Biodegradable Hydrogel Potentially Used As Injectable Medical Material

Tissue engineering has been a new growing point in the field of Science & Technology since the end of last century and might revolutionize Surgery. Biomaterial is one of the keys in tissue engineering. The subject of this work is an injectable polymer hydrogel potentially as tissue engineering materials. Compared with the pre-fabricated porous scaffolds, the injectable hydrogel has some advantages such as unnecessary to use scalpel and easy to fill any irregular and complicated caves in damaged tissues. As a kind of tissue engineering materials, injectable hydrogel is quite unique, albeit emerged lately.A typical and chief injectable hydrogel is triblock copolymer, PEO-PPO-PEO with the trade name, Pluronics. Its potential application in tissue engineering has, however, been widely patented by western researchers. After many discussions with surgeons focusing on tissue construction in the field of tissue engineering, we realized that the Pluronics hyrogel is, as its disadvantages, re-dissolved in the cultured medium and difficult to control the degradable rate. To resolve the two problems, we introduced the chemical crosslinking reaction into the preparation of hydrogel. The underlying chemical hydrogels were designed to be crosslinked after injection into the body and then to be biodegraded with tunable degradation rates under the physiological conditions.In this work, the macromer technique was employed to prepare hydrogel. The macromer has a hydrophilic central block polymer, extended with oligo(a-hydroxy acid) and terminated with acrylol groups. The mixture solution of macromers and cells are proposed to be, in the future, injected into the damaged tissues or organs, and the biodegradable hydrogel could be formed in situ at the body temperature; the hydrogel acts as a tentative cell scaffold; new tissues might be gradually formed with secretion of extracellular matrices and biodegradation of hydrogels etc. Although we have not yet peformed animal experiments so far, all of the work reported in thisthesis have been done to meet various requirements of tissue construction to the best of o ur k nowledge oft issue e ngineering. T he s ystematic r esearch i n s uch a k ind o f hydrogel is, as we know, rare in the world. Some new ideas have also emerged in the process of our researches.Our associated researches are listed mainly as follows:1. Synthesis of biodegradable macromer and in situ preparation of chemcially crosslinked biodegradable hydrogel. The synthesized macromer is composed of an FDA-permitted PEG or Pluronics as the central block, extended with oligo(oc-hydroxy acid) permitted also by FDA, and terminated with acrylol groups. Hydrogel is formed after polymerizaing crosslinkable hydrogel in aqueous solutions at 37°C. The redox initiation system was employed with persulfate salt as initiator and tetramethylethylenediamine (TEMED) as accelerator.Considering the possible requirements of injectable materials in tissue engineering, the hydrogel has been examined systematically such as the preparation condition, biodegradation behaviors, mechanical properties, swelling ratio and water absorption etc. For example, we have achieved approximate gelation time by adjusting initiator and accelerator concentrations; the temperature of the original macromer solution was elevated merely to 0.6°C in polymerization and thus in situ chemical crosslinking may e xhibit no serious thermal effect to surrounding tissues; elastice modulus of the chemical hydrogel is higher than the associated Pluronics hydrogel; The resultant hydrogel was verified to be biodegrable in PBS buffer solution at 37°C and the biodegradable rate could be adjusted in a large range by controlling the kind and the length of degradable oligoester groups, etc., which is important to meet the requirements to repair different kinds of tissues or organs.2. Evaluation of cytocompatibility of novel biodegradable hydrogel and modification of hydrogel. Biocompatibility of the above-mentioned novel hydrogel was examined by cell experiments. Cells used in our experiments were mostly 3T3 fibroblasts a nd p artly h uman e mbryonic chondrocytes a nd m esenchymal stem c ells from canine marrow. The results revealed that the accelerator of the redox system was the major cause to cytotoxicity after checking additives one by one. So we triedphotoinitiator, which is less cytotoxic. Cells exhibited round morphology and little biological adhesion to the hydrogels from both initiator systems. Nevertheless, cells maintained, after c omplete degradation, viability e ither implanted on the surface of hydrogel membrane or encapsulated in the hydrogel matrix. The hydrogel network is thus not cytotoxic istelf. To improve cell adherence to hydrogel, we prepared hydrogels covalently linked with lysine and RGDS polypeptides, which are believed to be bioactive according to the current research in Biology. The modified hydrogel exhibited improved cell adhesion, although further improvement is strongly called for.3. A novel intelligent biodegradable hydrogel (including microgel). We designed a novel thermosensitive and biodegradable microgel, and prepared the microgels by combination of the macromer synthesis and inverse suspension polymerization. The size of the microgels can be adjusted by controlling preparation conditions. The microgel had been proved to be thermosensitive and biodegradable with highly tunable degradation rates. Th volume variation exhibited LCST behavior and the phase transition temperature is just between normal refrigeration temperature and body temperature.4. The idea of the novel microgel as a novel protein drug carrier. The biodegradable intelligient microgels are suggested as controlled release carriers of growth factors in tissue engineering and are also an injectable biomaterial. Swollen in aqueous solution at low temperature and shunken at high temperature, the microgels might be employed to encapsulate protein drugs without any risk of organic solvent and high temperature. The potential application may not be limited in tissue engineering, but also affords a novel universal carrier for various protein drugs. BSA and insulin were used as model proteins in our experiments about encapsulation and release in vitro. The difference of protein release at different temperatures indicates that the microgel might be a promising new carrier of protein drugs. Further researches are worthwhile according to the preliminary experiments.In summary, the theme of this thesis is about a novel tissue engineering material, and a kind of chemically c rosslinked biodegradable hydrogel has been i nvestigated from synthesis, preparation, characterization to biocompatibility evaluation. The...