Review articleAtomic layer deposition on dental materials: Processing conditions and surface functionalization to improve physical, chemical, and clinical properties - A review
Graphical abstract
Introduction
There are different ways to categorize dental materials, the main three ways being: (1) based on the nature of the dental material; (2) based on their interaction with the body, i.e., either they are bioinert when they are implanted in the body or they are bioactive; and (3) based on the shape and form of the dental materials [1]. There are reports in the literature on the failure of conventional dental materials earlier than expected which increase the risk of second surgeries, and significantly higher costs for the patients. Also, simple dental materials cannot meet all clinical expectations. Studies in dental materials are actively proceeding in different aspects such as design, material and technique so that we come up with more suitable effective materials [2,3]. Therefore, there are several studies either on fabricating new materials or enhancing the properties of available dental materials. Property enhancement of current materials can be performed with known modification methods such as microstructural modification, conventional functionalization methods, e.g., adding nanoscopic fillers to dental material compounds [4], [5], [6] or performing novel functionalization techniques, e.g., surface functionalization [7], [8], [9].
Surface functionalization is indeed an increasingly promising approach for property enhancement of dental materials [10]. Due to the importance of the interface region in biological environments, surface modification has several advantages over bulk functionalization. The interface of the 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 at the interface of the biomaterial and its 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 dental material 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) [11], [12], [13], [14], [15], [16], [17].
ALD has several advantages over other thin film deposition methods [18,19]. CVD and ALD are both vapor phase thin film deposition techniques. In CVD both precursors are introduced into the reactor at the same time while in ALD precursors are sequentially introduced to the reactor each followed by an inert gas purge. This sequential process is a key difference between CVD and ALD, which consequently makes ALD a self-limiting reaction without gas phase reactions and gives unique characteristics to this method [20]. A detailed comparison of ALD and other thin film deposition methods is discussed in Ovirah et. al [21]. Outstanding advantages of ALD thin films over those from other techniques include high quality, high density, excellent reproducibility of the results, and control over the thickness down to sub-nm length scale.
The motivation of the current work originated in part from the shape and microstructure complexity of dental materials. Most of the modern dental materials are porous and/or have complex physical shapes. Also, at a smaller scale, these materials show 3D surface microstructure, which makes it hard to get them conformally coated and functionalized with other techniques. Other dental materials are resorbable in solutions, which makes it impossible to functionalize them via methods which include solvents (acid treatment, sol-gel treatment, chemical etching with hydrogen peroxide, or anodic oxidation). ALD is a unique and powerful approach for surface functionalization of substrates which results in uniform and conformal thin films, especially for surfaces with high aspect ratios, porous surfaces, and complex microstructures [17]. Further, processing conditions of ALD are tunable for the functionalization of materials with low degradation temperature.
ALD consists of sequential cycles. Each cycle typically consists of four consecutive steps with designated time durations, under vacuum, i) precursor pulse, ii) precursor purge, iii) oxidizer (or co-reactant) pulse, iv) oxidizer (or co-reactant) purge:
- (i)
precursor pulse into the reactor to react/interact with the substrate surface,
- (ii)
unreacted precursor molecules purge out of the reactor with only one monolayer of precursor covering the substrate surface,
- (iii)
oxidizer (or co-reactant) pulse into the reactor to react with the monolayer of the precursor,
- (iv)
oxidizer (or co-reactant) purge to remove the unreacted co-reactant and byproducts
Once the oxidizer (or co-reactant) is pulsed into the reactor, reaction between chemisorbed precursor molecules and pulsed oxidizer molecules takes place. One cycle of a typical ALD is illustrated in Fig. 1.
On the other hand, limitations of ALD are the slow deposition rate, sometimes expensive precursors, and the ALD of many metals and metal compounds have not yet been studied and developed well. Yet, the slow ALD rate has been increasingly addressed effectively with Spatial ALD (SALD) systems in which precursors are continuously supplied in different locations and kept apart by an inert gas region or zone [22], [23], [24]. Film growth is achieved by exposing the substrate to the locations containing the different precursors. Because the purge step is eliminated, the process becomes faster, being indeed compatible with fast-throughput techniques such as roll-to-roll, and much more versatile, easier and cheap to scale up [25]. Due to its higher throughput and growth rate, SALD is of interest for fast commercialized production. ALD on dental materials is still at the lab-scale research level but hopefully moving forwards from laboratory environment to commercialized production.
Precursors play a key role in the ALD reaction. Precursors must be volatile, chemisorb on the surface, and be thermally stable [26]. Precursors are mostly commercially available. An inert gas like nitrogen (N2) or Argon (Ar) can be used as carrier and purging gas [17].
By adjusting the number of cycles, one can tune the final thickness of the thin film. Each cycle of ALD includes chemisorption and self-limiting reaction, which means the entire surface gets exposed to the flow of the pulsed materials and therefore at the end of each cycle (depending on the growth rate), the entire surface can be covered with the targeted thin film. That is why ALD results in uniform, pinhole free, and conformal thin films, which are important advantages of this technique over other vapor phase thin film deposition methods. The final thin film is in the nanometer length scale and allows control over the thickness down to a few Ångstrӧm. The invention and introduction of the first ALD dated back to 1960 [27]. It was first used in the microelectronic and semiconductor industry. For each ALD process there is a temperature window, where the growth rate is constant and saturated [19]. For the fabrication of semiconductor devices via thermal ALD, this temperature window is wide and not suitable for heat sensitive polymeric and biological materials.
However after the discovery of processes and precursors for thin film deposition below 100°C, ALD has been increasingly used in the field of biomedical-advanced materials [28], [29], [30]. Biosensors, biomedical implants, and biomaterials are some of the examples that make use of ALD in the biomedical industry [28,31]. The present work provides a detailed overview of dental materials, which have been functionalized with ALD so far.
Section snippets
Methodology
For this literature review, an electronic search was conducted for papers in the databases Google Scholar, Pubmed, Scopus, Web of Science and SciFinder, using the keywords: atomic layer deposition, dental material, and dentistry. The notable published researches in the area of ALD on dental materials were reviewed first. Next, the search was expanded to their relevant references and citations. Among those references and citations, in the case of any relevant work, again its references and
ALD of metal oxides on metal and metal alloys dental materials
One of the challenges handling restorative dental materials is the unpredictable adherence of those materials to the instruments used in clinical work upon pull out from the cavity. Therefore, development of non-stick dental restorative instruments can be an answer to this challenge. Doped diamond like carbon coating (DLC) and Polytetrafluoroethylene (PTFE) coatings are two of the conventional methods to solve such unpredictable adherence problems. Leppaniemi et al. compared these two
Summary, perspectives, and future works
The translation of newly developed dental material can be categorized as the following four stages [101]:
Stage 1: fabrication of the material, microstructure characterizations, physical and mechanical properties.
Stage 2: In vitro tests for different cell types, cytotoxicity, genotoxicity.
Stage 3: In vivo implantation to study tissue viability, histology, and inflammatory effects.
Stage 4: Human trials for clinical feasibility and efficiency.
The ALD-fabricated materials, which were designed for
Conclusion
The current work covers how the promising ALD method can be used in the functionalization of dental material surfaces including metal and ceramic implants, dentures, maxillofacial prostheses, and orthodontic materials. The majority of works were done on TiO2 ALD; however, studies on the ALD of Ag, ZnO, ZrO2, Al2O3, and CaCO3 have also been reported. For removable dental materials, like dentures, challenges are easier: physical and mechanical tests have shown improvement in corrosion resistance,
Declaration of Competing Interest
None.
Acknowledgment
Partial support by the National Science Foundation, NSF 1309114, is gratefully acknowledged. Also, the authors would like to express gratitude for support from the Coordination for the Improvement of Higher Education Personnel (CAPES) in the PrINTProgram (#88887.373422/2019-00), through a scholarship to the author Faverani, L.P. Dove Medical Press is highly acknowledged as the original publisher of Fig. 4 of this manuscript and authors would like to thank them for giving permission to use Fig. 4
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