Biomaterial Surface And Microenvironment Engineering For Regulation Of Cellular Behavior Towards Biomedical Application

Biomaterials are aimed to interact with biological system so that they are bale to evaluate, treat, improve or regenerate certain tissue and organ or their functions. It is of great importance to understand the biomaterial-cell interface and to design biomaterials with optimized structure and function, which will benefit the construction of extracellular microenvironment and the practical implementation of biomedical engineering. This dissertation will investigate the effects of variable material-derived signals in extracellular microenvironments on the cellular behaviors, including surface charge, nanomaterials, biofactors and artificial extracellular matrix. This dissertation aims to demonstrate the interactions between extracellular microenvironments and cellular behaviors, the results of which will lay the theological and practical foundations for the design and construction of materials for biomedical applications.This thesis mainly includes the following aspects:1. Studying the effects of surface charge on stem cell behavior. The surface factors of the biomaterials including physical, chemical and biological properties have been demonstrated to individually, or synergistically affect cellular attachment, migration, proliferation and differentiation. To identify and assess the effects of individual niche components on cell behavior,2D biomaterial culture systems are often used as a simplified approach to reconstruct the niche. However, most of the studies were not able to conduct a single-factor study to investigate the effect of one particular factor on the cellular behavior. In addition, little attention has been paid to the response of stem cells to surface charge characteristics, particularly the differentiation of stem cells. Therefore, this study will demonstrate for the first time the use of spontaneous polarization generated surface charge as a 2D model to study the effects of surface charge on attachment, proliferation and differentiation of the mesenchymal stem cells. The results have shown that positively charged surfaces could promote the cell spreading and enhance the osteogenic differentiation of the stem cells cultured in osteogenic induction medium.2. Studying the response of stem cells to the nanocrystals in extracellular microenvironments. The application of stem cells in biomedicine, especially in the field of regenerative medicine, is still a challenging task, including the development of novel technologies to distinguish and control the various signals in the extracellular microenvironment as well as new ways to track and guide the implanted stem cells. Nanomaterials could be a powerful tool to comprehend and regulate cellular behavior due to their unique interactions with cells at molecular levels. For instance, biofactors could be released to targeted cells or tissues by using nanoparticle-based vehicle; the growth of cells or tissues could be guided along aligned electrospun nanofibers; and cells/biomolecules could be imaged or even tracked by using quantum dots or up-conversion techniques. With the application of a large number of functional nanomaterials in biomedical engineering, the response of stem cells to these artificially introduced nanoparticles into the microenvironment needs to be identified and studied. In this study, lithium niobate nanocrystals were prepared via a solvothermal method and introduced to the stem cell niche. By using NIR femtosecond laser as the excitation light source, the author observed the active internalization process of the nanocrystals into rat mesenchymal stem cells by two-photon microscopy and studied the effects of the internalized nanocrystals on stem cell differentiations by PCR and immunostaining techniques. This study demonstrated that the stem cells are active and responsive to nanocrystals in the microenvironment.3. Construction of biological microenvironment by sustained release of growth factors. The extracellular components responsible for controlling cellular behavior include not only the insoluble components of the extracellular matrix as described above, but also soluble molecules such as hormones and growth factors. Therefore, it is also an important way to construct the extracellular microenvironment by the combinative use of biomaterials and growth factors. Surgical repair of connective tissues such as tendons remains a clinical challenge, primarily due to the failure for the injured site to restore strength. There have been attempts to improve the outcome by increasing the strength of the suture material and by modifying the suture grasping method. Although these approaches can improve the initial strength of the repair, they cannot regulate the subsequent biology of healing. Therefore, this paper first proposed a porous suture-based sustained release of growth factors to construct a biological microenvironment for repairing of connective tissue. This study have developed a simple and versatile swelling-freeze-drying method for generating surgical sutures with highly porous sheaths without compromising their mechanical properties. Due to the presence of the porous structure, the voids inside the suture can be utilized to achieve a higher loading capacity of PDGF. The use of fibrin as carrier phase slowed down the release process and a sustained release of PDGF over a period of at least 11d. The released PDGF remained biologically-active and are able to promote the proliferation of human mesenchymal stem cells. This novel delivering system based on porous sutures provides a new strategy for the construction of biological microenvironment for tissue repair.4. Biomimetic construction of extracellular microenvironment by using natural extracellular matrix. Extracellular matrix (ECM) is a mixture with complex structures and functions, which plays a very important role in cellular behavior and phenotype. Thus, attempts have been made to obtain an artificial ECM in order to provide cells with microenvironment equivalent to natural ECM. The acellular matrix obtained by removing the cellular components of certain tissues has been increasingly applied in the field of regenerative medicine such as tissue regeneration and organ transplantation. However, the natural ECM scaffolds have a relatively low mechanical property and rapid degradation rate because the main component of the natural extracellular matrix is collagen, which restricts their application in tissue engineering, especially bone tissue engineering. Therefore, porcine acellular dermal matrix (PADM) was prepared by an improved decellularized method and the carbodiimide was used as crosslinker to enhance the PADM scaffold. The obtained scaffold retained the natural porous structure, retained the collagen, GAG and other active ingredients, and effectively removed the nucleic acid, SDS and other immune response sources. The crosslinking extent of carbodiimide was evaluated as well as the effects on the mechanical property and degradation rate. The mechanical strength and degradation rate of crosslinked PADM can be controlled by the crosslinking degree and the biomimetic scaffold as artificial extracellular microenvironment can also support the cellular migration to the scaffold inside. This study presents a possibility to explore the use of natural ECM to simulate and construct the extracellular microenvironment.In summary, this thesis devoted to studying the regulation of cellular behavior by biomaterial surface property and exploring the in vitro construction of extracellular microenvironment. These studies and explorations will further promote the application of biomaterials in biomedical engineering.