Hi guys! This week in class, we learned that the emerging field of tissue Engineering involves replacing, repairing or enhancing biological function at the scale of a tissue or organ by manipulating cells via their extracellular environment.From my understanding, tissue engineering intends to help the body to produce a material that resembles as much as possible the body’s own native tissue.

Scaffolds are often used in tissue engineering. They serve as temporary or permanent artificial Extracellular Matrices (ECM) to accommodate cells and support 3D tissue regeneration. They can also serve as delivery vehicles for exogenous cells, growth factors and genes and as a matrix for cell adhesion. They structurally reinforce the defect to maintain the shape of the defect and prevent distortion of surrounding tissues and act as a barrier to prevent the infiltration of surrounding tissues that may impede the regeneration process.

In general, the requirements of scaffolds for tissue engineering are:
(i) Three-dimensional and highly porous with an interconnected pore network for cell growth and flow transport of nutrients and metabolic waste.
(ii) Biocompatible and bioresorbable with a controllable degradation and resorption rate to match cell/tissue growth in vitro and/or in vivo.
(iii) Suitable surface chemistry for cell attachment, proliferation, and differentation .
(iv) Mechanical properties to match those of the tissues at the site of implantation.

Figure 1. Examples of trabecular scaffolds

Metallic scaffolds have been widely used in tissue engineering because of their good mechanical properties. The main disadvantage of metallic biomaterials is their lack of biological recognition on the material surface. To overcome this restraint, surface coating or surface modification presents a way to preserve the mechanical properties of established biocompatible metals improving the surface biocompatibility. Another limitation of the current metallic biomaterials is the possible release of toxic metallic ions and/or particles through corrosion or wear that lead to inflammatory cascades and allergic reactions, which reduce the biocompatibility and cause tissue loss.

One example of metallic scaffold is tantalum scaffold. Porous tantalum is a biomaterial with a unique set of physical and mechanical properties.It has a high-volume porosity (>80%) with fully interconnected pores to allow secure and rapid bone ingrowth. Studies have demonstrated substantial cortical bone ingrowth between the trabecular network as well as high levels of bone growth onto the scaffold itself. Initial stability of the trabecular metal itself is also higher than that of standard materials, such as cobalt chrome. Furthermore, this new material offers better osteoconduction than other technologies used for biological fixation.

Figure 2. Tantalum scaffolds

Titanium is found to be well tolerated and nearly an inert material in the human body environment.In an optimal situation titanium is capable of osseointegration with bone. In addition, titanium forms a very stable passive layer of TiO2 on its surface and provides superior biocompatibility. The nature of the oxide film that protects the metal substrate from corrosion is of particular importance. In general, porous titanium and titanium alloys exhibit good biocompatibility. Bioactive titanium meshes have been successfully used in spine fusion surgery for the past two decades.

Figure 3. Titanium scaffolds

Future directions of research in metallic scaffold will probably focus on the efficient combinations of osteoinductive materials, osteoinductive growth factors and cell-based tissue regeneration approach using composite constructs carriers to reconstruct and repair hard tissues. The goal is to obtain a functional replacement of the injured hard tissue in a procedure that avoids the step of bone harvesting.

References:
[1] Meyer, Ulrich, et al., eds. Fundamentals of tissue engineering and regenerative medicine. Springer Science & Business Media, 2009.
[2] Pallua, Norbert, and Christoph V. Suschek, eds. Tissue engineering: from Lab to Clinic. Springer Science & Business Media, 2010.
[3] Alvarez, Kelly, and Hideo Nakajima. “Metallic scaffolds for bone regeneration.” Materials 2.3 (2009): 790-832.