The scaffold provides a place for seed cells to grow, multiply, metabolize, and exchange substances, and can effectively control the growth location of transplanted cells:The scaffold can guide the regeneration of tissues, and its size and shape have an important impact on the structure and function of tissues; the scaffold can provide mechanical support for newborn tissues, resist external pressure, and maintain the original shape of tissues and the integrity of tissues; in addition, the scaffold can also serve as an active In addition, scaffolds can also serve as carriers of active factors, slow release of some bioactive substances such as growth factors, and promote the growth, proliferation and differentiation of cells.
The research and development of scaffold materials is the key to tissue engineering. In this paper, we review the current research results of tissue-engineered tendons and introduce the characteristics of ideal scaffolds, the development of scaffold materials and the development of scaffold manufacturing process, and provide the most suitable living environment for tendon cells through the comprehensive consideration of scaffolds, seed cells, and growth factors, so that tissue-engineered tendons will become an ideal and reliable method to repair tendon defects.
Tendon defect is one of the common clinical diseases. Tendon injury, if not repaired in time, often leads to limb dysfunction and even disability in severe cases. Therefore, the surgical repair and reconstruction of function after tendon injury or defect is one of the very important research topics in surgical surgery.
Tendon injury can be divided into two categories: non-defective injury and defective injury For tendon with defective injury, the following treatment methods are available:
1) Autologous tendon grafting to repair tendon defects;
2) Allogeneic tendon grafting;
3) allogeneic tendon grafting; 4) artificial tendon substitutes. The former is limited by the lack of donor tendon or immune rejection. With the development of cell culture technology and transplantation technology and biomaterials science, a new ideal tendon substitute, tissue-engineered artificial tendon, will finally solve the problem of repairing defective tendons.
1.Characteristics of tissue-engineered tendon cell scaffold
An ideal in vivo graft should meet the following points.
(1) The cell scaffold material must be non-toxic and have good biocompatibility;
(2) The material must be biodegradable and can be gradually degraded and metabolized in vivo with the proliferation of cells, and then absorbed;
(3) The degradation products of the material must be non-toxic and have good biocompatibility, and will not have adverse effects on the tissue and the organism;
(4) The material must have good processing properties and be able to be processed into the desired shape and structure;
(5) The scaffold must have an open pore structure, and its pore size must meet certain requirements;
(6) The scaffold must have the same shape and size as the tissue or organ to be regenerated or repaired;
(7) The scaffold must have good cell affinity, suitable for cell adhesion, proliferation, and matrix secretion;
(8) The scaffold must have certain mechanical properties, including strength, flexibility, etc.;
(9) The scaffold must be able to withstand sterilization without physical, chemical and biological force changes under conventional sterilization conditions;
(10) The scaffold should not only keep its shape during the cell culture operation, but also withstand the surgical operation in the implanted body to ensure that it will not break during the operation, fit the body, and will not form mechanical damage to the body tissue.
2.Materials of scaffold
2.1 Collagen
Collagen is the main component of the extracellular matrix (ECM); it can be extracted from animal bones and fascia through multiple processes such as boiling and hydrolysis. In the process of evolution, collagen retains its original amino acid sequence, which makes the scaffold material non-antigenic, biocompatible and permeable in vivo; moreover, since the tissue of tendon is mainly composed of thick bundles of collagen fibers arranged in parallel, their orientation is consistent with the traction force they are subjected to.
The collagen fibers are tough and resistant to traction; in addition it contains its own cell adhesion signal sequences that can guide the cells to specific recognition of the scaffold material; the collagen fibers are tough and resistant to traction Its preparation method has been developed over the years and is quite mature, commercially available, and has been approved by the US FDA for successful use as an extracellular matrix scaffold for tissue engineered tendons. Bellincampi et al. used autologous tendon cells inoculated with collagen scaffold and then implanted in the knee joint and subcutaneously in rabbits, and the complex was still visible after 8 weeks. They found that the tendons treated with MSC were thicker and had better collagen fiber assembly, seam characteristics and loading properties than the control group.
Award used type I collagen as a scaffold and implanted back into the autologous tendon defect, while the control group was implanted back with type I collagen alone, and the biomechanical effect of the experimental group was found to be significantly better than that of the control group after 4 weeks, but the histological examination was not significantly different from that of the control group.
2.2 Fibronectin ( fibronectin , FN)
It is mainly found in the extracellular matrix (i.e., cellular type), but also in the blood (called plasma type), and belongs to the class of glycoproteins. As a major component of the extracellular matrix, FN plays an important role in many biological processes, such as cell adhesion, cell proliferation and differentiation, cytoskeleton formation and apoptosis, and is also involved in numerous pathological processes of the body, including wound healing and inflammation.
Fibrin gels are three-dimensional mesh-like gels with plasticity, adhesion, degradability and biocompatibility, formed by polymerization of fibrin monomers under the action of thrombin, which can provide sufficient time for gel shaping by slowing down the aggregation of thrombin and therefore its transformation from liquid to gel. The fibrin gel releases platelet-derived growth factor (PDG F ) and transforming growth factor β ( TG F-β ) during polymerization, which have chemotactic and mitogenic effects and can further promote cell proliferation, adhesion and matrix secretion. However, it cannot improve sufficient mechanical strength, which is a common disadvantage of natural biomaterials, and furthermore it comes from blood:difficult to obtain, thus its application is limited.
2.3 Calcium polyphosphate fiber (CPFF)
CPFF is made of calcium dihydrogen phosphate or calcium metaphosphate as the main raw material. It is made into fibrous inorganic material by high-solution drawing. It has a fibrous appearance and mechanical properties similar to carbon fibers, but its histocompatibility and degradability are significantly better than those of carbon fibers, and it may become a new material ideal for building composite scaffolds for tendon tissue engineering in the future. Changqing et al [5] experimentally demonstrated that. It is degradable in aqueous solution in vitro. The degradation process is hydrolysis. Instead of enzymatic degradation. The experimental results showed that CPPF was completely degraded in vivo in about 16-20 weeks. Moreover, it has good degradation properties and histocompatibility; the fibers can also be degraded in a controlled manner by adjusting the ratio of synthetic raw material components.
2.4 Poly-alpha-hydroxy acids
Polyalpha-hydroxy acids include polylactic acid (PLA), polyhydroxyacetic acid (PGA) and their copolymers PLGA, PDLA, PLLA, PDLLA, which have three main structural forms (fibrous scaffold, porous foam, tubular structure); the degradation products of PLA and PGA are lactic acid and hydroxyacetic acid respectively, which are intermediate metabolites of the triple shuttle acid cycle. With good biodegradability and compatibility, they do not cause inflammatory reactions, immune reactions, and cytotoxic reactions, and are by far the most widely used degradable biomaterials, which have been widely used for tissue engineering of bone, cartilage, blood vessels, nerves, and skin.
Cao et al [7] inoculated tendon cells obtained from tendon tissues of calf shoulder and knee on a sorrel-shaped PGA mesh scaffold and implanted under the skin of nude mice after one week of in vitro culture and found that at 12 weeks, tendon tissues similar to normal tendon structure with some degree of biomechanical properties could be formed. Later, C ao et al [7] used autologous tendon cells + PGA + biofilm wrapping to repair a 4-cm tendon defect within the Leghorn muscle and found that the implanted tissue-engineered tendons were similar to normal tendons only in gross morphology and histology, and their biomechanical properties were 83% of those of normal tendons.
PLGA, a copolymer of PLA and PGA, not only has good biocompatibility and can induce up-regulated transcription of certain genes, but also its degradation rate can be controlled by changing the ratio of PLA to PGA, and it combines the high degradation rate of PGA and the high strength of PLA, so PLGA can also be used as a cellular scaffold for artificial tendons.
Ouyang and Goh et al. also used poly(lactic-co-glycolic acid) and poly(hydroxyacetic acid) complex [poly(lactic-co-glycolicacid), PLGA] as a scaffold and implanted it back into a 10 mm size defect of the autologous tendon, while the control group was implanted with PLGA alone, and the cell content was found to be significantly reduced after 4 weeks compared with that at 2 weeks, and collagen type I and III fibers were formed, and The experimental group was significantly stronger than the control group; at 8 weeks, the material was basically degraded; at 12 weeks, the defect was well repaired and no lymphocyte infiltration was seen, and the biomechanical strength of the experimental group was significantly higher than that of the control group, approaching normal tendon. This is consistent with the results of previous studies by Rodkey et al. and Sato et al. using PGA/Dacron as a scaffold.
3. Outlook
Tissue-engineered cell scaffold materials are a key focus and difficulty in the research of the Division of Tissue Engineering. Without a suitable scaffold, seed cells will be lost and die. Tissue engineering scaffold materials should have good biocompatibility, biodegradability, three-dimensional structure and plasticity with considerable mechanical strength in addition to good surface activity to facilitate the adhesion of seed cells and provide a good microenvironment for cells to grow and multiply on its surface and secrete matrix.
For tendons, tissue engineering scaffold materials are currently more studied as natural materials, synthetic materials and composite materials. Natural materials such as collagen have good biocompatibility, but have poor mechanical properties, degradation too quickly and poor processing and molding properties; synthetic materials such as bioceramics and polymers have low degradation rates, acidic degradation products cause inflammatory reactions and mechanical properties and other defects; these problems can be solved by the principle and method of composite materials, that is, two or more biological materials with complementary characteristics, in a certain ratio and manner. These problems can be solved by the principle and method of composite materials, which is to combine two or more biological materials with complementary properties in a certain ratio and manner, in order to construct a new composite material that can meet the requirements.
Tissue engineering of tendons has high requirements for scaffold materials, and the development of composite materials will continue to be a hot spot for future research, and further research is needed in the preparation process, design and optimal combination of properties of materials, which is one of the main directions for future research development of tissue engineering materials.