## 2 Technologies in DTx
### HCI Factors  
Regardless of the kinds of technology implied in DTx, user interactivity must be sufficiently satisfied. In the case of DTx, unlike conventional medical services, it is possible to empower users to participate actively in treatment programs. However, at the same time, since self-management is required, motivation must be provided for appropriate compliance [42; 43]. This empowerment and motivation can be provided through sufficient interactivity of the services. For this, there are a few suggestions about user-centered human-computer interaction components from earlier DTx studies. Furthermore, the therapeutic alliance is also considered necessary, especially in mental-healthcare-related areas where DTx has been most widely applied in recent years. The therapeutic alliance (TA), which refers to the rapport and the relationship between physicians and patients in existing mental health treatment programs, remains a crucial factor in digital formation [44]. Therefore, to secure such a therapeutic alliance, the HCI factors must be considered [45]. To establish a TA, the difficulty of non-face-to-face conditions in DTx can be offset by several HCI theories suggested by [PERSON] et al. [46]. The persuasive system design (PSD) contains four factors; primary task support, dialogue support, system credibility support, and social support, which can advocate empathy and build the relationship between DTx and the users [45; 46]. For primary task support, minimizing the user's participation effort must be accompanied by plenty of guides and explanations to change the user's attitude. In addition, it must provide personalized and trackable content to allow users to self-monitor their activities. In dialogue support, patients need to be provided feedback through words, images, symbols, or sounds, and it should be designed to appeal to users. Moreover, rewards, reminders, and suggestions that help to accomplish the task are essential in dialogue support. On the other support, system credibility, the information provided in the service must be truthful and appear knowledgeable and experienced. Moreover, although it is digitized, it still needs to deliver real-world feelings behind the service. At last, for social  
Figure 3: Top-down federated learning structure for digital therapeutics. The server distributes a model to the individual clients to update the global model. support, providing shared services that can promote social learning, comparison, cooperation and recognition are recommended [47]. If these HCI elements based on the PSD theory are incorporated into digital therapeutics, it will form a therapeutic alliance, which will increase the effectiveness of digital therapeutics. In addition to forming a TA, long-term regular use of DTx requires user satisfaction [48]. For this sustainability, the perceived usefulness, perceived ease of use, and attitude are suggested in the existing technology acceptance model (TAM) [49]. The TAM model states that in the acceptance of new technologies, the perceived usefulness and perceived ease of use influence the attitudes in a positive or negative direction toward the service. These attitudes are mediated to ultimately determine the intention to continue using the service or product [50]. Depending on the characteristics of the service or product to be applied, the variable of TAM is often expanded. In the DTx context, perceived enjoyment is often considered as a compelling variable, and it has been proved that it does affect the intention to use [51]. In this regard, successful digital treatment requires users to perceive the product as easy to use and recognize that the program is beneficial to them. In addition, if a positive attitude toward digital therapeutics is secured by delivering the perception that digital therapeutics is enjoyable, users' intention to continue using the product can be promoted. To sum up, these perceptions will further induce appropriate compliance and regular use of the DTx from users.