## 2 Related Work  
The term _user experience_ was introduced in 1995 [15]; the original idea behind its definition was to reflect the experience between a human and a system in terms of many aspects that go beyond the human interface or usability. According to the authors, the understanding of users' needs is the crucial issue that should guide UX; such needs should be met by designers providing users with positive feelings when using applications. Another relevant definition of UX was given by _the International Standard on Ergonomics of Human-System Interaction_ (ISO 9241-210) as _a person's perceptions and responses that result from the use or anticipated use of a product, system, or service_[16]. Anyway, a shared definition of UX is not available: as user experience is subjective by nature and related to an individual's perception (i.e., products or services) they interact with [17], characterizing it to understand if a given application will be acceptable is not simple. A recent work [18] has produced a very relevant review of the literature with the final purpose to propose a framework for classifying the different characteristics and approaches to UX developed over the last decades. They have classified UX typologies as follows:  
* _Ergonomic_: the UX is focused on the human-system interaction from the usability, performance, affordances, effectiveness, and user's behavior;
* _Cognitive_: the UX focus is on the system perception and understanding by humans, in particular from the functional and aesthetic points of view;* _Emotions_: the UX concerns how the users feel, in terms of pleasure, empathy, hedonistic values, social values, affection, and so on. With respect to the mHealth domain, it is known that the acceptance level is low [19]. _Care for the elderly_ is one of the most important fields. Wearable devices can be useful to monitor aged people living in critical conditions [20], increasing their autonomy level and supporting them to delay the negative effects of aging [21]. Wearing a device that monitors and tracks movements encourages the user to be more proactive in increasing their activity level [21]. Despite these great opportunities, wearables services are still behind elderly expectations [19]. The main reason for this is the objective difficulty in designing applications capable of meeting the elderly needs due to their low ICT literacy and adoption [22]. Many researchers, such as [23] have demonstrated how the inclusion of personalization aspects in the design and implementation of mobile health applications can have a positive impact on the involvement of older adults, with great benefits on the quality of life of caregivers too. Wearable devices have been demonstrating their usefulness in the management of chronic diseases too. People at higher levels of risk can be continuously monitored at home. Patients suffering from heart diseases, diabetes, and so on can collect data about their conditions and interact with doctors at a distance [24; 25]. They can wear sophisticated glucose monitors to check their status continuously [26; 27]. Psychological support is really important in the treatment of chronic patients [28]. Their motivation to follow therapies and good lifestyles must be continuously monitored and stimulated [29]. The proper support of caregivers is even more crucial: many studies, such as [30] point out the extremely heavy burden suffered by relatives of the patient, especially in the early stages of a chronic disease: the adoption of sensors and wearable devices can significantly decrease their burden, especially when chronic conditions are paired with aging and related complications [31]. As reported in [32], wearable devices are mandatory to implement personalization in healthcare and, consequently, to improve UX feelings; moreover, efficient and effective services must be designed to tailor the more and more sophisticated and powerful available technologies for the user needs.