  The successful amalgamation of cryptocurrency and consumer Internet of Things
(IoT) devices can pave the way for novel applications in machine-to-machine
economy. However, the lack of scalability and heavy resource requirements of
initial blockchain designs hinders the integration as they prioritized
decentralization and security. Numerous solutions have been proposed since the
emergence of Bitcoin to achieve this goal. However, none of them seem to
dominate and thus it is unclear how consumer devices will be adopting these
approaches. Therefore, in this paper, we critically review the existing
integration approaches and cryptocurrency designs that strive to enable
micro-payments among consumer devices. We identify and discuss solutions under
three main categories; direct integration, payment channel network and new
cryptocurrency design. The first approach utilizes a full node to interact with
the payment system. Offline channel payment is suggested as a second layer
solution to solve the scalability issue and enable instant payment with low
fee. New designs converge to semi-centralized scheme and focuson lightweight
consensus protocol that does not require highcomputation power which might mean
loosening the initial designchoices in favor of scalability. We evaluate the
pros and cons ofeach of these approaches and then point out future
researchchallenges. Our goal is to help researchers and practitioners tobetter
focus their efforts to facilitate micro-payment adoptions.
