# A Review of Blockchain Technology Applications in Ambient Assisted Living
## 2 Materials and Methods  
In carrying out our study, we have used the PRISMA methodology that defines the guidelines for conducting systematic reviews, which is widely accepted by most Web of Science (WoS) journals for organizing review-type articles [33]. More specifically, we have selected the \"PRISMA 2020 flow diagram for new systematic reviews which included searches of databases and registers only\" variant that features four main phases: articles identification, screening, eligibility, and inclusion. The goal of our systematic study is to create an overview of the domain of blockchain and IoT applications for ambient assisted living and to construct a snapshot of the state-of-the-art works for general or specific topics in this domain. This approach will also allow identifying current hot trends, future research directions, and research gaps. The first step in our research study was to clearly define search strategy in terms of research questions, keywords, or key phrases to cover the study targeted topic of blockchain, IoT, and AAL applications. The following research questions have been selected for our study:  
* Identify use-cases for blockchain and IoT applications in AAL;
* Identify applications, techniques, and tools developed for this domain;
* Highlight the challenges and limitations of blockchain and IoT in AAL;
* Find what are the main open research directions to be tackled  
This led to the definition of the following main search keywords to be used in the next stage of the study:  
* Blockchain IoT healthcare;
* Blockchain and Ambient Assisted Living;
* Blockchain and Active Assisted Living;
* Blockchain and Ambient Intelligence
* Blockchain and remote care;
* Blockchain data ownership in health care;
* Blockchain data sharing and analytics in health care;
* Blockchain and IoT security and privacy in health care  
Using the above, the second step of applying the PRISMA methodology was to select the scientific databases for the search process. In this context, we have selected Web of Science as the main database for our study since it is the most comprehensive scientific database widely recognized for including high-quality conference and journal articles from the most important publishers (MDPI, IEEE, Elsevier, ACM, Springer, Wiley, etc.). Using the WoS database allowed us to focus our search on a single platform while receiving results from articles from multiple publishers. To conduct the search, we have used the Clarivate WoS web platform [34]. As a search method in this platform, we have selected the Topic type because it covers the key information from the WoS indexed research articles: title, abstract, author, keywords, and Keywords Plus. The search keywords have been transformed into search strings in the platform, e.g., \"blockchain\" AND \"ambient intelligence\". Figure 3 illustrates the PRISMA 2020 flow diagram used to identify the articles that were included in the review. In the PRISMA identification phase, after aggregating the search results, we have identified 491 articles matching our search criteria. We have refined this set of articles and removed duplicate records (19 items), resulting in 472 records to be included in the Screening phase. In this phase, we have defined specific inclusion criteria for our study to further filter the results, thus, removing 312 records. Similarly, to further narrow the set of records in the Eligibility phase, we defined several exclusion criteria that helped us drop another 73 records. Both criteria are presented in Table 1.  
Finally, in the inclusion phase, we obtained 48 records to be considered in the study for an in-depth analysis of the presented work, concepts, approaches, and solutions for blockchain in AAL. Figure 4 presents the included papers distribution per publishing year. It can be noticed that most of the research around the studied topics has been accelerated from 2020 onwards.