Q. I read your article about the gel to use with the RF skin tightening machine, but not the one recommending the machine itself. What brand were you referring to and how has it worked for you? —W.B. A. I have been using the AOGNY Skin Tightening Machine since late summer 2021. There has been a lot of interest so I will provide a brief overview before I discuss the machine and its use. As we age, our bodies produce less collagen, which causes wrinkles and sagging skin. RF skin tightening uses high-frequency radio waves to penetrate and heat the skin, stimulating the production of collagen. Thi…