A biofilm has a unique structure composed of microorganisms, extracellular polymeric substances (EPSs), etc., and it is layered on a substrate in water. In material science, it is important to detect the biofilm formed on a surface to prevent biofouling. EPSs, the major component of the biofilm, mainly consist of polysaccharides, proteins, nucleic acids, and lipids. Because these biomolecules have a variety of hydrophilicities or hydrophobicities, the substrate covered with the biofilm shows different wettability from the initial state. To detect the biofilm formation, this study employed a liquid-squeezing-based wettability assessment method with a simple wettability index: the liquid-squeezed diameter of a smaller value indicates higher wettability. The method is based on the liquid-squeezing behaviour of a liquid that covers sample surfaces when an air-jet is applied. To form the biofilm, polystyrene surfaces were immersed and incubated in a water-circulated bioreactor that had collected microorganisms in ambient air. After the 14-d incubation, good formation of the biofilm on the surfaces was confirmed by staining with crystal violet. Although the contact angles of captive bubbles on the surfaces with the biofilm were unmeasurable, the liquid-squeezing method could distinguish between hydrophilic and hydrophobic initial surfaces with and without biofilm formation using the diameter of the liquid-squeezed area. The surface wettability is expected to be a promising property for in-situ detection of biofilm formation on a macroscopic scale.
Surface wettability is one of most important indexes for understanding the interfacial characteristics between a solid surface and a liquid, especially such physico-chemical properties as hydrophilicity and water repellency. Recently, non-contact wettability assessment by monitoring the behavior of liquid covering a sample surface by jet-flow application to the liquid has been proposed. This method is applicable to the in-liquid assessment of high wettability materials for which it is difficult to measure the contact angle, such as cultured cells in a liquid culture medium. However, when the flow is dirty, the contamination of the cell culture system by particles (e.g. microorganisms, virus, and aerosols) may occur. To solve this problem, we developed a contamination-free non-contact wettability assessment system that is composed of a clean-air source having a particle filter and an automated system for image processing instrumentation and motorized alignment device for sample positioning. We confirmed that the filtered air-jet contained no particles, although over ten thousand particles had been included in the air source. Furthermore, we detected no microorganisms on agar culture plates, which had been exposed to the air-jet before being incubated. With the image instrumentation, we automatically measured the liquid-squeezed size, which was the main index of surface wettability. The motorized alignment device allowed us to control the sample position without human direct operation. This developed system is compatible with wettability assessment in a high-level clean environment such as quality control of culture cells for regenerative medicine and in vitro assay of drug efficacy and safety.
This study proposed a concept for evaluating biological functions by its physico-chemical properties such as surface wettability. To realize the concept, we have to prepare two elements; (1) systems for assessing the physico-chemical properties of biological objects and (2) a database connecting physico-chemical properties to biological functions. In this study, non-contact wettability assessment system was used as an apparatus for assessing surface wettability, one of the most common physico-chemical properties. Because this system can eliminate a physical contact between a probe and an object, there is an advantage on contamination reduction. Furthermore, in-liquid evaluation is feasible by the system. Therefore, this study aimed to the evaluation of cellular functions via the surface wettability on the cultivated cellular tissues.
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