# A Review of Blockchain Technology Applications in Ambient Assisted Living
## 1 Introduction
In a blockchain-driven assistive living application, the users will join a blockchain network, and asymmetric encryption solutions will enforce the security of data sharing [26]. The IoT devices deployed in the user environment can be joined with  
Figure 1: Conceptual architecture of a cloud-based AAL system. smart contracts to automatically generate and sign transactions and forward them to the blockchain to be stored immutably [27]. The generated transactions are aggregated in blocks disseminated in the network and will be mined in the future blocks. To change a value in a block, the entire history of previously linked blocks needs to be rehashed, requiring a lot of computational power not being feasible these days [28]. To prove the ownership of the data, the IoT device provides a signature of the transaction, which is also useful for authentication and validation [29]. The updates are stored in chained blocks, taking advantage of the technology's properties such as reliability, availability, immutability, and consensus [30]. The blockchain enforces the provenance of data by a linked list of nodes; thus, data can be traced back by iteration of the chain [31]. In addition, securely storing the sensor's data and respecting the personal data regulations is difficult considering the perspective of the domain [32]. So, using a decentralized, user-centric approach regarding data privacy can address security and data ownership problems in developing ambient assisted living applications. Even though the research field is still at the beginning, relevant studies in the literature can be found. The motivation of this paper is to systematically review and organize them according to the research problems they address. To the best of our knowledge, there are no reviews conducted on solutions for integrating blockchain technology with ambient assisted living systems. To conduct this review, we have defined a search methodology with clear criteria for including or excluding papers, set up a reference interval, and focused on current important databases. We have included in the survey 87 papers on blockchain and ambient assisted living systems that were reviewed and organized. Thus, a reader would effortlessly gain insights into the current state-of-the-art research in the field. Nevertheless, there are still many gaps and open issues that would require increased attention from the research community in the coming years, such as data sharing and integration of care paths with blockchain, storage, and transactional costs, personalization of data disclosure paths, interoperability with legacy systems, legal issues, digital rights management, etc. The remainder of the article is organized following the Introduction, Methods, Results, and Discussion (IMRAD) structure: Section 2 presents the methodology and methods used in conducting the literature review; Section 3 illustrates the results by describing and organizing the most relevant research works; Section 4 presents a discussion on the survey findings, and Section 5 draws the conclusions. Figure 2: Features of blockchain technology desired for an ambient assisted living system.