Any object employed underwater in a natural aquatic environment will, in a short space of time, be covered by a composite layer known as biofouling. This layer consists primarily of small living organism , together with trapped detritus and particles. Membranes used with, for example, gas-sensors are also subjected to this phenomenon, which results in decreased performance. The membrane can be both chemically and mechanically damaged, with the main issue being clogging. The diffusion of substances through the membrane is drastically slowed, and in the worst case totally blocked. Furthermore, biological processes occurring within the biofouling may result in the production or degradation of the very substances which the sensor is designed to detect. Prevention of biofouling therefore represents a vital aspect for long-time sensor performance and reliability.
The development of adequate and quality membranes is therefore essential for improved sensor performance.

Silicone is the basic substance of the now available antifouling membranes, structured as a monolayer with an embedded reinforcement mesh (product sheet: „silicone-based membranes"). The membrane of an underwater gas sensor acts as a phase separator between the liquid measure medium and the detector room. The gas exchange from water-side to the detector side occurs from diffusion through the membrane.
The antifouling membranes utilise specific antifouling substances, which are embedded directly into the silicone during fabrication. Release occurs through diffusion from within the layer towards the surface, depending on the concentration gradient. The advantage of this mechanism is that only small quantities of antifouling substances are needed. The slow diffusion process guarantees that dispersion into the surroundings is absolutely minimal, as the substances are directly and immediately consumed on the surface by the biofouling organisms attempting to settle on the membrane.

Oceans, rivers, lakes and ponds. The membrane has also proven its efficiency in Sewage Water Treatment Plants based on natural decantation and degradation in rural areas. In open ocean or coastal areas, an active efficiency of up to 3 months has been achieved.

As the substances to be detected have first to diffuse through it, the membrane is a limiting factor. The quality of the measurements, and thus the functionality of the sensor, is determined directly to the functionality of the membrane.
The cost of maintaining the membrane can be high , particularly during long-term monitoring deployments. The antifouling membrane provides the solution to the problem of achieving high data quality at low costs, by significantly extending the period between maintenance activities.

Monolayer antifouling membranes are now available, with or without mesh reinforcement in a range of thicknesses. Other models like the antifouling silicone composite membrane are in development.
(For further information on the different membrane types, please see the product sheet silicone-based membranes).

The development was funded by the Technology Foundation Schleswig- Holstein, Kiel, Germany.
An international patent is pending.


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