People experiencing homelessness are socially excluded and face structural barriers to accessing healthcare, leading to high physical and psychosocial morbidity and mortality (1). In the UK, the definition of homelessness extends beyond the mere absence of shelter, and instead encompasses a range of interconnected aspects such as experience of extreme poverty, domestic violence, job loss, and inability to afford rent (2, 3). As a result, individuals who are experiencing homelessness face a myriad of interconnected challenges stemming from their diverse and complex health and social needs (4). These intricate physiological, socio-economic, and psychological issues require joint multi-sector efforts to fully comprehend and tackle (5). Gaining a better understanding of the context and social determinants of health that may be affecting individuals experiencing homelessness is crucial for practitioners, in order that practitioners feel equipped to embrace a more inclusive approaches that will engage this population, ensuring their continued involvement in health care interventions (6, 7). Previous research about community-based oral health interventions has confirmed that well-trained and motivated practitioners are a key component that leads to engagement of service users and subsequent behaviour change (6, 8).

Whilst it is crucial for practitioners to establish trust with marginalised populations, a lack of adequate information and/or specific training to aid with this has been reported (7). Therefore, improved training and educational resources could help practitioners to engage, build trust and therefore discuss a broader range of sensitive health topics (9). Alongside this, people with lived experience of homelessness have expressed that they could be listened to more and be better supported when accessing services (10).

Therefore it is vital to involve people with lived experience of homelessness and their support workers in the development of health educational and health promotional materials and interventions, to ensure the resources are meaningful and acceptable (11). It has been found that involving people with lived experience can lead to effective strategies to address health needs and improve policies to tackle health inequalities (12, 13). The World Health Organization (WHO) has recently launched a framework to support meaningful engagement with a view to enhancing policies and services (14). The framework includes principles such as power, equity, inclusivity, contextualisation, elimination of stigmatisation, and institutionalisation of engagement (14).

Co-design is a participatory approach that brings individuals together to collaborate and combine their knowledge, skills, and resources to accomplish a design task (15). Co-design transcends mere consultation, originating from participatory design (15), it involves the meaningful engagement of end-users who are recognised as experts by experience (16). This approach is particularly powerful for socially excluded groups, empowering individuals by acknowledging their views and experiences (11). Furthermore, co-design serves as a pivotal approach for tackling stigmatisation and promoting inclusivity, the creation of co-designed materials counteracts societal stigmatisation (17). Co-design techniques have been reported to result in increased applicability and acceptance of research questions, outputs, participants' engagement, increased knowledge of different contexts, and an improved community network for the researcher (18).

Hence, it is imperative to scrutinize existing literature regarding the involvement of individuals who are homeless and/or their support workers in the creation of health and/or oral health educational materials through a co-design methodology, to elicit evidence to support best practice. Prior to conducting this review, a search of the literature for existing reviews of any type found no evidence synthesis addressing our aim. In the absence of any review, a scoping review methodology was chosen to scope the literature and identify evidence gaps.

To accomplish the main aim, three specific objectives were outlined: (1)

To summarise the literature in the field of co-designed health and/or oral health promotion training/educational resources that involved people experiencing homelessness and/or their support workers.

This scoping review was undertaken following the methodology established by the Joanna Briggs Institute (JBI) (19). An initial search in April 2021 of Scopus, PROSPERO (International prospective register of systematic reviews) and Open Science Framework (OSF) found no existing scoping or systematic reviews on this topic. A protocol for this scoping review was registered within the OSF database a priori (number osf.io/7hbac). Due to lack of research team capacity in 2021 and impact of the COVID-19 pandemic the search for the included literature in our review was last done in August 2023. A scoping review is an essential first step to inform future studies related to co-design of health promotion materials for people experiencing homelessness.

The reporting of this review aligns with the PRISMA extension for Scoping Reviews—PRISMA-ScR, we used population, concept and context to develop the review question and the eligibility criteria (20). •

Population: People experiencing or at risk of experiencing homelessness and/or support workers that work with people experiencing homelessness.

A total of 1,105 papers were retrieved in the electronic literature search, and after the removal of duplicates, they were reduced to 435. Following title and abstract screening, twenty-eight papers were included for full-text screening. Twenty-two were excluded after full text screening, resulting in the inclusion of five papers (Figure 1). Two further resources were found via a grey literature search (n = 1) and the contact with experts/stakeholders (n = 1).

Our findings suggest that components of the co-design process such as inclusivity, safe environment, empowerment, and flexibility can increase participation of people experiencing homelessness in research and in the development of educational materials. We have identified enablers to facilitate this process, the included studies demonstrated that stigmatised and vulnerable groups such as people experiencing homelessness, despite being perceived as “hard to reach” groups, are willing to take part in research if they felt included and could have their voices heard in a safe environment. A review by Ní Shé et al. (32) found that engagement with seldom-heard groups needs to occur in safe, accessible, and inclusive spaces. Therefore, importance of providing an emotionally safe environment for positive participation based on principles of respect, non-judgmental listening, with meaningful opportunities for participants to feel that their views and lived experience have been acknowledged is required.

In our review, participants' feelings of being safe to express themselves within the research environment resulted in a feeling of empowerment, leaving them confident to share their views on issues that were important to them. There are other studies that reinforce the links between the provision of a safe environment and the empowerment of participants as enablers for participation when mutual trust, equity, and empathy are embedded in all phases of the research process. Schiffler et al. (33) identified clients were reportedly empowered to achieve their personal goals when co-designed mental health interventions were provided in their living environment, including home, work, and other places that they identified as safe and favourable. Flexibility was perceived as a key element to be applied across the different research's stages as an important strategy to involve people who might otherwise be excluded of participating. Life crisis and financial issues can be challenges for participation. The findings of our review suggests that incentives are an enabler in the codesign process, which concurs with the review finding by Ní Shé et al. (32) where necessary costing and flexibility in payment should be included when designing research with vulnerable groups. Flexibility related to researcher's attitude of being sensitive to participants' feedback and expressed needs during the process resulted in positive changes on research activities (time, duration, ways of delivering). Therefore, the context and needs of people experiencing homelessness are complex and diverse and research processes with less rigid structures can better allow the accommodation of necessary changes.

There were benefits in using co-design identified from the review. The included study by Rodriguez et al. reported impact from the co-design process with reported improvement in individual's critical consciousness, health literacy and behaviour change (9). It also helped strengthen their social interaction with service providers and their peers towards a more critical involvement with their communities. Social justice to achieve health equity should be core practices for health promotion interventions. Participants felt empowered when conditions for active involvement are in place and when they receive equitable treatment. These elements are essential to undoing oppressive forces existing in power structures (5, 34–36). Tindall et al. identified that co-design was helpful in balancing the power differential and providing support when participants usually feel reduction in their power especially in mental health settings where there are inherent power imbalances (37).

Health promotion interventions using participatory research methods such as co-design are successful because they consider the context and the specific needs of target audiences (38). Three of the included studies highlighted how important it is to have an in depth understanding of the context and needs of participants in order to tailor the research activities to enable participation (9, 25, 27). This led to empowerment of participants that felt more equipped to take informed decisions and change towards a healthier life. Health promotion is a process that enables people to increase control over and improve their health (39). Knowledge exchange programmes with public engagement activities have recommended the involvement of young people experiencing homelessness in the co-design of training resources to be used by practitioners (7). Adding to this, the participation of socially excluded groups, such as families, children and young people experiencing poverty and homelessness, using co-design approaches have benefited from the construction of new oral health and health knowledge (9, 40). Therefore, an alternative approach is necessary to empower people, enabling their active participation and to take charge of their own lives and environments (41).

The perceived barriers to codesign in research of increased time and financial expenditure are corroborated by Slattery et al. (18) e.g., there is not enough time allocated or enough focus on development of the skills needed to build trust and long-term partnerships within the community.

The evidence in this area is limited. This review provides foundations for further research to examine the impact of different components of co-design including the environment in which the co-design process is conducted. The identified enablers to co-design health and/or oral health educational/training materials suggest that an active and positive engagement with participants promotes meaningful experience of participation, resulting in participants' empowerment and increased knowledge. An in-depth knowledge of the diverse contexts and views of people experiencing homelessness through the investment of time and creation of good channels of communication, trust and positive interaction enables their voices to be heard, validated, and used to develop resources that can help practitioners with the non-stigmatisation of these groups in healthcare settings and society. Training or educational programmes/materials that include the views of people with lived experience of the health issues to be addressed have an increased chance of success in to improving service users' lives and wellbeing. Future endeavours should foster increased collaboration with individuals with lived experience of homelessness to co-design health and oral health promotion training/educational materials. Further studies with experimental design and reported using appropriate study design frameworks detailing active components of the co-design process would strengthen the evidence base in this area.