## 4 Discussion and Conclusions  
The performance shown by the two prototypes for the \"core\" characteristics are comparable to professional ventilators, however there are also some characteristics that are not implemented such as special sequences that help physicians evaluate the state of the patient. The volumes and pressures achieved during the tests are consistent with the clinical requirements, and both systems permit controlling the volumes and breathing rates within the ranges normally used in intensive care units. Furthermore the capacity to control tidal volume and PEEP allows the implementation of lung-protective ventilation strategies on both models. Of course there are still challenges to be overcome in future research:  
1. **Reliability of the mechanisms, software, etc. **: Since these were fast track developments, there has not been enough time to undergo appropriate long term reliability tests on these pieces of equipment. Although we did run tests for several days on both prototypes, appropriate tests for weeks or months under realistic operation conditions are still needed and will require a significant amount of time, incompatible with the urgent requirement of ventilators to deal with the ongoing international crisis. 2. **Mass production with appropriate standards**: Given that the concept of the devices presented here with the necessary adjustments for industrial production are expected to become medical equipment, the fabrication has to be performed in appropriate industrial environments with the necessary manufacture equipment and by trained personnel. In addition appropriate quality control has to be implemented. Once again, it is not clear how this could be done in a timely manner to face the COVID-19 pandemic. 3. **Regulatory approval or exemption**: Being medical equipment, the use of the devices presented here, or variations around them, requires appropriate approval by the health regulatory agencies of the different countries that intend to use them. Such a process is, under normal circumstances, time consuming, involves a significant amount of bureaucratic paperwork and financial investment, not to mention extensive preclinical and clinical trials. While some countries, such as the USA that have issued a declaration of \"liability immunity for activities related to medical countermeasures against COVID-19\" [16], most countries have not taken similar measures, therefore, the use of these devices depends either on the creation of similar policies of general application, or the expedited approval of specific devices. 4. **Additional features**: In addition to the central characteristics such as volumes, pressures, etc. required from a professional mechanical ventilator, there are a number of other characteristics that need to be met. For instance, the system has to be enclosed in an easy-to-clean fluid spill resistant casing, it requires battery support for the case of a power cut or for patient transportation, among other characteristics. The possibility to provide real-time pressure, flow and volume plots on a screen is also a characteristic that would be desirable as well as the possibility to program other inhalation/exhalation sequences that are of use for clinicians as diagnostic tools or for specific ventilation strategies such as pressure support ventilation. For completeness, we include here a reference Table 2 with a comparison of the performance parameters of our two models compared to a state-of-the-art commercial ventilator. It is worth mentioning that although not as flexible as the commercial system, our models meet, with some restrictions the characteristics needed for the treatment of ARDS(acute respiratory distress syndrome). In conclusion, we believe that our two proof-of-concept approaches to re-engineer and upscale simple low cost manual bag-value-systems to a low-cost ventilator were successful and promises, after further development, to be of benefit to patients in need of mechanical ventilation in the absence of other options.