Current stem cell research often relies on the use of growth factors to stimulate cell proliferation and differentiation for tissue engineering purposes. However, these growth factors are not only short-lived proteins but also expensive resources. Research in biodegradable magnetic nanocomposites is a promising rising field for biomedical applications due to the magnetic response properties of the materials that could provide external physical stimulations as an alternative. This project seeks to investigate the application potential of biodegradable magnetic nanocomposites for bone and cartilage tissue regeneration. Different weight percent of polyvinyl alcohol (PVA) was used to modify the surface of superparamagnetic nanoparticles to reduce aggregation and increase dispersibility in polymer solutions for the synthesis of magnetic nanocomposites. The results showed that 30 wt% of PVA coating was able to provide the best dispersibility compared to all other groups. Hydrogel-based magnetic nanocomposites were synthesized and the cytocompatibility of magnetic nanocomposite hydrogel with bone marrow-derived mesenchymal stem cells (BMSCs) was higher than pure hydrogel. However, both the hydrogel and magnetic nanocomposite hydrogel completely lost structural integrity within 24 hours of culture with BMSCs, which made it difficult for in vivo cell delivery. Another polymer, poly(glycerol sebacate) (PGS) was studied due to its reported elastomeric property. Initial study showed that magnetic PGS nanocomposites without any surface features had low cell adherence. Thus porous structures were created in magnetic PGS nanocomposites to increase cell adhesion. Different weight percent of magnetic nanoparticles (MNPs)-incorporated PGS nanocomposites were synthesized to investigate the effect of MNPs concentration on BMSC proliferation and differentiation behaviors with and without exposure to external electromagnetic field (EMF). The 3-week study showed that BMSCs cultured in magnetic PGS nanocomposite groups had slightly lower cell density but increased ALP activity, calcium deposition, protein and collagen secretion, indicating positive induction towards osteogenesis. This project presented promising porous magnetic PGS nanocomposite scaffolds for bone and cartilage tissue engineering applications.

Medical implants are devices that are often placed inside the human body to provide support to organs and tissues. Magnesium (Mg) shows great potentials as bioresorbable medical implants. Specifically, Mg is cytocompatible with many human cells in vitro and in vivo. Mg naturally degrades in the human body, which eliminates the secondary removal procedure of the implant. The primary degradation product, Mg ions, is an essential element which participates in over 300 enzymatic metabolisms. As a light metal, Mg possesses an adequate mechanical strength, even for the load-bearing condition in orthopedic device applications.

We conducted concentration-dependent tests of Mg degradation products, i.e., Mg ions, OH- ions, MgO and Mg(OH)2, with representative cell models. For human urothelial cells, pH of 8.6 showed cytotoxicity, and Mg ions of 10 mM showed a decrease in cell proliferation rate. We investigated three major strategies to improve Mg, that is, surface coating, alloying and composite. A bioactive, protective surface coating will control degradation rate, and possibly induce a positive tissue response to Mg. Alloying of Mg with other metal elements can potentially result in alloys with improved corrosion resistance and mechanical strength. Sintering Mg powders with calcium phosphates could result in Mg composite with lower degradation rate.

In an attempt to establish the consensus standard of the testing Mg-based implant, we design three different in vitro culture methodologies to mimic different in vivo cell-biomaterial interactions. Direct culture is suitable in vitro method when it is important to evaluate direct cell attachment on the biomaterial surfaces. Direct exposure culture is

desirable in vitro method for investigating the response of well-established cells in the body with newly implanted biomaterials. Exposure culture method is appropriate for evaluating cell-biomaterial interactions in the same environment, where they are not in direct contact. Due to the continuous degradation of Mg, cells that were direct contact or indirect contact with Mg substrates, both responded differently to different culture methodologies. A loading apparatus that we designed indicated that Mg under load had a significantly faster degradation rate. Thus, future evaluation of Mg should also take specific loads of different applications into account.

Magnesium (Mg)-based biomaterials have attracted increasing attention in biomedical applications, such as neural and orthopedic applications because of the biocompatibility, biodegradability, antibacterial properties, and excellent mechanical properties. However, rapid degradation of Mg is the major concern for many clinical applications. To address the challenge, the present dissertation developed the two approaches, including engineering proper surfaces and alloying Mg with other elements to control the degradation of Mg-based biomaterials and enhance the overall performances of Mg for neural and orthopedic applications. The first part of the dissertation developed a method to deposit a conductive polymer coating, poly(3,4-ethylenedioxythiophene) (PEDOT) onto the surface of Mg microwire for potential neural recording and stimulation applications. The optimized parameters were found for the first time. It was found that the corrosion rate of PEDOT-coated Mg microwire was much slower than the non-coated Mg microwire. The second part of the research reported the fabrication, characterization, degradation and electrical properties of biodegradable magnesium (Mg) microwires coated with two functional polymers, and the first in vivo evidence on the feasibility of Mg-based biodegradable microelectrodes for neural recording. The third part of the dissertation developed biocompatible, biodegradable Magnesium-zinc-calcium (Mg–Zn–Ca) alloys with similar mechanical properties to human bone for orthopedic applications. The objectives were to characterize Mg–2wt.% Zn–0.5 wt.% Ca (named ZC21) alloy pins microstructurally and mechanically, and determine their degradation and interactions with host cells and pathogenic bacteria in vitro and in vivo. Overall, the entire dissertation provided extensive knowledge regarding engineering proper surfaces and alloying Mg with other elements towards desired performances for neural and orthopedic applications.

Hyaluronic acid (HyA) hydrogels are promising in various biomedical applications such as tissue regeneration, drug delivery, cell therapy, and biosensing. Three-dimensional printing (3D printing) can precisely control the structures and properties of the HyA hydrogels, which is highly desirable for many biomedical applications. However, the crosslinking and 3D printing of HyA hydrogels usually require chemical modifications. This may raise toxicity concerns on the hydrogels, especially when regulatory approval is required for the clinical translation of the final products. This dissertation investigated the mechanisms of dynamic coordination and the relationships among the key parameters in controlling the tunable crosslinking, reversible phase transition, and 3D printing of HyA hydrogels for biomedical applications, without blending with other polymers or adding new functional groups. In the first part, tunable crosslinking and reversible phase transition of HyA hydrogels were achieved and demonstrated via dynamic coordination of Fe3+ ions with innate carboxyl groups. The concentrations of Fe3+ and H+ ions and the reaction time determine the coordination state, leading to the low-to-high crosslinking densities and reversible solid-liquid phase transition of HyA hydrogels. In chapters 3 and 4, three different 3D printing approaches for HyA hydrogels were developed, for the first time. Two 3D printing strategies, namely cold-stage and direct-writing methods, were achieved based on the tunable crosslinking and reversible phase transition of the HyA hydrogel. Direct writing of HyA solution in FeCl3 solution was also achieved by decelerating the solidification process of the hydrogel in FeCl3 solution. In chapter 5, the cytocompatibility of HyA hydrogels with different crosslinking densities and 3D-printed HyA constructs was investigated via the direct exposure culture method with bone marrow-derived mesenchymal stem cells (BMSCs). The last part of this dissertation investigated the incorporation of magnetic nanoparticles (MNPs) in the HyA hydrogels in situ. The MNP content and agglomeration in the magnetic hydrogels were tunable by controlling concentrations of Fe3+ and Fe2+ ions. Cell study results indicated that BMSC adhesion density decreased when increasing the MNP content in HyA hydrogels.

Electrical and ultrasonic stimulation have both shown great potential for promoting cell proliferation and tissue repair since piezoelectric materials can produce ultrasound with electrical stimulation and produce electrical signals with ultrasound stimulation. They can be a great option for tissue regeneration and biosensors. To evaluate the possibility of Polyhydroxybutyrate(PHB), a piezoelectric material to be used for medical use, cell experiments are carried out without any stimulation applied to the materials first to study their effects on cells. Results show that the materials do not affect the density of cells while altering the cells’ morphology which is related to the materials’ fiber structures. Since the material is cell compatible, further experiments with 3D-printing scaffolds and ultrasound stimulation can be carried out to test the effect of ultrasound and electrical stimulation on cells grown on the materials. In this thesis, we printed 6 different material groups including pure PHB scaffolds, PHB scaffolds modified with different percentages of either polyaniline (PANi) or reduced graphene oxide (rGO) to vary the materials’ piezoelectric properties. Future work will focus on using ultrasound to remotely stimulate 3-D printed Polyhydroxybutyrate(PHB) scaffolds and test their effects on cells’ proliferation.

Bioresorbable cardiovascular scaffold (BCS) is promising to eliminate the chronic complications caused by the conventional permanent scaffold by being completely absorbed in the body. Magnesium (Mg) as a bioresorbable metal has attracted considerable attention for BCS. However, Mg must be modified to achieve a slower degradation rate and to induce a desired endothelial cell response to prevent neointimal hyperplasia and endothelium abnormality. To address these challenges, the present dissertation developed polymer-based surface modifications, including different polymer coatings and nanopatterned polymer coatings, to enhance the overall performances of Mg. The first part of the dissertation is a comparative investigation of four representative biodegradable plastics as coating materials, including poly(L-lactic acid), two different poly(lactic-co-glycolic acid), and polycaprolactone. No previous literature thoroughly compared their functionality as coating materials for Mg. It was found that all four polymer coatings slowed the degradation of Mg, and PLGA (50:50) coating promoted the adhesion and spreading of human endothelial cells the most. The second part of the research explored the potential of a novel bioresorbable elastomer, poly(glycerol sebacate) (PGS), as an ideal coating material for Mg. The work verified the effectiveness of PGS coating in reducing the degradation rate of Mg and provided the first reference in the existent literature about the in vitro performances of PGS coatings on Mg. The third part of the dissertation pioneered novel nanopatterned polymer coatings on Mg. The nanopatterns are promising to further enhance the functionality of polymer coatings as a nonbiological and nondrug approach. For the first time, the parallel groove nanopatterns have been successfully fabricated on the PLGA (50:50) coatings and PGS coatings respectively. The nanopatterns on PLGA (50:50)-coated Mg presented a highly-effective biofunction to control the elongation of human endothelial cells without using any biological molecule or drug. The effort established an effective method to control endothelial cell activities on bioresorbable Mg. Overall, the entire dissertation provided extensive knowledge regarding polymer coatings and nanopatterned polymer coatings as surface modification methods for Mg towards desired performances for cardiovascular applications.

The mechanical and biological properties of magnesium (Mg) alloys are ideal for degradable bone implant applications, but excessively rapid Mg degradation in vivo limits their clinical translation. This rapid degradation may be prevented by controlling the bulk properties of Mg through methods such as alloy composition and processing, or by controlling the surface properties of Mg through methods such coatings and surface treatments.

The effects of alloy composition were investigated by the comparison of commercially pure Mg and a Mg alloy containing yttrium (Y), both of which had oxidized or polished surfaces. The Y alloy component could decrease degradation by forming a more stable passivation layer or increase degradation by forming a galvanic couple with Mg. These results demonstrated that the effects of the alloy parameters depended upon their interactions with each other and the working environment.

Nanocomposite coatings have the potential to control the degradation rate of Mg substrates while simultaneously performing additional functions, such as increasing the adhesion of bone cells. However, the duration over which such nanocomposite coatings remain effective on Mg substrates may be limited by delamination caused by gas evolution and surface instability. Improved engineering of nanocomposite coatings can increase their barrier properties and extend the duration of their effectiveness.

Nanohydroxyapatite (nHA)/Poly(caprolactone) (PCL), nHA/poly(lactic-co-glycolic acid) (PLGA), and nHA/poly(L-lactic acid) (PLLA) coatings were compared for their ability to reduce Mg degradation. The coating deposition method was refined, and the effects of post-deposition processing upon the coatings were characterized. All of the coatings significantly reduced Mg degradation and did not delaminate.

The nHA/PCL coatings reduced Mg degradation more significantly than the other coatings, and thus were tested for the adhesion of bone marrow stromal cells (BMSCs). The nHA/PCL coatings increased the density of adhered cells and their spreading areas, and prevented changes to their aspect ratios.

As an additional benefit from this work, guidelines for the rational design of nanocomposite coatings were elucidated, which will aid in the clinical translation of Mg-based degradable biomaterials for bone implant applications.

In recent years, Magnesium (Mg) and its alloys as bioresorbable metals have attracted increasing attention for orthopedic implant applications because of their promising mechanical and biological properties. However, the rapid degradation of pure Mg remains a key challenge toward its clinical translations. Additionally, implant infections continue to be a significant clinical problem for orthopedic applications. This dissertation develops two approaches to improve the overall performance of pure Mg. One method includes engineering a nanostructured oxide surface onto the Mg coupled with adding alloying elements into the Mg matrix, the other involves depositing a nano-micro and micro hydroxyapatite supplemented with gentamicin onto the Mg. The nanostructured oxides on Mg alloy and pure Mg resulted in a slower electrochemical degradation rate, and reduced adhesion of methicillin-resistant Staphylococcus aureus (MRSA) on their surfaces when compared with the corresponding non-treated Mg controls. Moreover, the soluble degradation products released from nanostructured oxides on the Mg alloy and pure Mg did not affect the growth of bone marrow derived mesenchymal cells (BMSCs) under indirect contact, but showed reduced cell spreading and adhesion under direct contact when compared with Ti control and glass reference. In terms of the nano-micro and micro hydroxyapatite supplemented with gentamicin coated Mg, the 12-week immersion study showed that the difference in the degradation rate between the hydroxyapatite coated Mg samples and the Mg controls is negligible because no statistically significant difference was found. The micro hydroxyapatite coated Mg samples did not affect the growth of BMSCs under indirect contact, but showed reduced cell spreading and adhesion under direct contact when compared with the Ti control and glass reference. The micro hydroxyapatite supplemented with 30 wt. % gentamicin coated Mg exhibited significantly lower adhesion of Staphylococcus aureus on the surface among all samples in vitro. Overall, this dissertation provides extensive knowledge regarding surface treatments on Mg and applying alloying elements into Mg to achieve desired performance toward clinical translations.

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Conclusion

Tissue engineering, the combination of cells, scaffolding materials, and biochemical signals is being extensively researched for tissue generation and regeneration. The potential to generate tissue and organs can combat the limited availability of organ and tissue donors. To guarantee successful tissue engraftment, a sufficient availability of oxygen and nutrients must be provided. This can be done by adding angiogenic potential to therapy. The expansion of existing vasculature to site of repair can increase success rates of transplant and engraftment. Bone marrow stromal cells (BMSCs) can allow bone repair and regeneration and also secrete vascular endothelial growth factor (VEGF), which plays an important role in blood vessel formation and endothelial cell recruitment. Hyaluronic acid hydrogels provide a 3D microenvironment that mimics in vivo conditions and enhances cell viability and function. Iron oxide magnetic iron oxide nanoparticles have been shown to maintain scaffold dexterity while also enhancing cell alignment, viability, and function. Curcumin is a naturally occurring protein that is being studied for wound healing and angiogenic properties. Curcumin coated iron oxide nanoparticles encapsulated in a hyaluronic acid hydrogel were evaluated in this thesis. The nanoparticles and magnetic hydrogel were synthesized and characterized. Cell adhesion and viability studies were performed with the materials, and post-culture media was evaluated for changes of VEGF release in samples with developed materials and controls. Results suggested the hyaluronic acid hydrogels and curcumin coated iron oxide nanoparticles allowed for enhanced cell adhesion and morphology, and increased the total amount of VEGF production by BMSCs.