**7.2 Pre-vascular scaffolds**

Angiogenesis is an important factor in the bone healing process in both intramembranous and endochondral ossification healing ways [157]. In endochondral ossification cartilaginous continues in the vascular network and the network provides cell sources, nutrition, oxygen, and growth factors needed for the repair process, and then osseous tissue formed [18]. Osteogenesis is closely related to angiogenesis during neo-bone tissue formation [158]. Therefore, efforts to create pre-vascular structures in order to create fully functional and perfusable tissue constructs for clinical applications have been made with the help of 3D bio-printing [159]. Kuss et al. [160] designed a pre-vascularized scaffold with the co-culture of Human adipose-derived mesenchymal stem cells (ADMSC) and human umbilical vein endothelial cells (HUVEC) on the polycaprolactone/hydroxyapatite scaffolds. Then, the pre-vascularized scaffolds were implanted in the nude mice, and vascularization capacity was analyzed. The results confirmed the microvessel and lumen formation and increased the vascular network formation. In addition, Nulty et al. [161] implanted the prevascularized 3D-printed polycaprolactone scaffolds to treat critical-size bone defects. The prevascularization was performed using both human bone marrow stem/stromal cells (hBMSCs) and human umbilical vein endothelial cells (HUVEC) in a fibrin base bio-ink. The *in-vivo* results showed increased vascularization and as a result bone regeneration.

*Recent Advances, Challenges and Future Opportunities for the Use of 3D Bioprinting in Large… DOI: http://dx.doi.org/10.5772/intechopen.111495*

### **7.3 Osteochondral scaffolds**

Osteoarthritis is the most prevalent joint disease that affects millions of people around the world. The growing elderly population increases its prevalence and needs extra attention [162]. The avascular nature and rare source of repair cells lead to the restricted self-regeneration capability of cartilage [163] and existing treatment methods mostly repair single cartilage tissue and are not suitable for the whole osteochondral replacement [164]. As a result, the search for the best osteochondral substitute continues.

A study by Gardner et al. [165] investigated the composition and the porosity of the clinically available polycaprolactone (PCL) and β -tricalcium phosphate (β-TCP) and proposed the selected scaffold for intervertebral disc replacement. The results demonstrated that β-TCP facilitated the proliferation and differentiation of the C3H10 cells. According to the results, the design consisted of 45% porous material with 60% of β-TCP for the topper and lower parts and the core consisted of polycaprolactone with 70–75% porosity to stimulate the cartilage section of the intervertebral disc (**Figure 3** (**A-B**)). In another study, Kilian et al. [166] introduced a novel combination

#### **Figure 3.**

*(A-B): Novel 3D printed polycaprolactone and b-tricalcium phosphate (β-TCP) scaffold for intervertebral disc replacement [165]. (C) the multi-component 3D plotted scaffolds designed by Kilian et al. [166] for osteochondral treatment consisted of three main parts to stimulate natural osteochondral structures.*

for osteochondral bio-printing. The bio-ink included the alginate-methylcellulose with calcium phosphate cement. The multi-channel 3D plotted structure consisted of three parts. The upper part contained the hMSC cell-laden hydrogel and partly mineralized calcium phosphate cement. The middle part was a biphasic interwoven network of both hMSC cell-laden hydrogel and calcium phosphate cement to mimic calcified cartilage and the bottom part was the subchondral bone that was created using pure calcium phosphate cement (**Figure 3** (**C**)). The results demonstrated high efficiency of cell survival with the potential of redifferentiation to produce cartilage extracellular matrix (ECM) components.

Shim et al. [167] described a 3D bio-printed construct with different ECM types for osteochondral tissue engineering. The bio-ink was constructed on the base of atelocollagen and supramolecular hyaluronic acid loaded with human mesenchymal stromal cells. The mixture of Cucurbit[6]uril/conjugated HA and 1,6-diaminohexane -conjugated HA ledto creating a biphasic stable construct without the need for chemical cross-linker and physical stimuli. The designed scaffolds were tested in the knee joint of rabbits and showed remarkable osteochondral regeneration after 8 weeks.
