Medical Design Briefs - November 2024 - 19
drug dose monitoring. Designers have options
for detecting these conditions.
One option for activation is a small
low profile, top-actuated, surface mount
pushbutton switch. Dimensions are 2.5
mm wide × 1.65 mm deep × 0.55 mm
high. Switches are rated IP67 against
ingress of moisture and dust. Pushbutton
switch life cycles can be as high as
300,000 cycles and contact resistance is a
low 500 mΩ. A second option is the type
of detect switch suggested for the dose
delivery detection circuit.
A third option is the use of a reed
switch. Designers can specify ultraminiature,
hermetically sealed reed switches
with variable sensitivities. Magnetic actuation
does not draw any power from the
circuit, and the current leakage through
the switch is at negligible pA levels.
A fourth option is a tunneling magnetoresistance
(TMR) switch. It consists
of a magnetic tunneling junction sensor
and CMOS circuits. This magnetically
triggered digital switch combines high
sensitivity and ultra-low power consumption.
Figure 5 shows a block diagram of a
TMR switch. A temperature compensated
regulator circuit ensures voltage stability
for the internal circuits. A Schmitt
trigger provides switching hysteresis for
noise rejection, and a CMOS push-pull
circuit provides the output drive.
Standard
ISO 11608-1
Title
Needle-based injection
systems for medical use.
Part 1: Needle-based
injection systems
ISO 11608-4
Needle-based injection
systems for medical use.
Part 4: Needle-based
injection systems
containing electronics
IEC 60601
Medical Electrical
Equipment Testing and
Certification
IEC 60529
Degrees of protection
provided by enclosures
(IP Code)
IEC 60068-2-XX
Environmental testing of
electrotechnical products
Table 1. Applicable standards for wearable medicine injectors.
Medical Design Briefs, November 2024
Vcc
Power regulator
with temperature
compensation
TMR
Offset
Sensor
cancellation
Latch
Vout
GND
Fig. 5 - Integrated switch containing a tunneling magnetoresistance sensor and drive circuitry. (Credit: Littelfuse)
The TMR switch is next to the bottom
of the injector's plunger chamber, and a
small magnet mounts on the injector's
plunger. As the motor pushes the plunger
toward the bottom of the injector and the
TMR switch, the switch produces an increasing
output voltage and switches logic
state. The switch indicates the plunger's
position. The microcontroller (MCU) uses
the position information to determine the
volume of the injected drug volume.
The TMR switch draws only 200 nA
when powered by 1.8-5 V circuits. Its
sensitivity can detect a magnetic field
General Scope
Specifies requirements and test methods for needle-based injection
systems (NISs) for single-patient use intended to deliver discrete
volumes (bolus) of medicinal product, which can be delivered through
needles or soft cannulas for intradermal, subcutaneous and/or
intramuscular delivery, incorporating prefilled or user-filled,
replaceable or nonreplaceable containers.
Specifies requirements and test methods for needle-based injection
systems (NISs) containing electronics with or without software
(NIS-Es).
Part 1-11: General requirements for basic safety and essential
performance - Collateral standard: Requirements for medical
electrical equipment and medical electrical systems used in the home
healthcare environment.
Developed to rate and grade the resistance of enclosures of electrical
and electronic devices against the intrusion of dust and liquids. It also
rates how easy it is for individuals to access the potentially hazardous
parts within the enclosure.
A collection of methods for environmental testing of electronic
equipment and products to assess their ability to perform under
environmental conditions including extreme cold and dry heat.
www.medicaldesignbriefs.com
as low as 5 Gauss. The CMOS output
enables direct connection to an MCU
which avoids the need for additional
components. The high sensitivity and
low power minimize battery drain, making
a TMR switch a superior solution for
wearable injector drug dose monitoring.
Control of the injector requires a microprocessor
or a microcontroller. To
save space and power, single-chip MCUs
can provide all the necessary functionality
and draw low power. MCU sleep
modes can significantly lower power
draw during periods of inactivity.
Market
Global
Global
Global
Global
Global
19
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Medical Design Briefs - November 2024
Table of Contents for the Digital Edition of Medical Design Briefs - November 2024
Medical Design Briefs - November 2024 - Intro
Medical Design Briefs - November 2024 - Sponsor
Medical Design Briefs - November 2024 - COV1a
Medical Design Briefs - November 2024 - COV1b
Medical Design Briefs - November 2024 - COV1
Medical Design Briefs - November 2024 - COV2
Medical Design Briefs - November 2024 - 1
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Medical Design Briefs - November 2024 - COV3
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