PowderJect Pharmaceuticals develop a range of devices aimed at needle-free injection of powders. One of their devices uses a flexible membrane to accelerate drug powders to sufficient velocities to penetrate the skin. The device consists of a cartridge of compressed helium gas which, when actuated fills a rupture chamber. When the pressure is sufficiently high, an aluminium membrane bursts, creating a shock wave that travels through the shock tube. The membrane, known as the inverting dome, is located at the end of the shock tube. The dome accelerates outwards, flipping inside out in the process, and takes the drug powder with it. When the dome stretches too far and slows down the drug powder, loosely attached to the dome by means of small hair-like fibres, continues towards the skin.
The inverting dome is manufactured from Hytrel, a commercial thermoplastic elastomer approved for medical applications. The original design of the dome was the result of experimentation and no attempts had previously been made to understand the process by which the dome inversion influenced the particle velocities. Needle-less injection using some of the other PowderJect devices was able to accelerate particles to 500-600m/s, while the inverting dome could only manage around 150m/s. However, the dome device was appealing for other reasons, for example the limited noise of operation compared with other devices, and the potential for use at the end of a catheter and for oral injections.
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