A more sensitive, less bulky, knitted circuit-embedded knee wearable for wireless sensing of joint movement in real-time has been developed which could highlight early mobility issues – potentially preventing or delaying functional decline.

The stretchable knee wearable has been designed by a team from the Singapore University of Technology and Design, and if commercialised could have a massive impact on how age-related joint changes are dealt with both by medics and older patients.

Our bodies become stiffer and more inflexible as we age because we make less synovial lubricating fluid, which acts like oil to keep our joints moving smoothly. Our cartilage also becomes thinner and ligaments shorten and lose elasticity.

Mobility limitation – which is an early sign of functional decline – can manifest as muscle weakness, loss of balance, an unsteady gait, and joint pain. This in turn can lead to reduced physical activity, possible weight gain, decreased balance and coordination, and social and psychological effects.

It has long been known that long-term and continuous monitoring of joint motion may prevent or delay this decline by allowing the early diagnosis, prognosis, and management of mobility-related conditions.

This is usually made possible through either wearable or non-wearable engineered devices.

Non-wearable systems are reliable, but need a laboratory environment and trained individuals to use, monitor and interpret them, so are impractical for daily use.  On the other hand, wearable systems are portable, cheaper, and much easier to use.

But typical wearable sensors tend to be inflexible and bulky.

A relatively new player are wearables made from soft, lightweight, malleable and non-invasive conductive fabric. They are comfortable to wear and can be used for long-term monitoring.

However, most conductive fabric-based wearables are prone to flagging up errors if they are removed from their intended location, and because they are attached externally to users’ clothing can also be cumbersome and restrictive.

The SUTD’s model boasts fewer external components, has more sensitive sensors, and because it’s made from a single section of highly stretchable fabric, allows the wearer more freedom.

The knee wearable has been developed by associate professor Low Hong Yee and her colleagues at the SUTD in collaboration with Dr Tan Ngiap Chuan of SingHealth Polyclinics. Their research has been published in the journal Advanced Healthcare Materials.

Professor Low said the team chose to concentrate on a wearable for the knee joint because of its importance for lower limb mobility.

But the team – which via the SUTD has filed patents related to the knee brace – is already looking to build on its work. The hope is to study the effect of sweat and humidity on sensor signals and to extend the research to include subjects from healthy and unhealthy populations in the future.

Professor Low said: “We have started working on extending the wearable to special user groups and to monitor other body joints, such as the shoulder. We’re also looking at securing an incubation fund to explore the commercialisation potential of the wearable.”

To develop the single-fabric circuit on the knee wearable, the team mechanically coupled an electrically conductive yarn with a dielectric thread of high elasticity in various stitch patterns.

Dimensions were customised according to the subject’s leg. The functional components – sensors, interconnects, and resistors – formed a stretchable circuit on the fully knitted wearable that allowed real-time data to be obtained.

However, putting together sensors, interconnects, and resistors in a single stretchable knit is difficult, as professor Low explained. “The synergy of yarns with different electrical and mechanical properties to achieve high signal sensitivity and high stretchability” was challenging, she said, as the desired properties for each component were vastly different.

Sensors need to produce a large change in resistance for enhanced sensitivity, while interconnects and resistors need fixed resistances of the highest and lowest values, respectively.

When worn, the knee brace converts changes in the knee joint to electrical signals, enabling the wireless and continuous real-time sensing of joint motion. Image: SUTD

As such, the researchers optimised yarn composition and stitch type for each component before connecting the functional circuit to a circuit board contained in a pocket of the wearable, allowing for wireless transmission of real-time data.

With a soft knee wearable developed, its components functional, and data transmission possible, it was time to test the performance of the wearable.

The team assessed the wearable through extension-flexion, walking, jogging, and staircase activities.

Subjects wore the knee wearable together with reflective markers that were detected by a motion capture system, allowing the comparison between sensor data and actual joint movement.

The sensor response time was less than 90 milliseconds for a step input, which is fast enough to monitor the human movements included in the study.

Additionally, the smallest change in joint angle that the sensors could detect was 0.12 degrees.

The research team say the potential impact of such a device in the medical field is huge.

Often, people ignore the early signs of mobility decline as they are not deemed serious enough to seek help. But wearable technology solves this problem by assessing a user’s mobility directly in real-time.

The team believe embedding a user-friendly sensor circuit into a soft and comfortable fabric may increase the public’s adoption of wearable technology – especially among the elderly.

Data can be gathered in real-time and translated into indicators that can detect mobility decline. When signs of mobility decline are found, preventive care, prognosis, and management of the healthcare condition can be given.