Multi-functional materials through surface modification
“Our research has been substantially supported by the Finnish metal industry in recent years. This shows that our industry stakeholders recognize the value of world-class research,” Mika Hirsimäki says.
Hybrid materials boast diverse functional properties that open up novel applications for stainless steel and other conventional materials that are widely used across large-scale industries. Potential uses range from electronics and sensors to life sciences.
Two research groups, the Surface Science Laboratory in the Optoelectronics Research Centre at Tampere University of Technology (TUT) and the Protein Dynamics Group at the University of Tampere, have joined forces to explore the functionalization of stainless steel surfaces using ultra-thin bifunctional silane layers.
Silanes are silicon compounds. Due to their insulating and hydrophobic, or water-repellent, properties, silanes are extensively used for surface treatment purposes, for example, to prevent corrosion, promote self-cleaning and provide electrical insulation.
Enhanced functionalization is not limited to steel
According to Professor Mika Valden, who leads the Surface Science Laboratory at TUT, conventional silane layers are relatively thick and have a disorganized structure. This makes their use in controlled functionalization next to impossible.
”In order for us to reliably predict and optimise the properties of the silane layer, we had to decipher the structure and composition of a highly organized silane layer that is deposited on a surface and identify the physical and chemical mechanisms governing the formation of the layer. The findings open up almost limitless opportunities to modulate the properties of different surfaces, not only steel.”
Bright radiation reveals the molecular structure of materials
Access to the national MAX IV synchrotron radiation facility based at Lund University, Sweden, proved invaluable in the investigation. The equipment at MAX IV is capable of producing ultra-bright synchrotron radiation, which was needed to peer into the structure of the silane coating. The molecules that are critical to the functionalization of the layer are sparsely distributed at the surface.
“A single layer of silane molecules is approximately 0.5 nanometres thick. If we had used other radiation sources, the results would not be as reliable,” says Senior Research Fellow Mika Hirsimäki from ORC’s Surface Science Laboratory.
Researchers from Estonia were also involved in the research. The Institute of Physics at the University of Tartu is one of the pioneers of synchrotron radiation in the Nordic and Baltic countries, and Professor Ergo Nõmmiste is a recognized authority in the field.
“We were very lucky to have our Estonian colleagues’ collaboration networks, synchrotron radiation experience and scientific expertise available.”
Diagnostic applications for the food industry and medicine
The researchers deposited a layer made up of two different silane molecules on a stainless steel surface. By varying the concentration of the molecules, which respond differently to their environment, the properties of the surface can be fine-tuned to match the requirements of a particular application. Silane coatings can be used, among others, to enhance the specific binding of proteins on steel surfaces.
“The aim of our surface modifications was to enable controlled protein attachment on the surface. We used the protein streptavidin, which is suitable for further deposition of a wide variety of biologically active components. This approach is called (strept)avidin-biotin technology, which enables the targeted delivery of drugs to affected tissues, the detection of antibodies and the purification of biomolecules,” Academy Research Fellow Vesa Hytönen from the Protein Dynamics Group says.
In addition, the resulting materials can be used in the food industry to detect harmful bacteria or to prevent bacterial attachment. Proteins that are modified to react to the presence of bacteria or the compounds they produce could be bonded to steel surfaces used during hygiene monitoring processes.
Proteins can also be attached to the surface of a stent made of stainless steel that is implanted inside a blood vessel to prevent it from becoming blocked again as a result of cardio-vascular disease. The downside is that the implantation of foreign material always leads to varying degrees of rejection. If, for example, RGD peptides were bonded to the surface of the stent via a silane layer, they could promote the adhesion of the right kind of cells.
The HYBRIDS programme of FIMECC Ltd, the Finnish Metals and Engineering Competence Cluster, includes one of the projects currently underway that explore the applications of silane layers in functionalized materials.
Paper examines the functionalization of stainless steel
The Surface Science Laboratory headed by Professor Mika Valden in the Optoelectronics Research Centre at Tampere University of Technology published a paper examining the functionalization of stainless steel in the October issue of Nanotechnology. The paper presents a new method for depositing bimolecular silane layers on stainless steel surfaces.
The results demonstrate that the structure and composition of the layers can be optimized to improve the biofunctionality of steel surfaces. A further aim was to ensure that the process can be easily scaled up from laboratory to full-scale production. The results are related to the dissertation research conducted by MSc (Tech) Leena Vuori. She will defend her dissertation at TUT in November.
Text: Marjut Kemiläinen
Photo: Virpi Andersin