In order to file for a patent, you must be the inventor. Now, if you find an existing product and you modify it, you apply it in a unique method, you might be considered the inventor and you can proceed with your own patent. You can also improve existing products, and if your improvement is seen as being new, useful, and it's not an obvious variation of what already exists, you could qualify for a patent. Some examples of obvious improvements that will not qualify: changes in size or changes in color or shape. Of course, if there are some obstacles that you're able to overcome by changing the size—for example, making the hearing aid significantly smaller or changing the shape or color allows you to overcome an obstacle—then these improvements could be seen as being non-obvious and you still might qualify.
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