## 6 Results of Functionality and Performance Tests  
A set of experimental test were done prior and after the assembly of the microinjector. The engine power must overcome the theoretically calculated viscous forces plus the power to overcome other friction forces, mainly the one between the rubber piston and the syringe wall. Frictional forces were measured experimentally using the dynamometer PCE-FB 20. Figure 10 shows typical results of the experimental forces observed, and Figure 10a shows the behavior of the friction force between the rubber piston and the wall for an air filled reservoir at 25 \({}^{\circ}\)C with an established flow of 13 \(\upmu\)L/s in a 1.0 mL syringe (piston displacement of 1.0 cm in 60 s). Due to the friction between the rubber and the plastic wall, the force is not constant, it jumps between 0.0 N up to a slightly variable maximum, \(|<\mathbf{F}_{Max}>|\approx 1.02\) N, in a kind of quasi-periodic behavior, with a general average force of \(|\mathbf{F}_{avg}|=0.548\) N. The average power applied by the dynamometer is given by  
\[P_{Piston/Wall}=\mathbf{F}_{avg}\cdot\mathbf{v}_{avg}=0.09\ \text{mW}, \tag{5}\]  
where \(\mathbf{v}_{avg}\) is the average piston velocity. Figure 10b shows the behavior of the force when the same syringe is filled with a commercially available formula of artificial tears with a quite low viscosity of 0.05 Pa-s (Lagricel Ofteno). Essentially, the dynamometer force behaves alike, but having higher magnitudes, for this case the average of the maximum peaks is \(|<\mathbf{F}_{Max}>|\approx 2.98\) N and the general average \(|\mathbf{F}_{avg}|=1.644\) N, which corresponds to a total power \(P=P_{Piston/Wall}+P_{viscosity}=0.27\) mW. These relatively small values are four orders of magnitude lower that the theoretically calculated power to push highly viscous tears through the capillary (\(\sim\)3.7 W), so the latter is the one that should be used as the minimum to be delivered by the selected engine. Combining together this experimental procedure to calculate the power needed to overcome friction forces (other than viscosity) with theoretical calculations of viscosity dissipated power assures that the selected engine has enough power to handle artificial tears with the higher reported viscosity, while being small enough to keep the size of the microinjector maneuverable. The selected engine Aiyima 15 delivers \(\sim\)14 W, a value reasonably higher than the minimum required, but not much higher to avoid wasting resources. A set of other verification and validation tests were done to establish the percentage error between the volume of fluid and delivery time programmed by an hypothetical user versus the values actually delivered by the microinjector. These performance tests were done at room conditions using air, distilled water, and commercial artificial tears of viscosity 0.05 Pa-s as testing fluids. Programmed volumes range from 120 to 1000 \(\upmu\)L and injection times from 2 to 16.33 h. For these tests, a silicon cylindrical capillary of inner/outer diameter of 600/750 \(\upmu\)m and 60 cm long was used. The results are indicated in Table 1, the first and second columns indicate, respectively, the programmed volumes of fluid to be delivered and the time to do so. The first result is in the third column \"Obtained volume\",  
Figure 10: Experimental pushing force of the syringe using a BD Plastipak Brand syringe. (**a**) Force measured with air. (**b**) Force measured with a commercial ophthalmic solution of 0.054 Pa-s of viscosity. within experimental error (1 \(\mu\)L), the microinjector delivered, for the three fluids, the programmed volume without any problem. Regarding the time, the fourth and fifth columns globally titled as \"Air\", indicate, respectively, the time taken to the microinjector to deliver the programmed volume and the percentage error. Once again, within experimental error (1.0 s), there is no perceptible delay. For the case of distilled water, the sixth column indicates that a measurable delay is now present.