The PlayStation Portable(PSP) was, hands down, one of Sony's most successful devices during the seventh generation of game consoles. The console was released in 2004 and by the end of its life cycle it had managed to sell an impressive 82 million units; deemed by Sony as a complete success.
Sadly, the same cannot be said about the PlayStation Vita; a console that many would even call ahead of its time. As the PSP's rightful successor, the PS Vita was a console that had a lot going for it, but that Sony didn't know how to manage; losing faith in its potential very early on to the point they simply gave up and abandoned what could've been a gold mine.
To the date, the PS Vita has barely managed to sell a rough estimate of 10-15 million units, which pales in comparison with the success of the PSP. It seems, however, that Sony still hasn't given up on handheld hardware jus tyet, as new information seems to suggest that they want to get back in the handheld market.
Recently, Sony has filed a patent for an "electronic game cartridge" that will, supposedly, be connected at the bottom of whatever device it is that Sony is working on. No more detailes were revealed about the patent, but this could suggest that, much like Nintendo did with the Nintendo Switch, Sony are planning on bringing cartridges to either the PlayStation 5, or to a PlayStation Vita successor.
Patents are filed pretty regularly, and most of them don't ever come to fruition. Companies like to file these patents as a "just in case" type thing before they actually consider bringing these ideas to live so this could really not mean anything, but given the success of the Nintendo Switch, it's only a matter of time until either Sony or Microsoft retaliate and come up with a portable device that can rival Nintendo's handheld hybrid console in terms of portability.