Medical Design Briefs - April 2022 - 46

last longer than lasers, are less expensive,
have fewer eye safety considerations,
and are compact, making cell
phone-sized qPCR devices possible.
n Delivery Optics Design
The purpose of illumination optics is
to collect the light from the source and
transfer it to the sample plane. The
transfer of energy from source to sample
is governed by an optical principle
known as the Lagrange invariant, given
by the following equation:
h0n0sinθ0 = h1n1sinθ1
The law states that the collected cone
of light from the source is conserved
throughout the optical pathway. In this
equation, h is the size of the emitter/
spot, and nsin is the numerical aperture.
Figure 2 shows a graphical representation
of the Lagrange invariant.
All real light sources have finite extent
and divergence. A challenge with LEDs,
compared to lasers, is the relatively high
light output divergence and larger emission
area, which results in low collection
efficiency. However, the difficulties with
divergence are readily solvable with the
right optical design and selection of illumination
optics. The illumination optics
design needs to capture the full extent
of the LED area and cone angle to
ensure the maximum amount of the
source radiation to the sample plane.
This capture generally is accomplished
by using aspheric condenser lenses or
TIR lenses.
Figures 1 and 3 show two common
illumination architectures used in qPCR
instruments. In Figure 1, the illumination
and excitation paths are separated
with a dichroic beam splitter. This architecture
is generally used when it is necessary
to have a tightly focused spot at
the sample and in instruments with one
or two detection channels to reduce the
complexity of the dichroic filter.
However, for applications that require
multiple target detection channels (i.e.,
up to 4 to 6 channels), the architecture
shown in Figure 3 is preferred since this
type of system layout is not dependent
on dichroic filters. The primary drawback
of the second system configuration
is illumination uniformity across the
sample. Additionally, in some cases, the
second configuration requires further
optimization to minimize stray light,
especially if the instrument developer
chooses to forgo collimation optics in
front of the LED to save on the cost of
goods. Designing the optical path to
minimize the effects of stray light from
contaminating the detection signal and
hence the accuracy of the test should
also be considered.
Finally, the thermal effect impact
from the LED thermal cycler module is
an important design consideration. The
temperature change can cause the emission
wavelength of the LED to shift,
leading to nonoptimal excitation of the
fluorophore and consecutive nonoptimal
emission of red-shift fluorescence.
Overall,
" wandering " LED emission
wavelengths due to thermal effects lead
to reduced fluorescence signal and
impact the test assay's reliability and
accuracy.
n Conclusion
Developing the right product for the
use case and achieving the needed performance
requires a comprehensive
understanding of the complete system
requirements and how to balance
design parameters to achieve the
desired result. For fluorescence qPCR
diagnostics, this starts with illumination
source selection and optimizing the
optical pathway to ensure that the test
results from the fully commercialized
product can be trusted time and again.
Taking a step further to select an LED
light source can save money and time
due to the ease of access with specific
wavelengths and powers, as long as the
team has the skills necessary to complete
an efficient and optimized illumination
optics design.
This article was written by Rekha Doshi,
Principal Optical Engineer, and Madilyn
Beckman, Business Development Manager
at Gray Optics, Portland, ME. For more
information, visit http://info.hotims.com/
82320-380.
Implant Offers Promise for the Paralyzed
A system enables patients
with a complete spinal
cord injury to stand, walk,
and more.
EPFL
Lausanne, Switzerland
A newly developed
system now
enables patients with a complete spinal
cord injury to stand, walk, and even perform
recreational activities like swimming,
cycling, and canoeing.
In late 2018, David Mzee, who had
been left paralyzed by a partial spinal
cord injury suffered in a sports accident,
got up from his wheelchair and began to
walk with the help of a walker. This was
the first proof that Grégoire Courtine
and Jocelyne Bloch's system - which
46
Cov
Patient with complete spinal cord injury walking at Lausanne University Hospital after five months of
rehab. (Credit: NeuroRestore/Jimmy Ravier)
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Medical Design Briefs, April 2022

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Medical Design Briefs - April 2022

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