Medical Design Briefs - April 2021 - 32

Surface Waves Help Nanostructured Devices Keep Cool
The surface waves provide a
thermal transport solution.
University of Tokyo,
Tokyo, Japan
The continuing progress in miniaturization of silicon microelectronic and
photonic devices is causing cooling of the
device structures to become increasingly
challenging. Conventional heat transport
in bulk materials is dominated by
acoustic phonons, which are quasiparticles that represent the material's lattice
vibrations, similar to the way that photons
represent light waves. Unfortunately, this
type of cooling is reaching its limits in
these tiny structures.

However, surface effects become dominant as the materials in nanostructured
devices become thinner, which means that
surface waves may provide the thermal
transport solution required. Surface
phonon-polaritons (SPhPs) - hybrid
waves composed of surface electromagnetic waves and optical phonons that propagate along the surfaces of dielectric membranes - have shown particular promise,
and a team led by researchers from the
Institute of Industrial Science at the
University of Tokyo has now demonstrated
and verified the thermal conductivity
enhancements provided by these waves.
" We generated SPhPs on silicon nitride
membranes with various thicknesses and
measured the thermal conductivities of

A research team led by the Institute of Industrial Science at the University of Tokyo finds that hybrid
surface waves called surface phonon-polaritons can conduct heat away from nanoscale material
structures. (Credit: University of Tokyo)

these membranes over wide temperature
ranges, " says lead author of the study
Yunhui Wu. " This allowed us to establish
the specific contributions of the SPhPs to
the improved thermal conductivity
observed in the thinner membranes. "
The team observed that the thermal
conductivity of membranes with thicknesses of 50 nm or less actually doubled when
the temperature increased from 300 to 800
K (approximately 27° to 527 °C). In contrast, the conductivity of a 200-nm-thick
membrane decreased over the same temperature range because the acoustic
phonons still dominated at that thickness.
" Measurements showed that the dielectric function of silicon nitride did not
change greatly over the experimental temperature range, which meant that the
observed thermal enhancements could be
attributed to the action of the SPhPs, "
explains the Institute of Industrial
Science's Masahiro Nomura, senior author
of the study. " The SPhP propagation
length along the membrane interface
increases when the membrane thickness
decreases, which allows SPhPs to conduct
much more thermal energy than acoustic
phonons when using these very thin membranes. "
The new cooling channel provided by
the SPhPs can thus compensate for the
reduced phonon thermal conductivity that
occurs in nanostructured materials. SPhPs
are thus expected to find applications in
thermal management of silicon-based
microelectronic and photonic devices.
For more information, visit https://
www.iis.u-tokyo.ac.jp. Contact Masahiro
Nomura, nomura@iis.u-tokyo.ac.jp.

Wearable Device Monitors Jaundice-Causing Bilirubin
and Vitals in Newborns
Device detects pulse rate
and blood oxygen
saturation in real time.
Yokohama National University,
Yokohama, Japan
Researchers in Japan have developed
the first wearable devices to precisely
monitor jaundice, a yellowing of the skin
caused by elevated bilirubin levels in the
blood that can cause severe medical con-

ditions in newborns. Jaundice can be
treated easily by irradiating the infant with
blue light that breaks bilirubin down to be
excreted through urine. The treatment
itself, however, can disrupt bonding time,
cause dehydration, and increase the risks
of allergic diseases. Neonatal jaundice is
one of the leading causes of death and
brain damage in infants in low- and
middle-income countries.
To address the tricky balance of
administering the precise amount of

32

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Cov

ToC

blue light needed to counteract the
exact levels of bilirubin, researchers
have developed the first wearable sensor for newborns that is capable of continuously measuring bilirubin. In addition to bilirubin detection, the device
can simultaneously detect pulse rate
and blood oxygen saturation in real
time.
Led by Hiroki Ota, associate professor
of mechanical engineering in Yokohama
National University's Graduate School
Medical Design Briefs, April 2021


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Medical Design Briefs - April 2021

Table of Contents for the Digital Edition of Medical Design Briefs - April 2021

Medical Design Briefs - April 2021 - Intro
Medical Design Briefs - April 2021 - Cov4
Medical Design Briefs - April 2021 - Cov1a
Medical Design Briefs - April 2021 - Cov1b
Medical Design Briefs - April 2021 - Cov1
Medical Design Briefs - April 2021 - Cov2
Medical Design Briefs - April 2021 - 1
Medical Design Briefs - April 2021 - 2
Medical Design Briefs - April 2021 - 3
Medical Design Briefs - April 2021 - 4
Medical Design Briefs - April 2021 - 5
Medical Design Briefs - April 2021 - 6
Medical Design Briefs - April 2021 - 7
Medical Design Briefs - April 2021 - 8
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Medical Design Briefs - April 2021 - Cov3
Medical Design Briefs - April 2021 - Cov4
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