  Internet of Things (IoT) services necessitate the storage, transmission, and
analysis of diverse data for inference, autonomy, and control. Blockchains,
with their inherent properties of decentralization and security, offer
efficient database solutions for these devices through consensus-based data
sharing. However, it's essential to recognize that not every blockchain system
is suitable for specific IoT applications, and some might be more beneficial
when excluded with privacy concerns. For example, public blockchains are not
suitable for storing sensitive data. This paper presents a detailed review of
three distinct blockchains tailored for enhancing IoT applications. We
initially delve into the foundational aspects of three blockchain systems,
highlighting their strengths, limitations, and implementation needs.
Additionally, we discuss the security issues in different blockchains.
Subsequently, we explore the blockchain's application in three pivotal IoT
areas: edge AI, communications, and healthcare. We underscore potential
challenges and the future directions for integrating different blockchains in
IoT. Ultimately, this paper aims to offer a comprehensive perspective on the
synergies between blockchains and the IoT ecosystem, highlighting the
opportunities and complexities involved.
