Can someone invest in a single product instead of an entire company?

yes. ofcourse. y not. but your product should be having USP(unique selling power) and quality edge. There are various products in the market which have single product line and also have investors.

Would it make sense to partner with one of or a group of your existing clients? So instead having an investor, you can develop for the clients. They will invest with you in the prototype, and get the full scale solution cheaper, and you can sell it to other clients for full price.

That way you don't have to worry about return on investment, equity or anything else. Your clients buy, but cheaper than the rest.

If you would like to discuss this solution further. let us talk. We have done this on different occasions in my business on smaller scale, and are right now doing it on a larger project.

Best regards
Kenneth Wolstrup

That is typically called a Joint Venture around a product, product family, market etc.... To really understand your investment this type of structure tracks separate financials of the JV only, so you can actually see your investment working.