## 6 Results of Functionality and Performance Tests
The top row indicates, for example, that 1.0 mL was programmed to be delivered in 16 h with 20 min, however the microinjector took 1 min 3 s longer, the time error being around 0.11%. As the viscosity increases the time delay is longer, as the last two columns for artificial tears show. For this case it took 1 min 30 s longer than the time programmed to inject 1.0 mL. These kinds of experiments were repeated with all artificial tears available in Mexican drug stores, for the most viscous, \(\mu=~{}80\) Pa-s, the time error was 2.2%. For practical purposes, this time error represents no major problem, since the user may easily get the flow he/she needs by proportionally decreasing the value of \(t_{inter}\). In general terms, the microinjector will deliver without any problem any requested flow within normal ranges of lubrication. There are at least two parameters that the user must specify to obtain a desired dose of tears according to his/her needs, the most natural may be to specify a volume of tears and the time to be injected, let us say for example 1.0 mL in 5 h, while another way may be to specify 1 drop (\(\sim\)50 \(\mu\)L) every 15 s. Ultimately, the user is specifying the average flow of tears he/she needs to feel better. From that flow, the number of steps the engine must rotate and the time to do so must be determined. The volume of tears injected by each engine step depends on the mechanical design of the toothed plunger, gears coupling, and syringe diameter, in any design it should be corroborated experimentally. For the present case, the injection of 1.0 mL takes the microinjector 7500 steps (0.133 \(\mu\)L/step). Taking as a common approach the practical medical equivalence of 20 drops equivalent to 1.0 mL, then it is necessary to rotate the engine \(\sim\)375 steps to have 1 drop of tears. For example, if the user needs half a drop every 15 s, he/she needs to enter into the Android interactive display \(n=188\) and \(t_{inter}=15\) s. The engine will take 0.56 s to inject the fluid. This operation will be repeated periodically every 15 s until the user stops the device. In case the user needs a larger volume of fluid he/she can double or triple the dose whenever he/she needs by increasing or decreasing the number of steps. Equivalently, the user may choose to increase or decrease the time interval between injections, it can be done anytime. The minimum possible volume of tears to inject is the equivalent to 1 engine step (=0.133 \(\mu\)L), the maximum is the total volume of the syringe (2.0 mL). For the mechanical design of the present microinjector, the relationship between the flow \(Q\), \(t_{inter}\), and \(n\) is given by  
\[Q=\frac{0.1333}{t}n\left(\frac{\mu\mathrm{L}}{\mathrm{s}}\right)=\frac{2.2 \times 10^{-3}}{t}n\left(\frac{\mu\mathrm{L}}{\min}\right). \tag{6}\]  
Even though the relationship given by Equation (6) can easily be established once the total number of steps needed to empty the total syringe volume is known, it is through experimental tests that this  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Programmed** & **Programmed** & **Obtained** & **Air** & **Distilled Water** & \begin{tabular}{c} **Lagricel Ofteno** \\ **(Ophthalmic** \\ **Solution 0.4\%)** \\ \end{tabular}  \\
**Volume** & **Time** & **Volume** \\ **(\(\mu\)L)** & **(h-m-s)** & **(\(\mu\)L)** \\ \hline
1000 & 16-20-00 & 1000 & 16-20-00 & 0.000 & 16-21-03 & 0.100 & 16-21-30 & 0.153 \\
960 & 16-00-00 & 960 & 16-00-00 & 0.000 & 16-00-43 & 0.070 & 16-01-31 & 0.157 \\
780 &parameter should be firmly established. Figure 11 shows the results of two of these performance tests, both done at \(t_{inter}=15.0\) s and \(n=15\) and \(30\) respectively, using 1.0 mL of Lagricel Ofteno as testing tears. Once the microinjector started running, the injected volume and the elapsed time were recorded and plotted. The figure shows a well defined linear behavior, where square and circle dots represent experimental data, while the solid line represents a linear fit.