Generator solves mixing and dispensing problem

A unique, patented disinfectant generator has been developed that, as well as helping to solve healthcare-acquired infection by sterilising medical instruments that cannot be autoclaved, could also be scaled up or down for a number of industrial applications, from water treatment systems and paint spraying machines, through to military de-contamination units, dentistry flushing equipment, wound irrigation and control of drinking water in Third World countries. The Tristel Company, a UK-based supplier of liquid chemical sterilising solutions to UK hospitals, worked closely on the design of the 'Tristel' generator with the help of product design and development consultancy firm Design Technology, based in Suffolk. The initial idea for the sterilising machine came from Paul Swinney, CEO at Tristel, who recognised a growing need for hospitals to sterilise medical instruments, such as endoscopes, that are too delicate and expensive to be sterilised in an autoclave. The normal procedure has been to clean endoscopes using sterilant liquids. Traditionally, this is done using gluteraldehyde, a chemical that is no longer permitted in hospitals because it can cause chest complaints, asthma and dry throats. Swinney had been selling his company's 'Medivator' machines to hospitals as a way of combating this problem. Endoscopes were placed in the machine for 20 minutes to sterilise. He saw an opportunity to use another chemical, a combination of citric acid and sodium chloride - chlorine dioxide in suspension. The business was based on shipping the base solution and the activator in separate bottles to hospital staff, who would then have to mix the two chemicals in a certain ratio to give a known concentration of chlorine dioxide. Tristel supply around 60% of all UK acute hospitals with these chemicals. But there were limitations to this process. Nurses had to be trained to mix and test the concentration of the sterilant at regular intervals and there was no traceability if infections occurred. So Swinney decided to develop a machine that would automatically mix the two chemicals in doses of 5 to 200 parts per million. The machine would need to be attached to the mains water supply to 'mix on the fly'. There was a huge stumbling block though. The two chemicals required a 30-second wait period to chemically react and change colour. This meant that the machine could not mix the two chemicals with the water supply until this occurred. Swinney approached Design Technology in the second Quarter of 2003 and the company carried out a six-week feasibility study to determine how it could make continuous batches of the sterilant mix. John Hawker, MD at Design Technology told Eureka: "The generator development was divided into three phases. The feasibility study, working prototyping and engineering. At the end of the feasibility study, we had a concept and had identified a patented method of mixing and delivering the sterilant to the mains water supply in controlled batches." The innards of the generator are all made from 316 stainless steel. There are two PTFE precision-manufactured diaphragm pumps (made by Swiss company KNF), complete with Viton seals. These were needed because of the aggressive chemicals being used. According to Hawker, the seals only need replacing about once a year. The stepper motors are fitted with optical encoders which are used to provide feedback to a control panel HMI. The solenoid valves (supplied by Parker Hannifin) and the manifolds are all made from 316 stainless steel. But the real innovation is the coil, which sits at the core of the generator. The two sets of chemicals are stored at the bottom of the machine within two plastic containers. The diaphragm pumps pump the chemicals in exact dosages through the coil. The coil ensures there is always sufficient reaction time for the two chemicals to mix (30s) before being dispensed into the water supply. According to Hawker, the idea for the coil came when he was "experimenting with a 250ml measuring tube and wrapping a tube inside it to watch how long it took to see the reaction and colour change of the two chemicals". The coil is made from 316 stainless steel and was manufactured by TT Tubecraft based in Bracknell. But another issue came to the fore. The pressure in the water lines (from the mains) would be subject to variation and would range from zero to 4.5 Bar. This meant that, as the Tristel generator pumped chemicals into the water supply, there might be a small amount or a large amount of back pressure. This unpredictable back pressure would mean that each time the diaphragm pump moved the bolus of liquid through the dispensing head, the amount of liquid would vary. Therefore, between the pump and the coil, a pressure control valve is used to maintain the pressure in the water lines at 5 Bar, regardless of mains water supply pressure. So the pumps always displace the same volume of liquid regardless of the back pressure in the mains water supply. Upstream of the coil is a digital flow meter, supplied by Omega, which feeds a digital pulse into the machine's microprocessor. Design Technology's patented software algorithm then calculates what ratio of chlorine dioxide is required per litre of water, to activate the pumps for a set period of time, to dispense the correct volume of liquid. Hawker assured Eureka that the system "instantaneously detects any change of flow rate". Once the design was finalised, Swinney applied for a DTI R&D research grant (Smart Award) back in April 2003. He secured £100,000 of funding, which helped move the project from proof of concept to pre-production prototype in October 2003. In August 2004, after several design iterations, the first pre-production unit was completed. The generator is capable of dosing chlorine dioxide on demand at concentrations of 1-200ppm. Accuracy is within 10%, which in parts per million terms is extremely accurate. Flow rates are from 2 to 20 litres/min and an ISE probe validates the concentration of dosed liquid and prints a validation ticket (or RS 232 data output to a file) so that hospitals have a traceable record of what was sterilised and when and in what concentration. Bottles of chemicals can be changed easily, the machine has a simple, menu-driven user interface and the machine can be wall- or floor-mounted or free-standing for mobile use. The product was launched in October 2004 to the UK's healthcare market at the 40th Annual National Association of Theatre Nurses' Congress and Exhibition in Harrogate. To date, 28 generators have been manufactured with a certified medical device manufacturer in the UK. Three of these systems are already in routine use in hospitals and a further 11 units are in the process of being installed.