This study aimed to formatively test the usability of the WEarable Artificial KIDney (WEAKID) for peritoneal dialysis (PD) in the device's intended user populations (i.e., nurses and patients).

End stage renal disease is a growing health problem worldwide, with a growing number of patients requiring a kidney transplantation. Recent years have seen the development of multiple novel devices for both hemodialysis and peritoneal dialysis (PD), one of these devices being the WEarable Artificial KIDney (WEAKID) for PD. WEAKID uses both sorbent technology and continuous flow PD to improve dialysis efficacy. Here, we present the results of a formative usability testing study of WEAKID among nurses and patients. We included 14 participants (n=6 nurses, n=8 patients) in three participating centres (Modena University Hospital, Italy; La Paz University Hospital Madrid, Spain; University Medical Center Utrecht, the Netherlands). Participants were asked to perform basic operational procedures with the device according to a usability task list. Their performance was observed and use errors were noted. Furthermore, participants were asked for feedback, both verbally and by means of completing the system usability scale (SUS), reflecting perceived usability on a scale from 0 (worst) to 100 (best). In total, 22 use errors occurred, 10 in nurses and 12 in patients. The mean SUS score was 70 (range 57.5 - 85) for nurses, and 71.9 (range 55 - 87.5) for patients. Despite the slightly above-average perceived usability of the device's current design stage, participants made a lot of suggestions for improvements, and indeed, usability must be improved to make the device ready for market introduction.

Inclusion Criteria:

• Nurses: aged 18 years or older, >1 years of professional experience with PD.
• Patients: aged 18 years or older, clinical diagnosis of ESRD, any treatment experience with PD, either current or in the past.

Exclusion Criteria:

• Nurses: none.
• Patients: severe physical and/or cognitive impairment, as judged by the researchers.

Participants represented two populations of intended users of the device, namely nurses and patients. The sample sizes were set at 6 (nurses) and 8 (patients). Prior work on the sample sizes of formative usability testing has shown that these sample sizes suffice to identify the most important design issues.

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