Porous Organic Nanolayers for Coating of Solid-state Devices
© Vidyala et al; licensee BioMed Central Ltd. 2011
Received: 31 December 2010
Accepted: 14 May 2011
Published: 14 May 2011
Highly hydrophobic surfaces can have very low surface energy and such low surface energy biological interfaces can be obtained using fluorinated coatings on surfaces. Deposition of biocompatible organic films on solid-state surfaces is attained with techniques like plasma polymerization, biomineralization and chemical vapor deposition. All these require special equipment or harsh chemicals. This paper presents a simple vapor-phase approach to directly coat solid-state surfaces with biocompatible films without any harsh chemical or plasma treatment. Hydrophilic and hydrophobic monomers were used for reaction and deposition of nanolayer films. The monomers were characterized and showed a very consistent coating of 3D micropore structures.
The coating showed nano-textured surface morphology which can aid cell growth and provide rich molecular functionalization. The surface properties of the obtained film were regulated by varying monomer concentrations, reaction time and the vacuum pressure in a simple reaction chamber. Films were characterized by contact angle analysis for surface energy and with profilometer to measure the thickness. Fourier Transform Infrared Spectroscopy (FTIR) analysis revealed the chemical composition of the coated films. Variations in the FTIR results with respect to different concentrations of monomers showed the chemical composition of the resulting films.
The presented approach of vapor-phase coating of solid-state structures is important and applicable in many areas of bio-nano interface development. The exposure of coatings to the solutions of different pH showed the stability of the coatings in chemical surroundings. The organic nanocoating of films can be used in bio-implants and many medical devices.
The interface between biomedical and nanotechnology is an area of intense research. Integration of biomedical micro/nanoelectromechanical systems (BioMEMS/NEMS) and materials offers tremendous potential to tackle medical problems in the areas of diagnostics, therapy, surgical implants and drug delivery . In past few decades, fluorinated coatings have seen many applications in the fields of biochemistry and tissue engineering [2–4]. These coatings are used to attain low surface energy and corrosion resistance properties in nano- and micro-structured devices [5, 6]. Organic composite films can be attained by many techniques, e.g. plasma polymerization, biomineralization, chemical vapor deposition (CVD) and self assembled monolayers (SAM) [7–13]. Two important goals of such coatings are biocompatibility and biostability; especially for the surfaces of medical implants. The biocompatibility and biostability can be achieved by modifying the surface characteristics of the substrates. Thus, surface modification of MEMS/NEMS structures has become one of the most important aspects of medically-related devices.
Structural stabilization of the coatings can be achieved from multiple covalent and hydrogen bonds using self organized silane films [14, 15]. Fluorinated surfaces have been studied to modify the surface energy, reduce cell adhesion, increase protein adhesion, and also in the development of organic-inorganic hybrid alloys [16–18]. 3-Aminopropyltrimethoxysilane (APTMS) and 1H,1H,2H,2H-Perfluorooctyl-trichlorosilane (PFTS) are non-toxic monomers commonly used to create fluorinated surface [7, 19]. APTMS, being hydrophilic monomer, is used as a linker in various applications such as for cell adhesion and DNA/protein attachment. It exhibits high coagulation activity [20–23]. APTMS film morphology has been shown to depend on the deposition method . PFTS is highly reactive and being a hydrophobic monomer has relatively low level of coagulation activity due to fluorine rich functional groups . Fluorine groups are inert, homo-compatible and are thermally resistive, which give reduced protein/cell adsorption on surfaces [19, 26].
Results and Discussion
Thickness of the nanolayer with respect to time.
Deposition Time (mins)
Thickness of the layer formed (nm)
Calculated Surface Energy
Concentration of APTMS: PFTS
Average Surface Energy (mJ/m2)
Spectroscopic Analysis of the Monomers
The chemical compositions of the organic films were analysed using FTIR. Nanolayers were made with 3 different ratios of APTMS and PFTS and their chemical composition was studied.
Coating of 3D Structures
Nanolayers of biocompatible coatings are the most desired properties for a number of device applications in medicine and engineering. Surface coating of the 3D micro and nano structures are reported using a simple method of vapor-phase vacuum chamber reaction. The coatings show biocompatible, low surface energy fluorinated layers which are ideal for many biomedical applications. The vacuum-based approach helps coating the inner surfaces of the devices and structures without need of any special equipment. This can be helpful in coating medical implants which need to be medicated on all sides of the device. Desired thickness and smoothness of the nanolayers can be acquired with respect to the type of application needed. The characterization showed that the nanolayers are stable at different pH solutions.
APTMS (hydrophilic) and PFTS (hydrophobic) were used as received (Sigma Aldrich). Silicon wafers were <100> orientation p-type doped, oxidized in a thermal oxidation furnace. The wafers were diced into small dyes and used as solid substrates to deposit the nanocoatings.
The substrate was kept in a vacuum reaction chamber and the two monomers were allowed to react in vapor-phase at a controlled vacuum and reaction time allowing the consecutive nanolayer deposition. Schematic diagram of this set up is shown in figure 1. The two monomers were placed on separate glass slides and a glass slide with chip to be coated was placed in between. Vacuum was maintained inside the chamber. The surface morphologies and the smoothness of the film varied with respect to the changes in concentrations of APTMS and PFTS monomers in the reaction chamber . For each concentration combination the film porosity also changed as the film grew thicker.
The thickness of the layer formed was measured with respect to time. The samples were made with different ratios of APTMS and PFTS for 20, 30, 40, 50 and 60 mins of deposition time (Table 1). After the deposition, the vacuum was turned off and the lid of the chamber was kept closed until the pressure meter indicator went down to 0 mmHg.
The films were made with different concentration ratios of APTMS: PFTS (1:1, 2:1 and 2:1) and the chemical composition of each were characterized using Fourier Transform Infrared Spectroscopy (FTIR). The spectrum was recorded in transmission mode on kBr crystals at a resolution of 4 cm-1 using Nicolet 6700 FTIR spectrophotometer.
The surface energies of the coatings were calculated from the contact angle measurements of the water droplet on the surface of the coated chip . To check the stability of the layer formed, the samples were immersed in the DI water and the surface energy was measured. Different pH solutions (pH 2, pH 4, pH 7 and pH 10) were prepared using HCl and NaOH and the stability of the nanolayer was checked by immersing the coated wafer in these pH solutions for 15 hours.
Coating of 3D Structures
The 3D micropore structures were coated using these layers. A Si wafer chip with a micropore of size 11.7 μm is shown as an example. The vapor-phase reaction was done using the two monomers. The coating formed a nanolayer on the pore covering all the sides.
The authors would like to thank Richard B. Timmons for help with the experiments, and acknowledge help by Rajendra R. Deshmukh in contact angle measurements and surface energy calculations of the nanocoatings. Partial chip characterization was carried out at UTA Characterization Center for Materials and Biology (C2MB). The work was supported by the National Science Foundation through CAREER grant (ECCS-0845669). Waseem Asghar was partially supported by a fellowship from the Consortium for Nanomaterials for Aerospace Commerce and Technology (CONTACT) program, Rice University, Houston, TX, USA
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