Plastics are increasingly being used in medical technology, especially for disposables. For many biological or medical applications however, the various surfaces first have to be modified in an initial step before they can for example be coated in a second step. Disposables often require bio-compatible surfaces in order to promote cell growth on the surface, specifically prevent it or to provide functional groups for the chemical coupling of biomolecules. This pretreatment is necessary but often also very expensive using conventional processes. Atmospheric pressure plasma processes are suitable tools for the cost-effective modification of surfaces. The application of microplasmas also makes it possible to very selectively adapt the surface to the desired application, or even to coat the already capped ducts of microfluidic components. The surfaces of 3D substrates such as microtiter plates, PCR tubes or cell culture bags can also be suitably modified with the help of these processes.
Creating biofunctional surfaces by the atmospheric pressure plasma process
Plasma methods based on dielectric barrier discharge (DBD) at atmospheric pressure offer a variety of possibilities for modifying the properties of surfaces to qualify them for use in biomedical applications.
In this method a physical plasma is generated in a gas gap between two electrodes by applying an a. c. voltage: this yields ions, high-energy electrons, reactive radicals and metastable excited gas particles. All these species enter into interaction with the substrate which is to be treated and modify its surface. The addition of layer-forming substances, so-called precursors, makes it possible to deposit coatings on the substrate which are typically not thicker than 10 to 100 nm. Depending on the selection of process parameters, such as the composition of the process gas or the treatment time, the following modifications or effects can be obtained from surface treatment by DBD:
- Activation of the surface to improve wettability with aqueous media
- Functionalization of the surface to generate chemically reactive groups
- Deposition of a coating to give the surface the desired function
Unlike other surface treatment methods, atmospheric-pressure plasma processes deliver a number of advantages: process times are short, neither solvents nor baths are necessary, and no expensive vacuum unit technology is required. Furthermore, atmospheric-pressure plasma processes scale and integrate into existing process chains very well. There is even great flexibility with regard to the materials and geometries which can be handled: not only polymer films, glass, silicon wafers or metals can be modified or coated but also porous materials, textiles or leather. Both sheets and moldings are suitable as substrates.
In-air plasma treatment – so-called corona treatment – of, for example, microtiter plates or cell culture flasks in order to improve the wettability of their surfaces is already very widespread in industry. At the Fraunhofer IST, atmospheric-pressure plasma processes create surfaces for promoting adherent cell growth in cell culture bags or on 3D framework structures known as scaffolds. Due to the use of gas mixtures containing nitrogen but no oxygen as process gas there is an increased formation of nitrogenous chemical groups on the surface of the substrate which favors the deposition of cells.
Chemically reactive surfaces for the covalent coupling of biomolecules, for example, proteins or antibodies, can be produced on very different substrates by the deposition of coatings containing epoxy, carbonyl or carboxyl groups.
A non-specific adsorption of biomolecules can be prevented by reducing the wettability of the surface with aqueous media. This is especially effective with so-called superhydrophobic coatings which are characterized by water-droplet contact angles above 145°.
Atmospheric-pressure plasma treatments can be whole-area or area-selective. The plasma printing process developed at the Fraunhofer IST in which the structure is, so to speak, stamped on by plasma is suitable for a area-selective surface treatment or structuring. In this way functionalities with dimensions limited to just a few 10 µm can be generated on the surface. Surfaces structured in this way play a major role in the production of biosensors or lab-on-a-chip systems.
For examining substrate surface modification, the Fraunhofer IST has a variety of analytical possibilities at its disposal, such as fluorescence analysis, infrared spectroscopy, laser-scanning microscopy (LSM), UV / Vis spectroscopy, x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM)/ energy-dispersive x-ray spectroscopy (EDX) or ellipsometry.
Furthermore, specific detection reactions are developed as required for the application in question. These enable quantitative information to be obtained regarding the efficiency and effectiveness of populating the surface with chemically reactive groups which in the reactions which follow interact with biomolecules or bind to them covalently.
One example is the development of a test for detecting amino groups which are, for instance, used for the biotinylation of surfaces.
- Whole-area and area-selective modification of surfaces
- Coatings for monitoring cell growth or adhesion to surfaces
- Setting wettability
- Generation of chemically reactive surfaces and development of functional layers
- Coating and process development
- Prototype production
- Layer characterization and surface analysis
- Advice and training