From Flexible Sensors to Smart Beds — A Journey in Patient Weight Monitoring
How to measure the weight of a bed-ridden patient?
Technical Keywords
Biomedical Instrumentation · Pressure Sensing · Pneumatics · Hydraulics · Control Systems · Data Acquisition · Patient Monitoring · ICU Devices · Translational Engineering
Background
This project began during my master’s studies at IIT Madras, where I worked as a Research Intern at the Healthcare Technology Innovation Centre (HTIC) in collaboration with Stryker Global Technology Centre.
The clinical problem was clear: how can we monitor the weight and fluid balance of bedridden or critically ill patients continuously and safely, without lifting or moving them?
In intensive care, subtle changes in patient weight (on the order of 1–2 kg) can reveal fluid retention, dehydration, or renal complications. Traditional systems rely on load-cell–based bed scales, which are accurate but mechanically complex and costly.
Our goal was to engineer a modular, low-cost, bed-integrated system capable of periodic or continuous weight estimation using compact sensing modules.
Phase 1: Flexible Polymer Sensors — Early Prototypes
The journey began with flexible piezo-resistive and piezo-capacitive polymer materials. We experimented with Velostat, Eeonyx conductive textiles, and CaplinQ films, aiming to create a conformable pressure mat.
Working principle
When force was applied, the polymer’s microstructure compressed, reducing its electrical resistance:
\( R = R_0 (1 - kP)^n \)
where \(R_0\) is baseline resistance, \(P\) is applied pressure, and \(k, n\) are material constants.
Electronics and DAQ
A Wheatstone bridge with an instrumentation amplifier (INA122) captured voltage variations, digitized using an NI USB-6002 DAQ (1 kS/s).
Challenges identified
- Hysteresis and drift due to viscoelastic relaxation.
- Temperature sensitivity and poor repeatability.
- Edge effects under distributed loads.
Key insight: the sensing medium must remain stable under static or quasi-static loads.
Phase 2: Air-Filled Mattress System — Proof of Concept
To overcome drift and non-linearity, we turned to air as the sensing medium.
Principle
At constant temperature, the relationship follows Boyle’s Law:
\( P_0 V_0 = P_1 V_1 \)
The applied weight causes a pressure increase \( \Delta P \) that correlates with patient weight.
System design
- Air-filled pillow and full mattress
- Pressure transducer: NXP MPXV5100DP
- Acquisition: NI USB-6002 DAQ, LabVIEW at 1 kS/s
- Calibration: \( \Delta P = aW + b \)
Performance highlights
- Linear fit: R² = 0.98 (pillow), R² = 0.99 (mattress)
- Coefficient of variation: 5–9%
- Sensitivity increased at lower inflation pressure
Limitations
- Air leakage and temperature dependence
- Baseline drift
- Slower transient response
Published as “Continuous Weight Monitoring System for ICU Beds Using Air-Filled Mattresses/Pads: A Proof of Concept” (IEEE MeMeA 2019).
Phase 3: Liquid-Filled Active Feedback System — The Breakthrough
The next generation introduced pressurized liquid-filled elastic channels. Liquids offered high stability, negligible drift, and linear pressure–force behavior.
Architecture
A hydraulic channel was coupled to a syringe-pump–based feedback mechanism. Piston displacement was measured using a linear potentiometer.
Control design
- Closed-loop PID controller in LabVIEW
- Real-time pressure regulation
- Transient recovery < 200 ms
Calibration & results
- R² = 0.993 between displacement and weight
- Operational range: 0–90 kg
- Mean error < 3%
- Repeatability within ±1.5% over 12 h
The system enabled detection of subtle vibrations related to cardiac and respiratory cycles, opening pathways to balistocardiographic (BCG) monitoring.
Published as “Periodic Weight Measurement for Bedridden Patients Using a Pressurized Liquid-Filled Channel System Integrated with Hospital Beds” (IEEE MeMeA 2024).
Design Evolution and Insights
| Phase | Sensing Medium | Key Advantage | Limitation | Outcome |
|---|---|---|---|---|
| I | Piezo-polymer | Flexible, low-cost | Drift, poor repeatability | Proof-of-feasibility |
| II | Air | Modular, non-invasive | Leakage, temperature dependence | Clinical proof-of-concept |
| III | Liquid | Stable, self-correcting | Higher complexity | Clinically robust prototype |
Translational Impact
The final liquid-based system transforms the hospital bed into a smart sensing platform capable of:
- Continuous weight tracking
- Real-time cardiovascular vibration sensing (BCG)
- Integration with AI-based health analytics
The design is scalable, modular, and low-cost, enabling retrofit into existing beds.
Collaboration
This project was carried out at the Healthcare Technology Innovation Centre (HTIC), IIT Madras, in collaboration with Stryker Global Technology Centre.
Related Publications
- Periodic Weight Measurement for Bedridden Patients Using a Pressurized Liquid-Filled Channel System Integrated with Hospital Beds , IEEE MeMeA 2024.
- Continuous Weight Monitoring System for ICU Beds Using Air-Filled Mattresses/Pads: A Proof of Concept , IEEE MeMeA 2019.
Gallery
