**2. Related Work**

Blockchain technology is not only underpinning cryptocurrencies, but also widely adopted in other industries, such as finance [22–24], IoT [14,25,26] and healthcare [27–29]. Although blockchain benefits the systems that use this technology, it brings scalability issues to them [30–32]. The methods to solve the storage scalability issue of blockchain are categorized into "on-chain" solutions and "off-chain" solutions.
