## 1 Introduction  
Since the term prosumer was first coined to refer to users who assume the role of a producer and consumer [1], several decades have passed until the technological and social context has propitiated the development of this concept. Factors such as the emergence of digital manufacturing, the growing trend of maker culture or the progressive network of platforms and services have allowed the end user to be included in the creative process through the co-creation, customization and manufacture of their own products [2]. Currently, prosumers use numerous digital tools, both individually and collectively, to be increasingly involved in the design or manufacturing process of a product [3, 4]. These actions often respond to needs and wants related to the user, such as self-realization, but also to external causes, such as the development of new technologies [5]. By providing users with effective and intuitive ways of production, the advances in ICT have enabled consumers to create goods in the digital field, thus initiating their transformation to prosumers [6]. From self-consumption to mass customization, we can differentiate several profiles depending on whether consumption (consumer) or production (prosumer) predominates [7]. This work focuses its attention on those users who seek the balance between consumption and production: they create their own physical products by buying part of them and designing/manufacturing the rest. Thanks to this balance, this user profile does not need advanced design and manufacturing knowledge as the product is not developed in its entirety (self-consumption). On the other hand, the level of production is adequate to facilitate the intervention in the creative process instead of making only a selection of characteristics (mass customization). Nevertheless, the use of physical tools by prosumers has been practically limited to 3D printing technologies, which have the potential to do the same as digital tools but in the world of physical objects [6]. In this regard, modular design is design based on independent and connectable modules that allow the configuration and variation of a product, making the participation of the end user in the product possible. In this way, it is an ideal physical tool that could facilitate the inclusion of the user in the creative process [8; 9]. It can be applied as a design approach tailored to build products with multiple components that enable rapid replacement and customization. In terms of sustainability, modular design can have a positive impact in areas such as environmental management or by prolonging the product life cycle by offering post-market modules [10]. If we focus on the end user, the inclusion of modular design and modules in eco-design principles has proved to be well-received from the consumer's perspective [11; 12]. Recent studies show that the key to resolving the conflict between customization and low cost is to implement modular design in physical products and services with the objective of meeting user needs [13; 14]. Combined with tools from the prosumer field, such as Open Access, modular design can allow the users to produce only the modules they need [15]. However, its benefits are not restricted only to the pre-consumption phases. Modular design also allows changes to the product through an adaptable interface that enables the inclusion of future updates or accessories [16]. In this way, the prosumer could take advantage of all these characteristics to intervene in the product through the design, manufacture or assembly of his own modules. Despite the obvious benefits that modular design can provide in the prosumer field, the possibility of focusing its application on this type of user has not yet been studied. Instead, previous research and methods developed for modular design have been geared toward developing and manufacturing the product from the company's point of view [17; 18; 19].