## 1 Introduction  
In orthopaedic rehabilitation following lower limb surgeries, such as fracture repair, total hip replacement, or knee arthroplasty, partial weight-bearing is widely recognised as a cornerstone of effective postoperative care. The clinical rationale for prescribing partial weight-bearing is twofold. On the one hand, applying a controlled mechanical load to the healing bone stimulates osteoblastic activity and bone remodelling, essential for the recovery and strengthening of the skeletal structure [1,2]. Conversely, limiting the load prevents excessive stress that could lead to complications such as fracture malunion, implant failure, or undesired alterations in gait biomechanics [3,4]. Despite its importance, achieving the recommended weight-bearing remains challenging. Traditional methods of teaching and monitoring partial weight-bearing, such as using home scales, verbal instructions, or tactile cues (e.g., placing a hand under the foot), have been found lacking in several respects. These static modalities fail to capture the dynamic nature of weight distribution during ambulation and daily activities, leading to poor correlation between estimated and actual load application [3]. Research has repeatedly reported that patients, regardless of repeated instruction, struggle to consistently reproduce the given weight-bearing limits over time, with notable deviations noted during dynamic activities such as walking [5,6]. The resulting discrepancies can shift the centre of gravity, adversely increasing muscular effort, particularly in the abductor muscles, and introduce compensatory movement patterns that further compromise rehabilitation outcomes [7]. In recent years, biofeedback systems have injected a new dimension into postoperative rehabilitation protocols. These systems, which include digital portable insoles and wearable sensor arrays, provide continuous, real-time feedback on the load exerted on the lower limbs. Unlike traditional static assessments, these devices enable patients to adjust their gait dynamically, ensuring that the load remains within prescribed limits during functional activities [5,7]. Despite the demonstrated efficacy of such digital solutions, their widespread adoption is hampered by two critical limitations: high costs and evidence of benefit restricted to short-term outcomes. Consequently, an unmet need remains for accessible, economically viable tools to deliver sustained improvements in load perception and weight-bearing accuracy.  
Several biofeedback solutions have been explored to help patients respect partial-weight-bearing (PWB) limits. Laboratory-based force-plate or visual/auditory treadmill systems can improve load awareness but are costly and bound to specialised gait labs [8]. Instrumented crutches with load-triggered sound give real-time cues during overground walking, yet they require batteries and deliver feedback only when crutches contact the ground [9]. Wireless smart-insole platforms (e.g., SmartStep(tm), OpenGo-Science) stream plantar-pressure data to a smartphone, but prices > EUR 600 and the need for Bluetooth pairing limit routine use [10]. More recently, capacitive sensor insoles with on-board electronics have been proposed, but they still depend on continuous power and visual monitoring [11]. Vibrotactile belts or smart glasses can offload visual demand, yet the wearables remain bulky, and their efficacy data are confined to balance-impaired populations rather than orthopaedic PWB [12].  
Our incentiviser costs < EUR 5 in off-the-shelf parts, is fitted in <5 min, functions without electricity, and delivers an unmistakable somatosensory cue regardless of lighting, ambient noise, or the patient's field of view. These attributes make it attractive for low-resource settings and unsupervised home rehabilitation, where electronic devices may be impractical.