## 5 Methods
### eARM Prototype  
Schematics of Prototype 2 (eARM) which is shown in Figure 15.  
Design considerations:  
1. A flexible and strong string is required (e.g., braided nylon cord), which should be able to carry enough weight, depending on the pressure which is to be achieved. Figure 14: Schematic representation of the assemble. The design will require adjustments. Figure 13: Schematic representation of the bush-bearing used at the long arm pivot position (**a**) is the main body of the bearing, (**b**) is a washer that maintains the bar in place. Sizes shown are just for reference and have not been optimized. The design will require adjustments. Figure 15: Three-dimensional render of the eARM prototype diagonally and from the front. Yellow: Motor with string roll, Orange: Squeezing arms, Green: Upper endswitch (volume and pressure control), Red: Lower endswitch (Timing control). The design will require adjustments. A spiral form in the string roll (see Figure 16) reduces jitter from string rolling up badly. Choosing a sufficient r2, in a way that squeezing is achieved in only a few turns, also helps avoiding jitter. 3. The string roll should enable easy installation/attachment of the string (e.g., twice through a hole in the string role). 4. An attachment on the arms (b2) helps pressing the bag-valve resuscitator while leaving space for the string roll (g \(>\) 2r2). 5. To avoid potential damages of the bag-valve resuscitator at the maximum squeezing, an additional space between the arms (dg) should be left. 6. Movable microswitches make setting up the device easier. The tunable position of the upper microswitch is essential for a manual volume and pressure control. 7. Two microswitches are needed to monitor and set respiration rate and inspiration to expiration ratio. Also if 1:1 ratio is the goal, keep in mind that opening and closing times differ. 8. Since bag-valve resuscitators are not designed for continuous long-term use, their characteristics and material can deteriorate with time. Regular rechecking and, if necessary, replacement are advised. Therefore, a design enabling relatively easy and quick replacement of it should be realized. 9. In Figure 17, the arm the length (l), spacing between the two rotational axis (d), bag-valve resuscitator position (h) and string roll radius (r) are shown. Accordingly a motor with appropriate characteristics (torque, rpm) have to be chosen in a way, that the squeezing process runs smoothly with the desired pace while reaching the desired pressure and volume. 10. By increasing l, the motor's leverage also increases, if h stays unchanged. This means that a motor with less torque could perform the squeezing. However, in this case the end points of the arms have to travel a greater distance from released to squeezed state, meaning that the motor has to rotate faster at the same r2 of the string role to maintain the same rate of squeezing (BPM). By increasing the r2, the speed of the motor could be kept unchanged, however, this would cancel out the leverage gained by increasing l. Nevertheless, these relations allow for some tunability of the design according to the needs. 11. The systems requires active control from a microcontroller and we propose the connection schematics shown in Figure 18. The microcontroller used was the ArduinoUNO platform and the code can be found in the supplementary information. Figure 16: Schematic of the eARM system with arms in the maximum compression position. The design will require adjustments. **Author Contributions:** Conceptualization, E.C.-C. and M.K. ; data curation, E.C.-C., J.O., G.H.-C., B.S. and J.T. ; formal analysis, E.C.-C., J.O., C.M., G.H.-C., B.S., J.T. and J.W. ; funding acquisition, E.C.-C. and M.K. ; investigation, E.C.-C., J.O., C.M., G.H.-C., B.S., J.T., K.K., C.S., F.H., G.K., P.M. and J.N. ; methodology, E.C.-C., J.O., C.M., G.H.-C., B.S., J.T., A.R.-M., J.W., T.W. and B.B. ; project administration, M.K. ; resources, E.C.-C. and M.K. ; software, J.O., J.T., S.K., C.S. and D.G. ; supervision, J.O. and M.K.