An extensive program of testing was undertaken to understand the mechanical behaviour of Hytrel, from which domes were made. An additional complication was the fact that the inversion happened in around 100 microseconds, and therefore the mechanical behaviour of interest was that at strain rates of the order of 10000 per second. This makes mechanical testing virtually impossible, and the behaviour has to be inferred from that at lower strain rates. It was also important to understand how the domes failed - a series of experiments measuring the tearing energy of the material, and the burst strength of the domes, shown below, were performed to shed light on this.
Tests were then performed on the inverting domes themselves. The domes exhibit a complex snap-through bi-stable behaviour, which was characterised through quasi-static tests. The dynamic behaviour was captured using a high-speed camera capable of taking exposures of 1 microsecond separated by a few microseconds, see below. Other methods were also employed, such as measuring the velocity of the drug particle cloud leaving the dome. Experimentally this area was particularly challenging.
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