The desire for biodegradable materials lead to intensive research on biodegradable magnesium and its alloys [1–3]. The field varies from cardiovascular [3–6] to orthopedic  applications. Biodegradable magnesium thin films are part of current research because they apply for different tissue engineering applications like stents or other scaffolds [3, 4]. Miniaturization of medical implants is one of the driving forces for the development of new fabrication technologies since it would allow a new generation of medical devices . This opens the research on new fabrication methods like magnetron sputter deposition which shows advantages in contrast to conventional bulk routes. Sputter deposition of magnesium was investigated by Schlüter et al. [9–12] for a variety of alloys, which showed that magnetron sputtered magnesium alloys exhibit mechanical and corrosion behavior comparable to bulk material and showed the possibility of depositing precipitate-free magnesium alloys [9, 11]. This is of particular advantage for biodegradable magnesium applications since its corrosion is highly susceptible to galvanic elements. Nieh et al. produced AZ31 magnesium foils with a thickness of 50 μm which showed mechanical properties comparable to bulk material . Production of scaffolds with magnetron sputtering confronts the fabrication process with two major problems. First is structuring thin films into scaffolds and second is releasing the structures from the substrate they were sputtered on. This leads to the development of different technologies suitable for specific desires. Structuring of magnetron sputtered NiTi was successfully achieved by Lima de Miranda et al. yet the process showed limited film thickness of maximum 50 μm . In addition the process required selective acidic etching of NiTi from copper which would not work for magnesium alloys. This work presents a fabrication method for freestanding magnesium alloys which combines the advantages of UV-lithography with magnetron sputtering. UV-lithography allows scaffolds with a feature size of 5 μm. The decisive part is the releasing method which allows a film thickness of up to 250 μm. In addition two different surface treatments were applied to achieve a reasonable surface quality for scaffolds produced with this technology.
Materials and methods
All magnetron sputtering processes were carried out with a VonArdenne 730S cluster machine. An Optispin 22 SSE was used for spin-coating of photoresists. The lithography was realized with a Süss Microtech MA6/BA6. An Ambios Technology XP-2-profilometer was used to determine the thicknesses of thin films. Optical and dimensional analyses were performed with a Nikon Eclipse L200 microscope and a Zeiss Ultra Plus SEM. Dry etching was performed with SenTech SI 500 inductively coupled plasma reactive ion etching (ICP-RIE) machine. Figure 1 illustrates the manufacturing process step by step. First (A) a 4 inch wafer (II) was structured with AZ 5214E image reverse photoresist (I). Next (B) a 50 nm thick aluminum layer (III) was deposited via DC-sputtering and afterwards the photoresist was released with acetone. The aluminum served as a hard mask for the ICP-RIE process (C). During step (D) the entire structure was covered with aluminum nitride (AlN, IV). Finally the Mg5W (V, supplied by FHR GmbH) was sputtered and afterwards released by selectively etching (F) the Al and AlN with potassium-hydroxide lye (KOH). Mg5W alloy was chosen due to a compromise between strength and ductility . The described processing route can also be applied for other magnesium alloys. For the surface treatment two different techniques were applied. First was microblasting using an airbrush air-eraser loaded with glass-beads (0–50 μm in size) at a distance of 10 cm for 2 min at a pressure of 2 bar. Second treatment was chemical etching using a solution based on ethanol and hydrochloric acid.
The thickness of scaffolds and tensile test samples was determined with a Mahr Extramess 2000 dial indicator. Tensile tests were performed with a Messphysik Beta 5-5/6×10 at a strain rate of 0.4% per minute and a preload of 0.2 N with samples of rectangular cross section of 500 μm times 26 μm. A Zeiss Ultra plus scanning electron microscope (SEM) was used for the optical characterization of surfaces. In addition a cross section of Mg5W film was prepared by focused ion beam milling (FEI Dualbeam Helios Nanolab). High angle annular dark field scanning transmission electron microscope (HAADF)-STEM was performed with a FEI Tecnai F30 STwin TEM to demonstrate magnetron sputtered microstructure.
Results and discussion
In order to demonstrate the fabrication process a series of SEM pictures was taken. Figure 2 A shows a structured wafer after dry-etching process with only aluminum on it (see Figure 1C). The wafer was dry-etched 50 μm deep for tensile test samples and 120 μm (respectively 300 μm) deep for scaffolds. The structure has a clear undercut which is of importance for later release of magnetron sputtered Mg5W-scaffolds. Part (B) shows the top on view of a released Mg5W scaffold while part (C) shows the side view. The sidewalls of the released scaffolds show the disadvantage of this method, since they disturb the surface quality. The deposited Mg5W grows not only perpendicular on the Al/AlN plateaus (see Figure 1 part D) but also on the sidewalls. These columns form a network which has grown partially on the sidewalls of the scaffold structure and their growth direction points to the sputtering target. The columns have a hexagonal microstructure and their diameter can reach up to 20 μm (see Figure 2 part D).
Surface treatment and tensile testing
Since the sidewalls of the structures were covered with columns which did not give a reasonable surface quality for scaffolds two different treatments were applied. The following SEM series (Figure 3) shows the surface of 80 μm thick scaffolds microblasted (B) and chemically polished (A) which can be compared to as fabricated in Figure 2B, C and D. Microblasting peels off the columns by kinetic impact of the glass pearls and also deforms scaffolds surface which can be seen (B). The impact resolves in cold forming of the material. Chemical polishing smoothed the edges removing all sidewall grown columns and giving an acceptable optical surface finish. Tensile test were carried out to demonstrate the effects of the surface treatments. Figure 4 shows the results. Since the tensile testing machine was operated without an extensometer, calculating young’s modulus was not possible. The Mg5W material reached as fabricated an elongation at rapture of around 14.35±2% and yield strength of 239±10 MPa which is comparable to prior results . The microblasted samples reached increased yield strength of 280±17 MPa which is due to cold work also leading to decreased elongation at fracture of around 6.9±2.7%. The chemically etched samples had a reduced cross section and showed lower yield strength at 131±12 MPa and elongation at fracture of 9.5±2.4%. Figure 5 shows STEM picture of a magnetron sputtered Mg5W cross section. The microstructure demonstrates the typical columnar (0002) magnesium orientation in the nanometer range already known from [9, 11, 12]. It also demonstrates the difference in grain structure between the beginning of the deposition and its end.
A new process was developed for the fabrication of thin, freestanding and micropatterned magnesium scaffolds. The process enables high reproducibility, spatial resolution of 1 μm and a high aspect ratio. The usage of UV-lithography allows a high degree of freedom in the scaffold design. In principle all kinds of two dimensional structures can be manufactured. Magnetron sputtering offers manufacturing thin films of high purity, low grain size and precipitation free alloys [9, 11, 12]. Chemical polishing enables high surface quality while microblasting allows adjusting of mechanical properties. This technology offers new options for the development of future biodegradable implants.
The authors want to thank the DFG for the financial support.
Funding: Deutsche Forschungsgemeinschaft, (Grant/Award Number: ‘QU 146/9-2’).
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