The paper by Prof A.T.K Assis called "Gravitation as a Fourth Order Electromagnetic Effect" written in 1995 discusses gravitation as an effect of electric dipol

The paper by Prof A.T.K Assis called "Gravitation as a Fourth Order Electromagnetic Effect" written in 1995 discusses gravitation as an effect of electric dipoles at the atomic level. He has a couple of papers discussing this on his website https://www.ifi.unicamp.br/~assis/papers.htm https://www.ifi.unicamp.br/~assis/papers.htm. What I found interesting with this idea is that it gives an explanation for only ever observing gravitational attraction and the size of the calculated resultant forces was at the experimentally measured order of magnitude. Whether or not it can be further developed and appropriate experiments designed, who knows.