# Sensors, Techniques, and Future Trends of Human-Engagement-Enabled Applications: A Review
## 1 Introduction  
As our reliance on technology has continued to intensify, societies and individuals have become dependent on digital technologies for their health, socialisation, work, and well-being. Re-assessing these reliant relationships by having a closer look at human engagement in this highly fragmented digital culture is important for building a digital future. Human engagement is a crucial construct in many domains: psychology, sociology, education, cognition, behaviour, and sentiment analysis. Reviews [1, 2, 3] have touched upon the conception and quantification of engagement. A universal definition of engagement remains elusive. Nonetheless, all would concur that engagement should be abstracted, scoped, and measured according to its context of use. The best recognized notion of engagement comes from [PERSON] et al. [4], \"the process by which interactors start, maintain, and end their perceived connections to each other during an interaction.\" Engagement is an interaction process between two or more parties. The process usually involves at least one human user and other human agents and/or apparatuses, software programs, autonomous agents, interfaces, etc. Albeit a widely used term, engagement is often not clearly defined in the literature among different applications. This article specifically focuses on human agents in a process of interacting with digital interfaces, apparatuses, software, and autonomous agents in pursuit of a productive goal. For instance, in manual takeover after automated driving [5], a driver may disengage to their current task of text messaging on a smartphone whilst commencing with driving. How to design robots to make them more attractive is also beyond the scope. We emphasize that engagement is a process with many quantifiable sub-processes, such as attention and motor response, as shown in Figure 1. We hypothesize engagement to be both idiothetic and allothetic. That is, the interaction or engagement itself is the driving motivation [6] and the engagement process should aim to achieve a certain goal through extracting information from the environment to fulfil it. Engaging in social media on a phone is positive from a phone app design perspective whilst negative from a workplace productivity perspective. We associate engagement with task and productivity as equivalent to _productive engagement_, proposed by [PERSON] [7]. We examine engagement as sub-processes persisting in humans' perception, appraisal, cognitive, motivation, and motor systems where the feedback cycle time is short. Importantly, this permits measurability and utility of engagement. Therefore, we do not take positive appraisal and cognitive load attention [8] without action or with actions that are not directed towards the interaction partner as engagement. This review focuses on engagement-enabled applications in which successful modelling and measurement will unlock countless opportunities to enhance intelligent systems, automation, safety, human-centric designs, and well-being in general. A number of applications, such as online learning, driver assistance systems, and human-robot interaction (HRI), are beginning to integrate automatic engagement estimation. They will be further empowered by more accurate, reliable, and privacy-friendly human engagement estimators. Unfortunately, we note a gap in the literature to thoroughly evaluate, contrast, and summarize sensors and techniques for human engagement estimation from an applicability point of view. To this end, this work presents a detailed review of sensing technologies for engagement estimation, common approaches, current practices, and future trends of engagement that inform future applications. The aim is to provide landmarks and guidelines on deploying human engagement estimators for researchers and practitioners in the field. This review is timely in this digital era when sensors, algorithms, and ubiquitous computing have reached a high level of maturity to deliver impact to change and enhance our daily lives.