48 | New Scientist | 19 March 2022
After a long search, in 1997, they finally
identified a protein called TRPV1. Receptors
for this protein are found in cell membranes,
mostly in nociceptive neurons in our skin.
These are nerve cells responsible for feeling a
certain kind of pain. TRPV1 alerts the brain to
both physical and chemical stimuli, such as
burning capsaicin and higher temperatures –
anything above 43°C. In response to these
triggers, TRPV1 opens an ion channel that
sends electrical signals to the brain.
Julius and others went on to spend decades
examining the intricate web of receptors,
other neurons, proteins and ion channels that
together give us the complex and varying
sense of touch we experience. Ardem
Patapoutian at the Scripps Research Institute
in San Diego, California, was one of the key
researchers, helping to discover the way we
feel cold sensations and pressure on the skin
(see “Under pressure”, opposite). This work
was vital in getting us closer to a complete
understanding of the sense of touch.
When Lu read about Julius and Patapoutian’s
work, it made her reconsider the way her lab
was using stimuli. “Before, I had just imagined
doing it externally – creating heat, providing
force feedback, generating vibrations,” she says.“But now, I was thinking of how to more directly
interact with the specific [cellular] channels that
regulate the perception of these sensations.”
In 2021, Julius and Patapoutian were
awarded the Nobel prize in medicine for their
work. On the same day, Lu and her team
published their paper introducing chemical
haptics. “It was actually quite a coincidence,”
she says. “Their work on detailing the receptors
that correspond to these sensations of hot,
cold and pain is foundational to our approach.”
In their paper, Lu and her team used
chemicals ranging from sanshool, a component
of spicy Sichuan pepper, to create a tingling
feeling, to capsaicin, to mimic warmth. Menthol
was used for cold, while a local anaesthetic,“ How we sense
the world is
critical for
pretty much
everything
in life”
HUMAN^ COMPUTE
R^ INTE
GRAT
IONLAB/UCH
ICAGOTouch a burning hot iron and
you will immediately know
to pull your hand away. This
sense, called discriminative
touch, is communicated to the
brain using nerves known as
A fibres, which provide almost
instantaneous information.
But there’s another group of
nerve fibres, called C fibres,
that act more slowly, taking
around a second to carry
a signal from your foot,
say, to your brain. These
communicate different
types of pain, such as
throbs and aches, rather
than stings or burns.
In the late 1990s, Åke
Vallbo at the University
of Gothenburg, Sweden,
discovered a specific type of
C fibre called C-tactile or CT
fibre. “It’s a lovely nerve,” says
Francis McGlone at Liverpool
John Moores University in the
UK. “It responds to exactly the
velocity of stroking you would
say is nice.” The CT fibres only
responded to slow, gentle
touch – 5 centimetres per
second – and they are only
found on hairy skin. The type
of touch that triggers the
CT nerve is called affective
touch, because it is used
to create social bonds.
Not getting enough of this
kind of touch has been linked
to depression and anxiety.
McGlone and his team
published a study in which
they stroked rats every day
for 10 minutes. One group
was stroked at the speed the
CT fibres respond to, while
the animals in another group
were stroked six times faster.
Then, they were put through
situations that would provoke
mild stress. The rats that had
been slowly stroked didn’t
respond to the stress. Whether
this works in people is yet to
be seen, but McGlone says
initial results are promising.
EMOTIONAL
TOUCH
Devices worn on the
skin can create more
realistic virtual worlds