that contained chondrocytes also reported the formation of cartilage after being implanted into the dorsal subcutaneous space of mice [67].

*In situ* bioprinting has been investigated by Cui et al. using human chondrocytes suspended in a poly(ethylene glycol) dimethacrylate (PEGDMA) solution [68, 69]. Scaffolds were printed directly into osteochondral plugs that had been harvested from bovine femoral condyles. Compared to scaffolds fabricated outside of the defect, the *in situ* bioprinted scaffolds had enhanced tissue integration. Another *in situ* bioprinting method has been developed by O'Connell et al. [70]. Based on an extrusion bioprinting technique, an "*in-situ* biopen" has been made comprising of two ink chambers and an extruder nozzle with a UV source that enables direct application of a bioink during surgery. Using the biopen, a human infrapatellar fat pad adipose stem cell (IPFP) laden gelatin-methacrylamide/hyaluronic acidmethacrylate (GelMA/HAMa) hydrogel scaffold was fabricated for *in vitro* testing. It was observed that the viability of biopen printed cells was 97% after 7 days.

#### **4.6 Muscle**

Bioprinting has shown great potential for building highly hierarchically organized cellular structures that comprise muscle tissue. Miri et al. created hierarchical cell-laden structures to mimic multicellular tissues [71]. Using extrusion-based bioprinting, multiple bioinks were deposited onto microfluidic chips. Structures resembling musculoskeletal junctions were printed using poly(ethylene glycol) diacrylate (PEGDA) and GelMA that were loaded with NIH/3T3 fibroblasts and C2C12 skeletal muscle cells. *In vitro*, the chips retained their printed interfaces

#### *Bioprinting DOI: http://dx.doi.org/10.5772/intechopen.81135*

and demonstrated adequate proliferation rates*.* PEGDA-framed chips that had a concentration-gradient of methacrylated gelatin (GelMA) ranging from 5 to 15%, were implanted subcutaneously in rats. After 30-days, enhanced cell proliferation was reported for regions containing 10% GelMA, signifying a great potential for angiogenesis. This study demonstrates the possibility to create hierarchically cellladen structures to mimic multicellular tissues.

Bajaj et al. used stereolithography to bioprint muscle tissues composed of mouse embryonic stem cells (mESCs) and mouse myoblast cell line (C2C12), embedded in PEGDA [72]. Live/dead staining showed a slight decrease in cell viability for the bioprinted scaffolds, possibly due to the non-adhesive properties of PEGDA.
