How to bond a life-saving medical device
BPR Medical, based in Nottingham is the firm behind the Bidirectional Firesafe Cannula Valve, used for in-home medical oxygen supplies to extinguish fires which can occur in the line between the concentrator and the user’s mask or nasal cannula.
Such fires can occur if the line comes into contact with ambient flame – such as a candle – or more commonly if the patient is a smoker.
BPR’s inline Bidirectional Firesafe Cannula Valve extinguishes the fire by cutting off the oxygen supply. It is a development of an existing, unidirectional design and acts as a thermal fuse whereby the oxygen supply is cut off when a fusible component softens as a result of the heat from an approaching fire in the oxygen delivery tube – making integrity an essential characteristic.
When BPR had chosen a two-part epoxy to bond both halves of the valve’s body, the next task was to find a dispensing solution that would assure deposition of a precise, repeatable volume of the adhesive, metered and mixed in the correct ratio, on to a cylindrical assembly. The resulting bond integrity would help ensure conformance for CE marking under European Medical Device Directive.
Technical director Ben Johnson’s team evaluated a number of possibilities, including premixing and dispensing via a pinch tube valve. This yielded inconsistent results due to changes in viscosity which begin to occur naturally as soon as epoxy is mixed, exacerbated by temperature fluctuations. It also took an unacceptable amount of set-up and clean-up time.
Having determined that volumetric technology would provide a better solution, BPR contacted Intertronics to arrange equipment demonstrations. Trials using the selected epoxy were conducted in a wide ambient temperature range, and rigorous pull, flexural and other testing of the assembled valve, revealed that the ‘preeflow eco-DUO’ precision metering, mixing and dispensing system suggested by Intertronics was the best solution. It offers ±1% dosing accuracy, >99% repeatability and can dispense volume flows of 0.2 to 32ml per minute, with a minimum volume of 0.01ml. The preeflow positive displacement technology means that the volume dispensed is not affected by viscosity changes in the material.
Ben Johnson and product development programme manager Mike Brudenell worked with Intertronics to configure the dispenser into its bespoke manufacturing jig, into which the two populated halves of the Firesafe valve body are manually loaded prior to being rotated whilst the epoxy is applied. Each of the 20,000 units per month produced at the time of writing is non-destructively tested, whilst regular samples are subject to a three-point flexural test.
The system operates an automatic purge at the end of the shift, eliminating the need for cleaning down. Control is via a preeflow plug ‘n’ mix interface and stored settings ensure consistency and repeatability, and eliminate daily set-up time. The volume of epoxy applied has been reduced to 0.05 g from the pinch tube valve’s 0.06 g.
Johnson said: “We had to go through a number of stringent processes to ensure the integrity of the bonding of the two body halves. An essential part of this was how the two-part epoxy adhesive was applied: to ensure a highly accurate, repeatable dose on a rotating jig, but also to maintain the correct dispense volume regardless of viscosity changes caused by temperature. After evaluating a number of options, we chose the eco-DUO from Intertronics as it provided the best results for both of these criteria.”
He continued: “Intertronics helped us choose a volumetric dispensing solution that was the best one for our needs. We’ve been very impressed with their expertise, they’re responsive and always on hand to support us with any queries or technical support we might need. The way they’ve helped us to integrate and set up the system means that we always achieve a consistently high level of process capability. Not only that, but we estimate our return on investment to be in the region of £2,000 a month.”