## 6 Future Directions  
As mentioned throughout this review, the possibility of freely sharing sensitive information between professionals and health institutions would allow a step forward for personalized medicine, taking advantage of the most advanced machine learning techniques that computer science is offering.  
We saw that there are different ways of thinking on how to share data on the Blockchain: storing data in its blocks (but very difficult actually due to scalability issues); exploiting data provenance, by basically storing positions of data in the Blockchains instead of data itself; and using distributed storages combined with the Blockchain, thus using them as off-chain storage. In each of the solutions, there is a potential to never move data across the network that could be simply accessed and used.  
The review focused on the ability of Blockchain of creating specific solutions able to enable the user to be the true owner of their data. At the moment, what is evident is the increasing interest in user-centric solutions even when it is not the goal of the research. Several solutions are trying to increase the level of ownership of users over their data, but the problem of creating a full decentralized user-centric system for health data remains. The usage of permissioned solutions owned by healthcare institutions is sometimes ambiguous, maybe enabling users on managing their data but finally forcing them to accept consortium rules over data management. Moreover, this specific problem affecting security in Permissioned solutions also has an impact on the usage of decentralized storages that risk being wasted since they lose their value. If the Blockchain risks being compromised by participants, then the solutions fail to deliver immutability and even if user data are on decentralized storage, they will not be able to guarantee a user-centric system. Several implementations underline this.  
Finding a solution to this problem, maybe through hybrid implementations between Permissioned and Permissionless implementations, could be even more impressive when these networks can reward the participants making them able to be active contributors to a community with their data as the one of healthcare. This vision also settles very well with a concept that has been found in one review only: rewards for data contribution. As said previously, whenever an individual uses an IoMT system or an IoT device, it is never rewarded for the contribution it makes. Typically, individuals pay to get professional help, but their contribution is higher than the performance received. In fact, in principle, they contribute to enhancing scientific knowledge. In an increasingly Blockchain-enabled world, the Internet of Blockchains could definitely enable data sharing and data crowdsourcing.  
However, we would need to move the architectures from a system-centric perspective, where a user is the consumer of the application, to a user-centric perspective, where the user is more than a consumer but an active participant. It could be exciting having a solution in which each user physically owns data, for example, on a small device in their home or their smartphones. If these devices act as safe, decentralized storage, then every unknown entity could access easily the data stored in it, respecting rules posed by the users that are owners of what they share.