1 Introduction
For good reason, extremely high demands are placed on medical technology products and surfaces relevant to medical technology in general. These requirements vary greatly depending on the place of use and application. For example, syringe bodies must not interact with the active ingredients of the injection solution. Implants, on the other hand, should grow into the body well and with as few complications as possible - fixators or other osteosynthesis materials, on the other hand, should be easy to remove again after a certain period of time.
In order to meet these requirements, it is often necessary to modify the material surfaces. However, there are often other requirements that need to be met by the products at the same time. In the case of syringe barrels for so-called prefilled syringes, for example, this is the need for the plunger to glide evenly. This is achieved through siliconization. Small amounts of silicone oil are introduced into the glass syringe barrels and distributed evenly over the surface. This reduces the sliding friction forces of the plunger stopper when the syringe is used. With certain medications, however, an interaction between excess silicone oil and the active ingredient can lead to undesirable reactions. To avoid such reactions, so-called baked-on siliconization was developed, in which, on the one hand, significantly less silicone oil is introduced into the syringe barrel and, on the other hand, additional thermal fixation takes place [1, 2]. On the other hand, this process can reduce the sliding friction effect if too little silicone oil is used, so that the required uniform movement of the plunger stopper can no longer be fully guaranteed. Against this background, omniphobic coatings are an interesting alternative [3].
Implant materials are of particular interest in medical technology. In addition to stainless steel (V4A steel), either pure titanium or corresponding titanium alloys (e.g. Ti-6Al-4V) are used as osteosynthesis materials [4, 5]. Some of the implants used remain in the body for a long time or, ideally, permanently, but other implants such as bone screws or metal Kirschner wires often have to be removed again after the healing phase. Although titanium materials are characterized by excellent osteointegration, their good ingrowth behavior can be a disadvantage if the material needs to be removed, as it is then necessary to remove surrounding or ingrown bone material [6]. In these cases too, at least for temporary osteosynthesis materials, an appropriate surface modification in the form of a thin coating could be advantageous.
Other substrate materials of interest in the medical field are plastics. These are used in medical technology, particularly for syringes, catheters, hygiene accessories and dressing materials. In particular, polyethylene, polypropylene, polystyrene and PVC are used as materials for disposable items [7]. Plastic housings are also frequently used to protect medical equipment or devices. Faceshields are currently being used as an alternative to textile face masks - these are made from PET and are suitable for multiple applications. It is therefore important that they are easy to clean.
In order to meet the increasing requirements, e.g. in terms of sterilizability and resistance to aggressive cleaning agents, plastics manufacturers are working on modified formulations for the respective plastics. On the other hand, however, targeted coatings could also be used to achieve corresponding optimization, e.g. to repel various liquids and thus make cleaning easier.
For various other surfaces in the medical field, e.g. patient chairs, wet rooms in hospitals, etc., hydrophobic surfaces, on which most liquids roll off and thus reduce the tendency to soiling, can also be advantageous.
2 Material and methods
A wide variety of materials were used as substrate materials, such as non-siliconized glass syringe bodies and corresponding planar borosilicate glass substrates, various plastic plates (ABS, POM, PET, PMMA, PA6, PP) and plates made of Ti-6Al-4V alloy.
The chemicals used to produce the omniphobic coating solution were sulphuric acid, dimethoxydimethylsilane (95 %) and isopropanol. As the omniphobic coating only forms a covalent bond with the substrate on glass-like substrates, a Pyrosil adhesion promoter layer was applied to all substrate materials except glass in one series of tests [8]. This was followed by wetting with the coating solution, which can be applied by spraying, dipping or polishing. The samples were wetted with the coating solution, dried for at least 15 minutes at room temperature, then rinsed with isopropanol and finally with distilled water.
The layer formed is only a few nanometers thin and is chemically a type of silicone oil bound to the surface. The layer structure is also referred to as a "polymer brush", as the polymer formed during coating protrudes from the substrate surface like the bristles of a brush [9]. Due to this layer structure, the layer is not only hydrophobic, but also acts as a sliding layer. This is demonstrated below using the example of syringe bodies.
To investigate the wetting properties, the water contact angle was measured on both the uncoated and coated substrate surfaces. The optical contact angle measuring device and contour analysis system OCA-15plus from Dataphysics GmbH (Filderstadt) was used for this purpose. The wetting properties were evaluated with the SCA 20 software from Dataphysics. The abrasion resistance of the coatings on the planar substrates was carried out by means of a load test according to ASTM D2486 as a wet test using nylon brushes (Ti-6Al-4V) or a microfiber cloth (plastic plates). The water contact angles on the substrates were measured again after various abrasion cycles.
Sliding friction tests were also carried out on the coated syringe bodies. For this purpose, the plunger was set manually on empty syringes without needles and the breakaway and sliding friction effect was tested using a universal testing machine (Hegewald and Peschke, Nossen) with a special testing tool. In order to be able to better classify the values, corresponding tests were also carried out on syringe barrels equipped with spray siliconization (Nipro Pharma Packaging
GmbH) were also carried out.
-to be continued-
