I'm doing a project on healthcare data management Dapp. My project contains the actors as the doctor, patient and hospital. Actors have the attributes them self like name, id, age, hospital registration number and more. The project have the functionalities that are adding actors and updating actors. i'm using the IPFS to store the files related to the each actor as an off-chain storage. My question is how to store the private data of each actors? Storing the data on ethereum blockchain cost much amount of gas. Are there any ways to store the data on any off-chain database? Let me clear about how and what are the databases i can use to store the data as off-chain.