Deep dive into the fundamental physics and chemistry that make smart textiles possible
At the most fundamental level, electrical conductivity in smart textiles follows the same physical principles as conventional electronics, but with unique challenges posed by the textile environment.
The relationship between current flow and material properties is described by:
Unlike rigid conductors, textile-based conductors show significant temperature sensitivity:
σ(T) = σ₀[1 + α(T - T₀)], where α > 0 for metals. Resistance increases with temperature due to increased phonon scattering.
For carbon-filled polymers, α < 0. Resistance decreases with temperature as more charge carriers are activated.
Body heat (37°C) can change circuit resistance by 2-5%, affecting sensor accuracy and LED brightness.
The piezoresistive effect describes how mechanical strain affects electrical resistance:
Where GF is the gauge factor (material sensitivity) and ε is strain (fractional length change).
Understanding the chemical processes that affect smart textile performance is crucial for designing durable systems.
Silver-plated conductive threads are susceptible to tarnishing:
This reaction produces a black tarnish layer that increases resistance. Prevention strategies include protective coatings and controlled storage conditions.
UV radiation breaks polymer chains through free radical formation:
Chain scission leads to mechanical property loss. UV stabilizers and antioxidants can slow this process.
Water molecules attack polymer backbones, particularly in polyesters:
This is especially problematic in humid environments or during washing cycles.
When different metals are present with moisture, electrochemical reactions occur:
Zn → Zn²⁺ + 2e⁻ (preferentially dissolves)
Cu²⁺ + 2e⁻ → Cu (protected from corrosion)
Avoid mixing dissimilar metals in humid conditions to prevent preferential dissolution.
Repeated thermal expansion and contraction creates mechanical stress:
-20°C to +60°C at 1°C/min maximum rate to avoid thermal shock
Resistance drift, mechanical stress from thermal expansion, phase transitions
Use flexible interfaces and account for differential expansion between materials
Moisture absorption changes both electrical and mechanical properties:
Hygroscopic materials absorb water, changing dielectric properties and creating unwanted conductive paths.
Fiber swelling can change fabric dimensions by 2-5%, affecting circuit geometry and component alignment.
10% to 95% RH at 25°C ± 2°C, measuring property changes every 10% increment.
Repeated loading leads to cumulative damage and eventual failure:
Where N = cycles to failure, Δσ = stress range, A & m = material constants
Explains conductivity in carbon-filled polymers:
Small changes in filler concentration near the threshold cause dramatic conductivity changes. This explains the sensitivity of pressure sensors.
Current flows between nanoparticles through quantum tunneling:
Distance changes exponentially affect current - the basis for highly sensitive pressure sensors.
Polymer behavior depends on loading history:
Where G(t) is the relaxation modulus. This integral describes how current stress depends on the entire deformation history, explaining hysteresis in stretch sensors.
The Butler-Volmer equation describes electrode reactions:
Where η is the overpotential. Important for understanding corrosion rates and battery integration in wearable devices.
Performance data for common smart textile materials:
Resistivity: 0.001-0.01 Ω·cm | Gauge Factor: 2-3 | Max Strain: 5-10% | Fatigue Life: 10³-10⁴ cycles
Resistivity: 0.01-0.1 Ω·cm | Gauge Factor: 2-4 | Max Strain: 2-5% | Fatigue Life: 10⁵-10⁶ cycles
Resistivity: 1-100 Ω·cm | Gauge Factor: 10-50 | Max Strain: 50-200% | Fatigue Life: 10⁴-10⁵ cycles
Resistivity: 0.0001-0.001 Ω·cm | Gauge Factor: 1-2 | Max Strain: 10-30% | Fatigue Life: 10⁶+ cycles
Following the systematic approach developed by Hannah Perner-Wilson and Mika Satomi:
Map all forces: tension, compression, shear, torsion. Consider how each affects electrical properties.
Account for body heat, ambient temperature, component heating, and thermal expansion effects.
Consider exposure to sweat (pH 4-6.8), detergents (pH 9-12), and environmental pollutants.
Source, batch numbers, and storage conditions for all materials used
Techniques, thread tensions, and environmental conditions during assembly
Document failure modes and their physical causes for future design improvement
Long-term monitoring of key metrics under realistic use conditions