Full length articleSurface and subsurface film growth of titanium dioxide on polydimethylsiloxane by atomic layer deposition
Graphical abstract
Introduction
Polydimethylsiloxane (PDMS) is nontoxic and widely used in medical applications [1]. PDMS is chemically and physiologically inert which makes it a good choice for long-term implants in the body such as cardiac pacemakers, mammary implants, maxillofacial, voice, and finger joint prostheses, drainage tubes, and catheters [2]. It has also been used in the fabrication of soft contact lenses [3] and has numerous applications in microfluidic devices [4]. Although its chemical inertness makes this polymer very attractive for biomedical applications, the lack of functional groups along its backbone imposes limitations on its applications [5].
TiO2 is well known for its photocatalytic, antibacterial and antifungal properties [[6], [7], [8]]. Further, TiO2 has hydrophilic properties that lead to protein adsorption and make it a well-established material for biomedical applications. Several processing methods such as chemical vapor deposition (CVD), spray pyrolysis, magnetron sputtering, radio frequency sputtering, and atomic layer deposition (ALD) have been implemented for the synthesis and deposition of metal oxides (e.g, TiO2, Al2O3) on solid surfaces like silicon, glass, and fluorine doped tin oxide coated glass [[9], [10], [11]]. Among these vacuum deposition techniques, ALD results in outstanding features of deposited films in electronic applications [12]. ALD is a unique method for low temperature vapor phase thin film deposition. This deposition method is a cyclic process, where each cycle includes precursor pulse, precursor purge, oxidizer pulse, and oxidizer purge. Ideally, at the end of each cycle, one atomic layer of desired thin film is deposited [13]. By varying the number of cycles, one can control the thickness of the thin film according to the final application. Another interesting feature of ALD is uniform thin film coverage on the substrate surface; therefore, deposition of the film would be conformal onto irregular surfaces. This feature makes ALD ideal for coating complex morphologies and microstructures of polymers [14]. Despite the limitation of precursor volatility and thermal stability, many metals and metal oxides have been deposited with conventional ALD where these materials exhibit outstanding features in their intended applications. In this study, the focus is the early stages of the deposition of TiO2 on PDMS.
The biocompatibility of PDMS along with the bioactivity of TiO2 makes their combination ideal for biomedical applications. Previous studies have investigated depositing thin films of metal oxide on PDMS. For example, Boudot et al. [1] deposited TiO2 on PDMS surface via a vacuum arc plasma technique. They showed that 150 nm thick TiO2 on top of PDMS resulted in stable surface properties of PDMS over time in water and under sterilization methods such as gamma irradiation (radiation dose 25 kGy) and ethylene oxide (45 °C, 5 h, 680 mg/L, aeration time 80 h). Furthermore, it did not impose any toxicity to the PDMS and was reported to improve cell adhesion to the surface by 260–380% for 7 out of 10 samples which makes it a good candidate for biomedical applications. Pessoa et al. investigated [15] the effect of ALD grown films of TiO2 on polyurethane (PU) and PDMS substrates as a means of improving anti-yeast properties. Thin films of TiO2 were deposited on PDMS (growth rate: 0.10–0.11 nm/cycle) and demonstrated the effect of TiO2 ALD films on the inactivation of Candida albicans yeast. Their results show that TiO2 on PDMS decreased the colony forming unit (CFU) by 59.5%. This study shows promising clinical applications for TiO2 coated organic substrates; however, no discussion was presented on the formation and growth characteristics of TiO2 on the PDMS.
After any surface modification, hydrophobic recovery of PDMS is one of the drawbacks that has been reported in several studies regardless of the method of surface modification [[16], [17], [18]], including ALD [19]. Spagnola et al. [19] coated both native PDMS and ultraviolet/O3 treated PDMS with Al2O3 via ALD. For both substrate types, hydrophilicity of the surface increased significantly right after exposure to <100 cycles of ALD; however, after 48 h, the water contact angle for both samples increased upon air exposure. This effect had been observed by other researchers and correlated with out-diffusion of organic species (e.g., oligomers, low molecular weight chains) [19,20] and/or hydrocarbon adsorption from lab ambient atmosphere onto the surface [21]. To limit this drawback and make the hydrophilic property of the ALD treated surface more stable, Gong et al. used metalorganic infiltration process to suppress the apparent hydrophobic recovery [21]. That is, enough time was given to the metalorganic precursors to infiltrate/diffuse below the surface resulting in subsurface reaction and nucleation. The steps of metalorganic precursor infiltration were as follows: first exposure to metalorganic precursor (trimethylaluminum) for 5 h followed by argon purge for 30 min and then exposure to water vapor for another 30 min. After the infiltration treatment, PDMS was exposed to typical ALD cycles where trimethylaluminum (TMA) precursor was pulsed/purged for 1 s/30 s and oxidizer (water) was pulsed/purged for 1 s/30 s at reactor temperatures of 60–120 °C and reactor pressure of 1 Torr. Infiltration of precursor followed by ALD cycles is believed to create an organic/inorganic hybrid interface which may provide a pathway to address the hydrophobic recovery problem of PDMS [21]. Water contact angle of the coated PDMS showed that the hydrophilicity of the Al2O3 coated PDMS lasted considerably longer, up to 200 h, demonstrating significant impediment to the hydrophobic recovery of PDMS [21]. In 2015, Yu et al. [22] also deposited AlOx via ALD onto PDMS; In their work, PDMS was exposed to sequential infiltration of precursor, which effectively extended the precursor pulsing time. Their results showed that by increasing the TMA pulse time, deep infiltration of the Al precursor took place due to good solubility of TMA in PDMS and it was concentrated within the top ~3 μm region.
ALD overall is a relatively slow vapor phase deposition method, except for some of its modified versions, like spatial ALD (S-ALD) [23]. Therefore, adding another time consuming step, like metalorganic vapor infiltration would make this overall method even slower. However, PDMS is an important polymer in many biomedical applications and modification of its surface with ALD is a challenging issue; there are still unanswered questions about the early (nucleation and growth) stages of metal oxide ALD on its surface as well as within the polymer matrix. More specifically, the apparent mechanism of nucleation and consequent ALD of TIO2 in the absence of functional groups on PDMS has yet to be understood [24]. In this work, the objective is to study the early stages of TiO2 nucleation and growth on PDMS substrates with different surface properties. Both X-ray photoelectron (XPS) and X-ray absorption near edge structure (XANES) spectroscopies are used to further investigate the formation of TiO2 films on such polymeric substrates. XPS probes only the top (external) surface while XANES extends analysis deep within the (interior) subsurface of the material structure. This is the first study on the early stages of ALD film formation on thermoset elastomers like PDMS. These two techniques are then complemented with scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDX) to provide a more complete characterization of ALD TiO2 nucleation and growth on plasma and non-plasma treated PDMS.
Section snippets
Materials
Platinum-catalyzed, vinyl-terminated poly (dimethyl siloxane) elastomer (Factor II, Inc., A-2000) combined with functional intrinsic pigments (Factor II, Inc., FI-SK: Functional Intrinsic Skin Colors – Silicone Coloring System) were used as a maxillofacial prosthetic substrate material for deposition in this study. During preparation, the monomer was combined with a polymethyl hydrogen siloxane cross-linking agent at a 1:1 ratio by weight. The molds were placed in a convection oven and held at
Methods
PDMS samples were degassed prior to ALD at <180 mTorr for 6 h at 190 °C so that any potential non-reacted monomers or other volatile impurities could be removed. Next, PDMS substrates were rinsed with deionized water (>17.5 MΩ), dried with N2, and cut into 2 cm × 1 cm pieces (the time elapsed in-between degassing and ALD was less than two days). The ALD of TiO2 on PDMS was done in a commercial ALD reactor, ALD-150LE™ from Kurt J. Lesker Company®. Deposition pressure and temperature was
Results and discussion
TGA of this material under inert N2 atmosphere showed two different degradation temperatures, 475 °C and 650 °C (Fig. 2); both are much higher than the ALD temperature (120 °C), and therefore no significant thermal degradation is anticipated during deposition.
The temperature dependence of TiO2 ALD growth was probed on Si and can be seen in Fig. 3 using pulse/purge times previously presented in the Method section in the range of 90 °C to 130 °C. It is noted that all runs at 120 °C were done with
Conclusions
Nucleation and growth of TiO2 thin films deposited on PDMS via ALD were investigated for PDMS substrates with different surface properties. O2 plasma-treated PDMS was used as a hydrophilic surface and pristine PDMS was used as a hydrophobic one. The precursor/oxidizer used were TDMAT and ozone. XANES was used to probe the diffusion of precursor/oxidizer onto the subsurface of the polymeric substrate. XANES data of ALD TiO2 on pristine PDMS showed significantly more Ti than that on
Acknowledgements
This work made use of instruments in Electron Microscopy Service (Research Resources Center, UIC) and Nanotechnology Core Facility (NCF, UIC). We are thankful to Dr. Robert Klie for SEM/EDX related discussions and to Dr. Salman Khetani for allowing us to use his lab equipment for plasma treatments of our samples. Portions of this work were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS) with special thanks to
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