Team Discussion
Our interpretation of the world around us is constructed from the information that we collect via these sensory routes. More sophisticated sensor technology is on the way. This technology will improve synthetic methods for perceiving the external world, heralding the next generation of cochlear implants, visual prosthetics which can restore sight, and limb prosthetics with built in haptic feedback.
The opportunity exists not only to develop devices to compensate for lost senses but to stretch our existing senses so that we can see in the infrared; use built-in magnetic-receptors to navigate; detect sound at a wider range of frequencies; and even detect and translate radio signals. In this article we take a closer look at some of the senses and explore how developments in sensory devices can be exploited to mimic, enhance, or even extend our in-built sensory mechanisms.
What is a ‘sense’?
Any system that consists of a sensory cell type that is able to respond to specific internal or external stimuli. The sensory mechanism (sight, sound, touch, etc.) corresponds to a particular region of the brain where the sensory signals are received and interpreted.
The human touch
Just when we thought that wearable healthcare extended only as far as smart watches and fitness devices, the world of miniaturised electronics has stepped up (or should that be stepped down?) to make wearable sensors a viable healthcare proposition. Technical developments such as ‘on-board’ health monitors and prosthetics which provide haptic feedback are in progress.
‘Smart’ skin which senses pressure, temperature and humidity and can support nerve stimulation is moving into the realms of reality; and a combined Japanese and German research team are working on an electronic skin which can detect magnetic fields.1 Whilst magneto-sensitivity is useful to homing pigeons and migratory birds, what possible use is introducing this sense to humans? Well, one of its inventors believes that magneto-receptive skin could act as a proxy for proprioceptive sense – the body’s ability to know where limbs are located in space – in prosthetic limbs. Others believe that magnetic sensing will be of most use in refining the functionality of surgical robots.
Most of us take for granted our ability to interact with and learn from our environment via the sense of touch. One area in which we are reminded of its importance is in surgery, in particular laparoscopic surgery, where it is not possible for the surgeon to use a hand to ‘feel’ tissue, let alone distinguish tissues of different textures.
Scientists at the University of Hiroshima, in Japan, have devised a simple solution to the lack of haptic feedback. Their technique allows a minimally invasive instrument to be adapted to provide ‘tactile’ feedback to the user by the addition of a small piezoelectric actuator attached to the instrument’s handle, which gently vibrates at a constant frequency. This vibration is not felt by the user of the instrument, until the instrument comes into contact with tissue, at which point the user receives a distinct feeling in their hand. The beauty of this solution is that it works with almost any existing instrument. Researchers have also observed that the vibration frequency does not need to be varied from user to user to achieve optimal tactile sensation.2
“a sensor has to react instantly, vibrating on immediate contact with a hot liquid or object”
Sensing danger
Congenital insensitivity to pain (CIP) – a medical condition in which pain signals are diminished or switched off altogether due to a mutation in gene SCN98 – is a dangerous condition for its sufferers. Children growing up with this condition are especially vulnerable to trauma, due to the lack of pain cues, often resulting in proliferation of wounds, infections, burns and broken bones that may go undetected.
One of many initiatives to develop solutions for alerting patients with CIP and other peripheral neuropathic conditions in which sensation to hands or feet is reduced, is BurnAlert.3 This wearable prototype device – consisting of temperature-sensing thermistors – vibrates when the user touches a hot object. The thermistor sensors are strapped onto the user’s fingers and attached to a vibration motor worn on the wrist.
However, the subtleties of the human sense of touch are not easy to replicate in the design of thermo-haptic warning systems such as BurnAlert. For example, a sensor has to react instantly, vibrating on immediate contact with a hot liquid or object; as happens in the real world, reaction is instant, there is no latency.
Despite the miniaturisation of components, devices are likely to be obtrusive – unlike the body’s natural built-in sensors – requiring strapping to the hand and/or wrist.
Sniffing out trouble
Electronic-nose (also known as an e-nose) devices, which mimic the human olfactory system, are not new, but they have undergone recent development in the field of point-of-care diagnostics (Figure 1).
Figure 1: The key principles of an e-nose
Human breath, for example, contains many volatile organic compounds (VOCs) in a gaseous state; metabolites and abnormal chemicals that are present directly in air expired from the lungs. Some of these VOCs are specific biomarkers, or indicators of diseases such as pneumonia, kidney conditions and even some cancers.
Although many designs of e-noses exist they all operate on the same principle: a set of sensors are exposed to the captured sample (e.g. exhaled breath); the VOCs cause a reaction on each sensor (for example, a change in electrical resistance); the sensitivity of this measurement is related to the chemicals present. A computing system resolves the output from each of the sensors and compares this to a database of ‘breath profiles’ from healthy patients to provide a diagnostic result.4
The advantages of deploying an e-nose to diagnose disease, versus more traditional analytical techniques, are clear; the method is non-invasive, portable, and has a rapid response time and lower operating costs. The problems of the e-nose approach include: not being able to identify individual compounds in complex mixtures, the sensor’s sensitivity to water vapour, a relatively short sensor lifetime and, correspondingly, lower sensitivity than analytical chemistry. Perhaps the greatest challenge to address is that the correlations between many proposed biomarker metabolites and disease are still tenuous – the body’s underlying biochemical mechanisms behind the disease-related VOCs are largely unknown.
“A computing system resolves the output from each of the sensors and compares this to a database of ‘breath profiles’ from healthy patients to provide a diagnostic result”
Mine eyes smell onions
The condition is unlikely to have been known about in the early 1600s, but Shakespeare could have been describing synaesthesia in his quotation, “Mine eyes smell onions”.
One of the most idiosyncratic conditions involving the senses, synaesthesia is a phenomenon in which stimulation of one sensory pathway leads to an effect in a separate sensory pathway. In short, the senses appear to be cross-wired. Synaesthesia manifests as a spectrum of different types, but experts are agreed that there are four main forms of the condition (Figure 2). The most common form is understood to be seeing sounds, music or voices, as colours. For others, different musical sounds may be felt as physical sensations on different parts of the body.
The rise of the (nano-)robots
What next for sensor technology? Imagine a future in which a nanometresized sensory robot in a human body can continually monitor physiological levels and detect health problems. Systemic sensors could be developed which dynamically sense cardiac enzymes which suggest a possible heart attack, monitor glucose levels for a diabetic patient, or track biomarkers which may indicate ovarian or breast cancer in women carrying the BRCA1 or BRCA2 gene mutation.
What if they could feed information back to an application, or even trigger appropriate therapeutic responses via a brain–computer interface? Developing sophisticated sensor technologies such as these sounds like the stuff of science fiction, particularly as we still have a great deal to learn about even our basic senses and how they work.
Figure 2: Predominant types of synaesthesia
Sources of information
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