Multi-Functional Scaffold With Shape Memory Effect For Bone Tissue Engineering

Biomaterials scaffolds integrated with additional functionalities such as shape memory effect (SME) are highly desired clinically for transplantation of a particular tissue-engineered product through minimally invasive surgery. Electrospinning has recently emerged as one of the most attractive enabling nanotechnologies to produce nanoscaled fibers that are able to emulate the morphology and structure of the natural extracellular matrix (ECM) for engineering tissues. A broad variety of thermo-sensitive shape memory polymers (SMPs) have been developed so far to direct cell functions such as adhesion, migration, proliferation, and differentiation. However, only a few reports address the issue of electrospun biomimetic fibers or structures with SME functionality for tissue engineering, and little attention have been focused on the SME of biomimetic nanofibers as well as the efficacy of using the SME-capable biomimetic nanofibers for bone regeneration (e.g., for the case of repairing bone screw holes).In this thesis, first of all fibrous scaffolds of biodegradable poly(D,L-lactide-co-trimethylene carbonate)(denoted as PDLLA-co-TMC, or PLMC) with shape memory properties are fabricated through an electrospinning technique. By varying the chemical composition of DLLA:TMC (i.e., hard to soft monomer ratio) from5:5to9:1, fineness of the resultant PLMC fibers is attenuated from ca.1500nm down to680nm. This also allows for readily modulating the glass transition temperature Tg (i.e., the switching temperature for actuating shape recovery) of the fibrous PLMC scaffolds to fall between19.2and44.2℃, well suitable for implantation in the human body. The PLMC fibers exhibit excellent shape memory properties with shape recovery ratio of Rr>94%and shape fixity ratio of Rf>98%, and macroscopically demonstrate fast shape recovery (-10s at39℃) in the pre-deformed2-D and3-D configurations. The SME-capable fibrous PLMC also demonstrates the desired bone formation oriented outcomes by supporting cell proliferation, alkaline phosphatase (ALP) expression, and mineral deposition of the osteoblasts derived from mouse calvarial tissue.Based on the above obtained results, the following chapter of the thesis is to develop a bioactive scaffold with shape memory effect by incorporating hydroxyapatite (HAp) into PLMC nanofibers, and then to investigate the effects of HAp introduction on SME and bone formation ability of the PLMC nanofibers. Composite nanofibers of PLMC/HAp with different PLMC:HAp ratios via electrospinning are prepared. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), mechanical tests, dynamic mechanical analysis (DMA) are carried out to examine morphology, thermal properties, mechanical properties, and shape memory effect of the PLMC/HAp composite nanofiber, respectively. Moreover, the bone formation ability of PLMC/HAp nanofibers is assessed. The results show that HAp nanoparticles can be favorably dispersed within the PLMC nanofiber matrix, but some aggregations are observed in the SEM and TEM images of the PLMC/HAp nanofibers with HAp content above20%. By varying HAp mass ratio, Tg of the fibrous PLMC/HAp scaffolds can be modulated from45.7to51.5℃. The PLMC/HAp nanofibers possess an ultimate tensile strength of up to five times of that from neat PLMC nanofibers. And also, introduction of HAp gives rise to improved shape memory effect with respect to the PLMC based nanofibers, which is advantageous for in vivo applications in mechanotransduction mediated boen regeneration. Biological experienments demonstrate that the biomimetic nanofibrous PLMC/HAp scaffolds support and enhance the proliferation, and particularly osteogenic ability of the osteoblasts. The potent biocompatibility, as substantiated by the outstanding cell viability and bone formation ability, together with the integrated SME functionality makes the PLMC/HAp composite nanofibers well suitable for the bone tissue engineering application.When SME capable biodegradable scaffolds and/or implants are designed for uses in physiological conditions, finding a safer and effective stimulus other than direct heating in the body to accomplish remotely controllable triggering on shape recovery and the integrated functionalities, should be another pivotal factor for consideration. The fourth part of the thesis is to demonstrate synchronized capabilities in SME and on-demand drug delivery of a scaffold, from using ultrasound as the sole trigger. As a proof-of-concept, chitosan (CTS) functionalized poly(lactic-co-glycolic acid)(PLGA) microspheres containing a model payload, lysozyme (Lyz), are prepared by a water-in-oil-in-water emulsion method, from which cylindrical shaped rod (5mm in diameter) is fabricated by sintering the composite microspheres in a mold. High intensity focused ultrasound (HIFU) is then employed as a unique technique to enable shape memory and payload release effects of the3-D structure. It is found that incorporation of CTS into PLGA microspheres could regulate transition temperature Ttrans of the microspheres from45to50℃and affects shape memory ratio of the fabricated cylindrical rod to some extent. Shape memory test and drug release assay prove that HIFU could modulate the shape recovery process and synchronize the release kinetics of the encapsulated Lyz in the rod in a switchable manner. Moreover, the two processes can be manipulated by varying the acoustic power and insonation duration. Mechanical tests of the microsphere-based rod before and after ultrasound irradiation reveal its compressive properties in the range of trabecular bone. Examination on degradation behavior indicates that introduction of CTS into the PLGA microspheres also alleviates acidic degradation characteristic of the PLGA dominant cylindrical rod. Thus, the results in this part have laid a solid foundation for using ultrasound to regulate the SME and drug release behaviors of drug-loaded biomimetic composite PLMC/HAp nanofibers for bone (e.g., bone screw hole) regeneration, which can be performed in future researches.