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Surface Functionalization and Enhancement of Properties of Biomaterials through Atomic Layer Deposition
thesisposted on 01.05.2021, 00:00 by Sarah Hashemi Astaneh
Atomic layer deposition was introduced in 1960s as a method for thin film deposition in semiconductor industry. Several precursors were synthesized and different types of ALD reactors were developed to fabricate metal oxide and metal thin films. Uniformity and conformality at nanometer and Angstrom scale are the major advantages over other thin film deposition methods. Gradually, ALD expanded its application into surface functionalization of materials in other field of engineering and science. Biomaterials with complex, 3D, and porous micro/nanostructures were one essential group of materials which have taken advantage of ALD functionalization. Surface functionalization is indeed an increasingly promising approach for property enhancement of engineered materials, including biomaterials. Due to the importance of the interface region in biological environments, surface modification has several advantages over bulk functionalization. The interface of this biomaterial and the biological environment has a major role in the effectiveness of the biomaterial. Therefore, instead of mixing nanomaterial with the matrix that eventually yields to random distribution of nanomaterial on the interface of biomaterial and biological environment, focusing on the surface modification can result in an efficient distribution of the functionalizing agents at that interface. There are several chemical nanofabrication techniques (alone or combination of them) used for biomaterial fabrication and modification: (i) anodic oxidation (anodization), (ii) acid treatment, (iii) alkali treatment, (iv) chemical etching with hydrogen peroxide, (v) sol-gel treatment, (vi) chemical vapor deposition (CVD), and (vii) atomic layer deposition (ALD). The focus of this thesis is on development, optimization of ALD processing conditions and fabrication ALD thin films on materials which has application mostly in dental industry. Several surface characterizations were done to study the metal and metaloxide ALD films, and different invitro and invivo tests were done to evaluate the potential applications of these materials for in body environment. Polydimethylsiloxane (PDMS) is widely being used in medical applications due to its mechanical properties and inertness. Surface functionalization of an inert surface is challenging and has not fundamentally studied. In this study, atomic layer deposition (ALD) of TiO2 on polydimethylsiloxane (PDMS) is investigated at the early stages of nucleation and growth of TiO2 on both O2-plasma treated and non-plasma treated PDMS. X-ray absorption near edge structure (XANES), scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), and X-ray photoelectron spectroscopy (XPS) are used for material characterization. Results indicate that ALD-like surface growth of TiO2 takes place on plasma-treated PDMS; for non-plasma treated pristine PDMS, apparent favorable diffusion/infiltration of TiO2 into the subsurface of the polymer is obtained, without noticeable external surface deposition for at least the first 25 ALD cycles. A simple physical masking method is presented for selective ALD. Iron powder and a magnet are used as a masking pair. This method is easy and efficient for depositing patterned thin films with feature sizes estimated 0.3 mm or larger on any substrate. In this work, using iron powder held in place by a magnet would mask part of the silicon wafer substrate, while no iron powder is on the unmasked part. A silver particulated thin film was deposited on the masked Si wafer. X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy were used for silver characterization on the masked and unmasked parts of the sample. Results indicated that an ALD-like surface growth of a silver thin film occurred on the unmasked Si wafer, and no growth was detected on the masked portion of the Si wafer For the first time, thermal atomic layer deposition (ALD) of silver was carried out on commercially available collagen membrane to improve the bioactivity of collagen. The aim of this research is to study the chemical and morphological changes of the collagen membrane and investigate the bioactivity of collagen after its functionalization with silver. For two groups of samples, the role of metaloxide interlayer in the silver ALD was also investigated. All thin film depositions were done via ALD. Four different sample groups were investigated: pristine collagen, silver-coated collagen (Ag/Collagen), and two groups of silver ALD (650 cycles and 1300 cycles) on top of ALD TiO2-coated collagen (Ag/TiO2/Collagen). XPS and SEM were used to characterize the chemical and morphological changes of the collagen membrane after silver ALD. XPS of the silver-coated collagen showed the corresponding double peaks of Ag 3d and Ag 3p. Particulated silver thin film appeared on collagen fibrils with an average diameter of ~16 nm. Comparison of gingival cells cultured on pristine collagen, silver-coated collagen, and silver-coated samples with an interlayer of TiO2 demonstrated that the silver nanoparticle size and concentration are below the toxicity level of silver in an oral environment. This novel material can be used in surgeries as bone grafting agent with improved antibacterial properties.