This document discusses challenges in integrating biometric sensors into wearable devices from engineering perspectives. It addresses questions product managers, mechanical engineers, and software engineers may have around sensor placement, form factor considerations, electrical design, software integration, testing, and validation. The document provides recommendations on sensor size and positioning, attachment methods, interface choices, power supplies, metrics, algorithms, and production testing protocols to optimize sensor performance for different use cases.

1. An engineering perspective
on biometric sensor
integration in wearables
© 2017 Valencell, Inc
Dr. Michael Aumer
November 2017

2. © 2017 Valencell, Inc
Software Test
Product & Project
Managers
ElectricalMechanical
You may have seen our previous webinars on The
Systems Approach, Signs Your Biometric Wearable
Project Needs Help and more.
But today, we’re taking a deeper dive into your product development cycle.
What questions will your team ask?
What challenges will you face?
What’s the best way to address those challenges?

3. © 2017 Valencell, Inc
PM Questions
• How will product development process be impacted
by adding biometrics?
• How many design cycles will it take?
• How do I know when I’m done?
• Why shouldn’t I make my own solution?

4. © 2017 Valencell, Inc
Impact to Product Development Cycle
Kickoff
Design
Concept
Mockup
Prototype
Functional
Alpha
Prototype
Functional
Beta
Prototype
Final
Internal
Testing
RFQ
RFQ ODM Kickoff
Technology
Transfer
Engineering
Sample
Runs
Test
Production
Runs
Mass
Production
Ship
➢ Validate form/fit/function of sensor in mockup stage, before ID freeze
➢ Electrical design reviews and power supply validation before engineering samples
➢ Manufacturing test plan development and implementation

5. © 2017 Valencell, Inc
Mechanical Questions
• Where can a sensor be positioned for a biometric
wearable?
• What is driving the size of the sensor front end?
• How should the sensor be attached to the
wearable?
• Is the sensor water proof?

6. © 2017 Valencell, Inc
Top 5 Challenges in OHRM
Source: “Optical heart-rate measurement’s top 5 challenges” Dr. Steven LeBoeuf; EDN Magazine; 8-25-15
Optical Noise Skin Tone Blood Perfusion
Sensor Location Crossover Problem

7. © 2017 Valencell, Inc
PPG Signal and Body Position
*Tur, Journ. Invest. Derm. 1983
➢ Extremities (fingers, ears, head) generally
have higher perfusion (better signal)
➢ Limbs tend to have a lower signal
➢ Sensor design and SW must be optimized for
solutions at different parts of the body

8. © 2017 Valencell, Inc
Mechanical: Sensor Size
Epidermis
Dermis
Subcutaneous
Emitter Detector
Close (2 mm) source-detector
spacing
Wide (4 mm) source-detector
spacing
Emitter
Detector
S/N  0.05 S/N  0.25
Wider spacing captures less total light, but it
captures a much higher ratio of “good light”
(blood-flow scatter) to “bad light” (motion-
related scatter).
Epidermis
Dermis
Subcutaneous

9. © 2017 Valencell, Inc
Ultrasonic Weld Considerations
• Resin & Surface. The opaque frame of the Benchmark is molded
from Polycarbonate. This PC frame will ultrasonically bond most
readily to a PC housing. Blends of PC and ABS also weld well, but
some testing and process development may be required.
• Fixture. Use aluminum fixture for part holding and consider a vented
horn and fixture. Minimize/eliminate contact to the front lens surface.
• Energy Director. A triangular energy director of 0.5mm wide and
0.4mm tall is provided for welding. A recess should be designed into
the outside of the case of the device into which the module is placed
for welding.
• Power-On-Self-Test. It is possible for accelerometers to become
damaged during out-of-control welding parameters, so it is a good
idea to manage the process through the use of the POST.
• Glue. As an alternative to ultrasonic welding, glue may also be used
as the attachment method for the module, but do not use
cyanoacrylate.
Ultrasonic
Weld
Energy
Director
Material
Callout

10. © 2017 Valencell, Inc
Form Factor-Specific Mechanical Questions
Hearables
• Which ear is best to place sensor?
• Does it have to be in contact with skin?
Wearables
• Does module orientation matter?
• How tight must the band be?

11. © 2017 Valencell, Inc
Electrical Questions
• Which interface method should be chosen?
• Where should the MCU be placed?
• How to ensure that electrical noise doesn’t degrade
biosensing performance?

12. © 2017 Valencell, Inc
MCU Placement and Choosing the Interface Method
Host System
WAKE
POST
UART_RX
UART_TX
0 R
PerformTek
Sensor System
0 R
TP TP
VDD
Host System
WAKE
POST
I2C_SDA
I2C_SCL
0 R
PerformTek
Sensor System
0 R
TP TP
VDD
2K2
2K2
VDD
UART option
I2C optionConsiderations
▪ Availability of interface at host
▪ Power consumption
▪ Preference for a clocked interface
▪ Need for other peripherals on same bus

13. © 2017 Valencell, Inc
Power Supply Recommendations
• Account for current demand dynamics (peak vs average)
• Isolate the supplies from the rest of the circuit as much as possible
• When using switching supplies, isolate the ground plane for the supply
• VDD supplies used with success include
• TPS79933 (3.3V Linear)
• TPS63031 (Buck / Boost)
• MAX14676 (1.8V)
• VLED supplies used with success include
• TPS79933 (3.3V Linear)
• TPS63031 (3.3V Buck / Boost)
• TPS60150 (5V Charge pump)
• MAX14676 (5V)
• Direct connection to the battery

14. © 2017 Valencell, Inc
Software Questions
• What information must be supplied to the sensor
technology?
• What is directly measured and what is a derived
metric/assessment?
• How to implement assessments?
• How to optimize performance for different use
cases?

15. © 2017 Valencell, Inc
Software: Metrics and Assessments
• Fitness levels
• Training load
• Stress levels
• Overhearing /
hydration
• Sleep metrics
• Core
temperature
• Diet planning
Firmware manages the process from the timing of LED firing and signal extraction to the biometric data
output that feeds the user experience.
• Are the algorithms optimized for different activities?
• Is the device firmware upgradable for future enhancements?

16. © 2017 Valencell, Inc
Software: Optimizing for the Use Case
The PerformTek Interface Protocol Document and User Guide cover detailed information
required to integrate the module into system level software. Areas defined include:
• Design Approach
• Protocol Commands
• Example Data Flows
• Algorithm Configuration
Feature activation and algorithm configuration are the typical methods by which power consumption is
optimized for different use cases. Examples include:
• Workout session: Continuous HR, activity, and energy expenditure algorithms are running
• Daily living: HRM is configured to be in a different activity mode
• Stress assessment: RRI feature is activated to enable HRV statistical analysis

17. © 2017 Valencell, Inc
Testing Questions
• How accurate is my device?
• How will devices be tested in production?

18. © 2017 Valencell, Inc
Testing: Protocols and Independent Validation
•Testing protocols that match the use cases: resting, lifestyle activities, mild exercise, aggressive
exercise, interval training, etc.
•Minimum 30 participants of multiple physical habitus, gender, & skin tone per prototype variant
– hundreds of data sets per product launch
•Biometric analysis that includes regression analysis (R2) & Bland-Altman analysis
•Analysis that includes true positives, false positives, true negatives, false negatives, & total
positives & negatives
•Ideally, there is independent validation of each metric

19. © 2017 Valencell, Inc
Testing: Production Test Fixture
• Goal is to verify that the device-under-test (DUT) is manufactured within specification
• Individual tests are performed to cover potential failure modes
• Electrical failures
• Optical emitter issues
• Optical detector issues
• Mechanical issues in the optical path
• Benchmark modules provided by Valencell are 100% tested with a custom test system

20. ©2016 Valencell. Inc

21. ©2016 Valencell. Inc
Dr. Michael Aumer
VP, R&D
aumer@valencell.com
www.valencell.com