Here is the very high level specification that the government has provided. It refers to various further specifications and regulations that the device must comply with in order to be accepted by medics.

https://assets.publishing.service.gov.uk/government/uploads/...

You can see that the hardest part isn't so much the mechanics, but the parameters within which the device must operate, including safeguards on multiple conditions.

From a software point of view alone, I'm sure most of us here can appreciate the difficulty in producing this from scratch. e.g. "It must have 100% duty cycle for up to 14 days".

I suspect a huge challenge will be sourcing components, such as pressure sensors, that meet the required specifications. These seem likely to be specialised parts.

I read this the other day[1]. See challenges listed in the middle. It does seem like it's not very easy at all.

[1] https://hackaday.com/2020/03/25/ventilators-101-what-they-do...

Indian companies are working on some simpler versions which would cost about 100$.

https://twitter.com/anandmahindra/status/1243074972660027392

I think the hard part is that it has to run continuously, reliably and within tight tolerances or someone dies.