Johanna Buschmann

Additional information is available at the end of the chapter Johanna BuschmannAdditional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73507

#### **Abstract**

Critical size tendon defects demand for tissue samples replacing the missing tissue and guiding an effective healing. Autografts, allografts, or xenografts represent viable options; however, limited availability and donor site morbidity go along with this approach, representing big disadvantages. Tissue engineering of tendon tissue is a further strategy fulfilling this need. Basically, an appropriate scaffold material is developed and tested for its biomechanical suitability as a graft material. In addition, cell seeding might improve biointegration of the tissue engineered construct (TEC). Different cell sources as well as different cultivation procedures can be applied in order to tune the envisioned primary strength of the TEC. In this chapter, *in vitro* fabrication protocols and mechanical tests as well as animal *in vivo* experiments will be presented—covering various (bio)materials, cell types, and cultivation procedures.

DOI: 10.5772/intechopen.73507

**Keywords:** tendon, graft, scaffold, biomechanics, gene therapy, growth factor

#### **1. Introduction**

Tendon injuries as encountered by accidents may end up in complete ruptures, going along with tissue defects that have to be replaced with the aim to regain full function—without pain. In order to offer the body suitable substitutes for what it has lost, materials are needed that guide and stimulate the healing process and finally lead to a fully integrated and sufficiently stable tissue. Main problems occurring after tendon rupture repairs are insufficient strength (leading probably to re-ruptures) and adhesion formation (leading to a diminished range of motion) [1].

Best grafts for the reconstruction of injured tendons are obviously tendons themselves, however, although sometimes possible, tendon grafts are very limited in terms of availability and have to be decellularized before application if they are allografts or even xenografts to avoid

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

transplant rejections. Only autografts are easily transplanted—but the donor site morbidity may cause a lot of pain and go along with impaired function. In addition, other disadvantages of autografts are reported to be insufficient strength [2] because other tendons than the one to be replaced might be different in strength, cellularity as well as gliding capacity [3].

In this chapter, natural and synthetic materials as well as combinations of them are presented. Moreover, different types of cells seeded onto TECs are compared and their performance *in vitro* and *in vivo* [12] are discussed in a step-by-step manner with criteria set as evaluation milestones [13]. Although many of these approaches are highly promising in animal studies, they did not yet find their ways into clinical application because the success of new graft materials is finally dictated by clinical outcomes of studies where graft materials are implanted into the human body—and clinical trials are not only expensive, they also take a long time to be performed. Tissue engineered constructs that were developed 10 years ago might only now be ready to be judged and tested in terms of clinical success or failure.

Tissue Engineering of Tendons

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http://dx.doi.org/10.5772/intechopen.73507

Before we turn our interest toward tissue engineering of tendons, a brief summary of what native healthy tendon tissue is composed of and of the characteristics of human and selected animal tendons is given here. The hierarchical structure of the tendon tissue is very well-known and has been characterized by multiple imaging and analysis techniques [14]. Starting with the smallest molecular entity, tropocollagen molecules assemble to form microfibrils. Covalently connected, these microfibrils form sub-fibrils and fibrils of the collagen which is typically seen in histological sections of tendon tissue as slightly waved "crimps" [15, 16]. Fibrils form bundles resulting in fascicles. Between the fascicles, there are cell-rich layers called endotenons that can be very well seen in histological sections, as the tenocytes form "lines," one behind each other

In addition, there are some tendons that have a peritenon around the whole tendon. The peritenon is a thin sheath around the tendon, but should not be confused with the tendon sheath on intrasynovial tendons [20]. More information on the organization of the tendon tissue are found in several articles [14, 21, 22], with a special emphasis on the extracellular matrix (ECM) [23]. In the ECM, the main component is collagen I (around 95% of the dry weight). The noncollagenous part of tendons is composed of proteoglycans like lubricin, decorin or biglycan, glycosaminoglycans (GAGs; typically encountered as chondroitin sulfate, dermatan sulfate or heparan sulfate) and glycoproteins such as fibronectin. Proteoglycans are important for tissue hydration (especially decorin) and it was found that they are essential for limiting the viscoelastic behavior by preventing tissue fatigue [24]. When GAGs of an extracted fascicle were enzymatically digested, they exhibited higher reductions in failure stress and more stress relaxation, supporting the regulation of viscoelasticity [24]. Noteworthy, water makes up 60–80 wt% of the entire tendon tissue and is—together with the GAGs—a highly important component regulating viscoelasticity [25]. Moreover, elastin has not to be neglected although it makes up only 2% of the tendon dry weight. Elastin fibers are found closely to the tenocytes—

Tenocytes are the mature tendon cells, while tenoblasts are the immature ones. Tenoblasts build up the ECM components. They are spindle-shaped, very similar to fibroblasts—and their

connected by gap junctions that are important for mechanotransduction [17–19].

**2. Native tendons**

**2.1. Structure and composition of tendons**

the cells in the tendon tissue [26].

Hence, an excellent alternative to decellularized tendons is the tissue engineered construct (TEC) aimed to be attached to tendon stumps [4] (**Figure 1**). In this field, tissue engineering has covered natural materials like collagen constructs [5], combinations of natural and synthetic components as realized in PLGA and alginate [6] or entirely synthetic polymers such as PCL ± PEO [7]. Many reports on seeding cells onto the corresponding materials have determined their impact, including extracellular matrix deposition and inherently going along changes in stability [8]. Other strategies include growth factors implemented in the graft material [9, 10] with the ultimate aim to be released sustainably to the repair site in order to support and accelerate the innate healing process [11].

**Figure 1.** Fabrication of a tissue engineered construct. As a first step, a scaffold material is used and processed as exemplified by electrospinning. Then, cells may be seeded onto the construct. After that, the cell-seeded construct may be cultivated under static conditions or under perfusion in a bioreactor before being implanted into an appropriate animal model. As a final step, performance of the TEC is assessed in a clinical trial.

In this chapter, natural and synthetic materials as well as combinations of them are presented. Moreover, different types of cells seeded onto TECs are compared and their performance *in vitro* and *in vivo* [12] are discussed in a step-by-step manner with criteria set as evaluation milestones [13]. Although many of these approaches are highly promising in animal studies, they did not yet find their ways into clinical application because the success of new graft materials is finally dictated by clinical outcomes of studies where graft materials are implanted into the human body—and clinical trials are not only expensive, they also take a long time to be performed. Tissue engineered constructs that were developed 10 years ago might only now be ready to be judged and tested in terms of clinical success or failure.
