From 1 August 1999 medical syringes were required to be fitted with safety needles to protect healthcare workers from needlestick injuries. So for the Zero-Stik 3ml safety syringe, made by NMT Group in Scotland, the unity of the needle and hub is critical. Without such total integrity the syringe would suck in air when being charged; or leak or expel the needle when the syringe plunger was depressed. Of even greater importance is the absolute need for the needle to remain in the hub when this automatically retracts after the syringe has been used.
`Originally,’ explained John Targell, R&D director, `we wanted to injection mould the hub around the needle, but because of the sharpness and fineness of the needle, there was a risk of damage during assembly. Also we couldn’t produce the homogenous needle and hub units fast enough to achieve economies of scale.’
Next bonding the needle and hub was considered, and the Loctite Technical Service Group was approached to discuss the design, which called for a needle pull-out force of 20N. Tests using the chosen adhesive showed that the bonded needle would require around six times this force, ie, 120N, to separate it from the hub.
Targell continued, `Trials also showed that circular grooves cut into the hub bore (which our production people thought would be necessary to increase bond strength and ensure the needle was sealed) were unnecessary. `In fact the only change we made to the design, at Loctite’s recommendation, was to increase the chamfer where the needle entered the hub. This facilitated the accurate deposit of the adhesive during fast mass production and guarded against the adhesive contaminating the hub exterior.’
The needle, drawn from 304 stainless steel, has a 0.5mm exterior diameter, a 0.25mm bore and is 29mm long. When inserted into a hub of KR-01 resin there is a 0.04mm radial gap between the parts, which is filled automatically during manufacture with approximately 0.0002g of Loctite 3001. This is a one-part, solvent-free, ultraviolet-curable adhesive specifically developed for bonding medical devices. Consequently it has USP Class VI Certification and can be sterilised by ethylene oxide (EtO), Gamma radiation and a 30min steam cycle at 170 degrees C to 1175 degrees C.
Loctite 3001 has a low viscosity, which is vital if the adhesive is to wick into and fill the 0.04mm gap at the speed needed to produce multi-millions of units every year. After a 5s cure by ultra violet light, all the subsequent transparent bonds are automatically tested on-line.
Loctite UK Tel: 01707 358800