Breathable RF‑E‑Textiles: Flexible Conductive Fabrics for Wearable Communications

By Mario D’Auria, John Greenwood, and Chris Hunt at Pireta, together with Martin Salter and Nick Ridler at the National Physical Laboratory (NPL), we present a pioneering method for depositing conductive tracks directly onto fabric. This breakthrough opens a path to a broad spectrum of wearable devices.

In the RF domain, significant effort has focused on high‑performance substrates that reduce losses and extend operational bandwidth. While many options are now available, they are predominantly rigid or only semi‑flexible, overlooking markets where extreme performance is unnecessary but mechanical compliance is essential.

As devices shrink and cost drops, wearable technology is becoming a central focus for industries ranging from healthcare to defense to fitness. Conventional manufacturing relies on rigid components, making miniaturisation challenging, especially for RF systems where geometry is tightly coupled to frequency.

Large, bulky wireless wearables limit user freedom and comfort. Our conductive‑fabric technology provides spatial and design freedom while preserving flexibility and breathability.

Pireta’s proprietary process, compatible with natural and synthetic fibers, consists of five immersion steps—cleaning, sensitising, seed‑layer printing, electroless metal deposition, and passivation—alongside a printable seed layer that gives geometric freedom.

The method is scalable and shares steps with roll‑to‑roll digital printing, enabling large‑scale production. Metal coats the fibers without compromising handle, drape, stretch, or breathability.

Transmission lines, essential for RF evaluation, were fabricated on cotton drill fabric using the Pireta process. Two coplanar strip sections (5 mm wide tracks, 2 mm spacing) were made in lengths of 50 mm and 80 mm. The tracks were first seeded with silver, then plated with copper, and finally passivated with a silver layer.

After fabrication, the actual track width measured 5.5 mm and the gap 1.7 mm. SMA female connectors were soldered to the ends, a process made possible by the uniform metal coating.

VNA Measurements

Measurements were performed at NPL with a Keysight PNA‑X vector network analyzer across 10 MHz–10 GHz. Precision 3.5‑mm connectors (rated to 33 GHz) were used; SMA connectors are common up to ~12 GHz. A short‑open‑load‑thru calibration preceded the tests.

Figures 2 and 3 show the S‑parameters for the 50‑mm and 80‑mm lines. Reflection coefficients (S₁₁ and S₂₂) are poor above 100 MHz, likely due to printing resolution limits. Implementing an impedance transformer could mitigate this mismatch. The near‑identical S₁₁ and S₂₂ values confirm good connector repeatability.

Transmission coefficients (S₁₂, S₂₁) remain acceptable up to 2 GHz, suggesting the lines are functional once mismatch is addressed. Table 1 summarises the S₂₁ loss at selected frequencies.

Using the standard attenuation formula, the corrected loss per unit length (α_d) was calculated (see Figure 4). For short sections, losses are remarkably low: ~0.20 dB cm⁻¹ from 10 MHz to 100 MHz, rising to ~0.32 dB cm⁻¹ near 1 GHz.

Enhanced Metallisation

To reduce ohmic losses, a copper electroplating step was added after passivation, creating an electroplated (EP) variant alongside the original electroless (EL) lines. Both sets had similar appearance, with a slight increase in stiffness.

The EP lines show a 0.2 dB cm⁻¹ improvement over EL across 10–100 MHz, with losses rising gradually beyond 100 MHz. At 1 GHz, the EP loss reaches 0.3 dB cm⁻¹. Remaining loss is attributed to geometric imperfections and fabric roughness; finer fabrics and refined designs should further improve performance.

Interaction with Human Tissue

When the lines contact skin directly (Figure 7a), performance degrades due to the lossy nature of tissue. Introducing an insulating spacer (Figure 7b) yields similar losses, whereas adding a second conductive layer beneath the line restores performance (Figure 7c). These results indicate that, with proper design, body effects can be largely mitigated.

Effect of Fabric Distortion

Figure 9 displays attenuation per unit length across all five conditions. The lines exhibit minimal variation, with only a modest increase in the wiggle configuration, likely due to inter‑segment coupling.

Results and Future Work

The data confirm that conductive‑fabric transmission lines can operate effectively up to at least 1 GHz, covering AM, FM, RFID, Wi‑Fi/Bluetooth, and satellite radio bands. By neutralising the influence of human tissue and showing resilience to distortion, the technology is well‑suited for wearable RF applications. Future work will optimise the planar geometry, investigate substrate dielectric constants, line thickness, and address non‑uniform current paths relative to conventional solid metal tracks.

Conclusions

Pireta’s e‑textile process, though still emerging, demonstrates the ability to meet RF requirements of many telecom applications, including sub‑6 GHz 5G, while preserving textile properties such as handle, drape, and breathability. This dual capability opens new avenues for product innovation across diverse sectors.

References

1. R. Garg, I. Bahl, M. Bozzi, Microstrip Lines and Slotlines. London: Artech House, 2013, pp. 376–377.
2. IEEE Std 287‑2007, “IEEE Standard for Precision Coaxial Connectors (DC to 110 GHz).”
3. IEC 60169‑15:1979, “Radio‑frequency connectors. Part 15: R.F. coaxial connectors with inner diameter of outer conductor 4.13 mm (0.163 in) with screw coupling – Characteristic impedance 50 Ω (Type SMA).”
4. S. Rehnmark, “On the Calibration Process of Automatic Network Analyzer Systems,” IEEE Trans. on Microwave Theory and Techniques, Apr 1974, pp. 457–458.
5. F. L. Warner, A. E. Bailey, “Attenuation measurement,” in Microwave Measurements, London, U.K.: IEE, pp. 132–134, 1989.