It's a billion dollar industry already with plenty of professionals hoping to differentiate their product from the rest of the pack by producing a condom that is significantly more pleasant to use. If someone has a promising prototype that Bill Gates would grant $100k for, an established condom company would probably pay many multiples of that to buy it. I don't see what additional incentive Bill Gates is offering here. I'm sure there's no shortage of effort here as it is. The hurdle is likely not one of human ingenuity, but plain physics.
The problem is that many promising developers struggle to get the basic seed money to even get to the basic development stages. It requires capital just to do the basic research. What Gates is offering is a much more accessible form of capital for an inventor than what a company will generally offer. It's not easy to walk up to a company and just say "Hey I have a great idea, give me a small chunk of seed money to get it off the ground." 100K is peanuts but that's the point. It's a relatively accessible 100k (that could later turn into 1 million dollars) for a prototype and inventors would probably get to keep the rights to their product instead of selling off the rights to a company in exchange for funding. It's just a different method of venture capital and its one that some inventors might be much more comfortable with. Money isn't the sole driver of innovation. (although certainly its probably the biggest)
I don't know if cheaper condoms are more meaningful than somehow trying to stamp-out the pre-industrial socioeconomic model that incentivizes excess procreation while scandalizing non-matrimonial female sexuality.
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