# Dry Test Methods for Micropumps
## 4 Discussion  
The combination of a Design-for-Test and specific pneumatic test methods provides fast, reliable, and quantitative characterization of the micropump after packaging. The main output parameters of the micropump can be estimated, and the determination of the device functionality is straightforward: the test results are compared to specifications, and any deviation leads to a failed test, and the micropump is then discarded. The data analysis is based on considerations about the micropump dynamics. Experimental results were coupled with numerical simulations to determine, for instance, at which time of the test sequence the valve prestensions \(P_{val~{}IN}\) and \(P_{val~{}OUT}\) will be evaluated to match the theoretical value of \(\pm 100\) mbar [28; 29]. Indeed, the first positive peak of pressure at about \(t=2.5\) s corresponds to the opening of the outlet valve when the pressure in the pumping chamber exceeds \(P_{val~{}OUT}\). The gas flows out of the pumping chamber during this peak of pressure. The pressure decrease corresponds to a progressive closure of the outlet valve. A pressure plateau at about 100 mbar follows the peak of pressure. Valve leakage can be evaluated at this stage (see e.g., the exponential pressure decay in Figure 11b), but a more reliable estimate is obtained during compression test #1, which is about 8 times more sensitive due to the larger pressure in the pumping chamber. An equivalent leak rate as low as a few nL/h of insulin can be measured during this test. From Figure 10, the estimated inlet and outlet valve prestensions of +105 mbar and \(-101\) mbar, respectively, together with the compression ratio value of 0.816, indicate that the tested micropump meets the specifications [28; 29]. It is important to note that the purpose of the dry test is not to determine the exact value of specific mechanical characteristics but rather to verify that the device is safe and effective. Indeed, the large variability of critical parameters, including the valve prestensions, is due to manufacturing tolerances (up to \(\pm 30\%\) at \(3\sigma\)). The designer will consider this variability as a design input so as not to make patient safety conditional on too narrow  
Figure 13: Experimental pressure profile during the extended dry test of a functional micropump. The different pressure gradients refer to the compression tests described in the previous sections. \(P_{val~{}IN}\) and \(P_{val~{}OUT}\) are the inlet and outlet valve prestensions, respectively. tolerance and thus acceptance criteria for the functional test. As an example, a direct and noncontact measurement of the membrane displacement upon actuation provides a better estimate of the stroke volume that its derivation through compression test #1. On the other hand, the control of the etch process of the pumping cavity, coupled with the dry test described previously, provides good confidence that the micropump will infuse insulin with accuracy. However, to improve test accuracy, e.g., for a sake of process control, it is possible to calibrate in situ the integrated pressure sensors. The outlet valve is forced open, while a pressure controller generates the test pressure. Pressure sensor offset and linearity can be measured during this test to make the contribution of these sensors to the overall dry test error negligible. The functional dry test can further provide strong evidence of the failure mode as illustrated in Figure 11. However, this analysis must be performed in a precise order because some failures can mask others. The presence of a large leakage flattens the pressure signals, and the test method is no longer effective. In addition, for a micropump that successfully passes electrical tests, the determination of the compression ratio is only valid if the leak rate is within specifications, the valves are not stuck, and the pneumatic control of the outlet valve is operational. Note that electrical failures are discarded prior to the dry test.