Disposable Sensors - have you lost your sensors?

One of the operational advantages of most disposable systems is the relative simplicity of the process paths. It is great to be able to see into your system without requiring the x-ray vision capability of Superman. But frequently, it is helpful or necessary to have a little help from a sensor. Typical needs are pressure, flow, temperature, conductivity, UV, bubble detectors, and turbidity.

In this post, let’s talk about the current state of available sensors. In a separate post, I will talk in greater depth at a later point about flowrate determination and about particle size determination and turbidity. However lets discuss the other sensors….

An ideal functional specification for our preferred sensor would be that it is light weight, has NIST traceable calibration, is linear and highly accurate, materials are all inert and documented, that the sensor comes as part of a pre-sterilized drop in place tubing assembly, and is sufficiently low cost that it can realistically be single use.

In an ideal world, we might also want the instrument to talk invisibly to us either via wireless technology or at least a clear easy to read display. Cables are both a trip hazard to the user and generally clutter the process area. Despite the inherent containment provided by the bag set to the fluid – most people still prefer to operate in true or near clean room environments. Again cables are bad news for a number of reasons including particle shedding. An ideal would be a 100% sealed and stand alone, battery driven datalogger, using ASIC or other low power digital processors.

In a real world, many sensors that are used are not disposable but form part of the framework that gets reused for each new bag set. For example proximity sensors (ultra-sonic, optical or capacitance) for determining fluid level are often mounted on the frame and register fluid level within a tubing set of bag

The actual sensor business end for at least temperature, pressure and conductivity need not be very expensive – either. Small micro-strain gauges and the associated Wheatstone bridge and printed circuit boards can have very low manufacturing components costs. However, to date no manufacturer has either had the sales volume or desire to provide a really low cost component.

In my opinion, for the most part Bag manufacturers are still in the learning stages about instrumentation. One supplier (an equipment verses bag supplier), who is perhaps the furthest ahead in this direction, is SciLog. I have used a number of their disposable, pre-calibrated sensors for conductivity, temperature, and pressure and can confirm they work. Typically we are not seeking very high accuracy and SciLog products are a reasonable flow through option. They even provide a wide range of connection options. However, at least for now you are still on your own when it comes to coupling your tubing set to the sensor component. Here is a quote from their published marketing:

For truly disposable sensors, bioprocessors should look into products from Scilog (www.scilog.com), which introduced a line of single-use disposable fluid-path products pre-fitted with. Sensors for additional process parameters are in development.
SciLog also holds several patents on single-use manifolds for automated, aseptic liquid transfer for liquid-intensive separates such as filtration, tangential flow filtration, ultrafiltration, reverse osmosis, sterile dispensing, bioreactor feeding, and chromatography.
These are smart sensors, says product manager Julie Schick. That means the devices contain embedded memory in which they store identification numbers, lot numbers, and calibration data. The sensor transmits this information to the SciLog monitor, which feeds back to the automated equipment and data collectors.

2 January 2007 | Uncategorized | Comments