PET vs TPU Electrode Substrate: How to Choose for Flexible and Stretchable Printed Electrodes

JASPER certifications: ISO 9001, ISO 13485, IATF 16949, and ISO 14001.

1. Quick verdict: PET for stable geometry, TPU for elastic motion

PET usually wins the production decision when registration, conductor geometry, die-cut location, and low post-cure movement carry more risk than tensile deformation. TPU wins when a wearable circuit has to follow extension across skin, fabric, a joint, or another soft surface. A hybrid stack can assign those jobs separately.

A wearable biosensor patch construction should therefore start with a strain map, not a polymer preference. If the conductive path sees bending but almost no axial extension, PET may remain the cleaner choice. If the path must elongate, TPU becomes plausible only with a stretch-compatible conductor and geometry.

2. A flexible printed electrode substrate is not automatically stretchable

Flexible describes a structure that can curve. Stretchable describes one that can increase in-plane length and recover while retaining its required electrical and mechanical functions. PET can be very flexible at low gauge, yet its role is normally a stable film rather than an elastomer. TPU is elastomeric at room temperature, but the printed conductor, dielectric, encapsulant, adhesive, and connector do not inherit that elasticity automatically.

This distinction changes the engineering question. A circuit wrapped once around a 25 mm housing radius experiences a different strain field from a skin patch stretched 15% across a knee. Thickness and neutral-axis location govern the first case. Ink binder, trace shape, substrate modulus, strain direction, recovery, and cycle profile govern the second.

Cahn and co-authors demonstrated the interface problem with two DuPont silver inks on polymer films. At 35% applied strain, the conventional flexible 5025 ink reached roughly 15 times its initial normalized resistance, while polyurethane-binder PE874 reached about five times under the reported construction. The study tied the difference to strain localization and ink mechanics, not to a simple “TPU is better” label.

The substrate choice is therefore shorthand for a system decision:

motion → substrate strain → ink/trace strain → resistance change → signal or power consequence

A medical sensor patch substrate also sits inside an assembly. Skin adhesive, hydrogel where applicable, dielectric coverage, snap or tail transition, encapsulation, liner, and housing can shift strain into a narrow area. Film selection without that stack is incomplete.

3. PET vs TPU electrode substrate: side-by-side grade data

The table below uses named commercial grades to make the comparison auditable. It is not a generic property table. Mylar Specialty Films' Melinex ST506 represents heat-stabilized, print-treated PET; Covestro's Platilon U 4201 AU represents a polyether TPU film. Their supplier tests differ, so values should not be used as direct qualification limits.

Melinex ST506's reported 0.10% machine-direction shrinkage is 1.0 mm over a 1,000 mm reference length under the supplier's 150°C-for-30-min test. That simple conversion shows why even a “stable” film still needs artwork compensation and measured fiducials when circuits, coverlays, adhesive apertures, and die cuts must align.

Platilon U 4201 AU's 500% break strain should not become a circuit design target. Covestro reports it as a typical film result under DIN EN ISO 527-3 for a 50 µm reference film, with product variation noted by the supplier. A conductive path may leave its acceptable resistance band at a small fraction of the substrate's break strain.

Ink data reinforces the same point. Henkel's PET-oriented LOCTITE EDAG 725A (6S61) specifies 15 minutes at 120°C and 0.008–0.017 Ω/sq at 25 µm. Stretchable LOCTITE ECI 1014 specifies 0.010 Ω/sq/25 µm after 15 minutes at 120°C, or 0.013 Ω/sq/25 µm after five minutes at 100°C. Those values belong to different formulations and test conditions. They define starting process windows, not a universal winner.^4

4. Where PET wins

Registration through print, cure, and converting

Heat-stabilized PET is usually easier to treat as a geometric reference. That matters when a flexible printed electrode substrate carries close electrode spacing, multiple printed layers, dielectric openings, connector contacts, or a die-cut outline referenced to circuit features. ST506's grade-specific shrinkage data gives an engineer a measurable starting point; it does not remove the need to inspect machine-direction and transverse-direction movement on the actual line.

The practical advantage appears across the process chain:

film conditioning → first print → dry/cure → second-layer registration → lamination → die cutting → final resistance map

Each heat and tension step can move a web. PET normally makes that movement easier to control than an unsupported elastomer. This is one reason a stable carrier is often retained during operations on a stretchable electrode film.

Fine, repeatable conductor geometry

A smoother, more stable web helps hold screen-to-substrate registration and printed line geometry. For electrode arrays, geometric repeatability can matter as much as bulk sheet resistance because active area, spacing, and connection width influence the intended measurement. The appropriate acceptance characteristics are the customer's functional dimensions and electrical map—not an unsupported universal line-width capability.

PET is not the best choice when the installed trace crosses a zone of real tensile extension. Thin PET can wrap around a curve and survive controlled flexing, but bending strain rises as thickness increases or bend radius decreases. Creases, compound curvature, and unsupported transitions concentrate strain. A stable print process cannot compensate for a mechanically wrong installed state.

Established polyester ink windows

LOCTITE EDAG 725A provides a concrete polyester example: Henkel lists a 15-minute, 120°C drying cycle, 0.008–0.017 Ω/sq at 25 µm, and a 5B adhesion result on polyester after drying. That combination is useful for process planning because film and ink suppliers both publish temperature-related data.

It is not a released manufacturing recipe. Oven airflow, loading, actual part temperature, solvent removal, deposited thickness, and web restraint can change the result. Adhesion must also survive dielectric printing, lamination, cutting, storage, and the product's motion—not just the initial supplier test.

5. Where TPU wins as a stretchable electrode film

Tensile compliance and body conformity

TPU becomes the stronger candidate when the electrode must extend with its mounting surface rather than merely curve around it. Covestro offers Platilon U 4201 AU across 25–1,000 µm and reports 500% strain at break for a 50 µm reference film. This confirms a large elastic-material range, but the usable strain of a printed circuit remains an assembly property.

A 2016 Scientific Reports study illustrates the distinction. Suikkola and co-authors screen-printed silver-polymer conductors on 50 µm TPU and measured a mean initial sheet resistance of 36.2 mΩ/sq. Half of 30 samples remained conductive to about 74% monotonic strain, and none failed before 50% under that experiment. Those are feasibility results for one construction. They do not establish a cyclic design limit, wear period, or JASPER capability.

Lamination into soft or textile assemblies

Covestro describes Platilon TPU films as printable substrates and as layers that can be heat-laminated into smart-textile and wearable constructions. That can reduce the mechanical discontinuity between a soft garment or patch and its circuit. TPU may also serve as substrate, encapsulant, bonding film, or one layer of a coextruded structure, depending on grade.

TPU is not automatically the best choice for a body-worn patch. A soft film can creep under static load, relax after stretching, distort during cure, or transfer force to a rigid connector. The skin adhesive and liner may dominate handling. Moisture, sweat simulant, cleaning chemistry, edge geometry, and occlusion can change assembly behavior. Those questions require the actual full stack.

Ink–substrate modulus matching

Stretchable ink normally uses a polymer binder designed to deform with an elastomeric substrate. Henkel identifies LOCTITE ECI 1014 as a stretchable silver-filled ink with a thermoplastic polyurethane binder and names TPU among its key substrates. That binder match is a useful starting signal, not proof of adhesion to every TPU surface.

The Cahn study gives the failure mechanism practical weight: a conventional flexible acrylic ink localized strain more severely than PE874's polyurethane-binder system under the reported tensile loading. A meander trace can lower effective axial strain, and a dielectric can redistribute it, but geometry cannot rescue a brittle ink from every motion profile.

6. Printability, cure window, and ink adhesion

A print trial should answer four linked questions: Does the ink wet and resolve on the conditioned film? Does the cure achieve resistance without moving the substrate beyond registration limits? Does the interface survive converting? Does the electrical path remain inside its acceptance band after the specified motion?

Henkel's two reference inks show why cure cannot be reduced to one temperature. EDAG 725A lists 15 minutes at 120°C for its polyester process. ECI 1014 reaches a lower listed resistance at 120°C for 15 minutes than at 100°C for five minutes, but the shorter, cooler condition may reduce thermal exposure to an elastic web. The team must decide whether the resistance difference matters and whether the hotter cycle changes dimensions, adhesion, or recovery.

Surface preparation also needs an expiry and handling rule. Hassan and co-authors treated PET, PI, and TPU with oxygen plasma at 50 W for one minute before screen printing. Their TPU was 300 µm while PET and PI were 25 µm, so the work supports a process principle—not an equal-gauge material ranking. Plasma settings from a paper should not be copied into production without checking wetting, surface damage, treatment decay, and adhesion on the actual grade.

For test-method planning, IEC 62899-202-5:2018 addresses repeated bending and electrical-property evaluation of a printed conductive layer on an insulating substrate. IEC 62899-202-7:2021 defines a 90° peel method for a printed layer on a flexible substrate and constrains comparisons to equivalent substrate and thickness conditions. A project may adapt or supplement these methods, but it should state specimen geometry, conditioning, rate, radius or strain, cycles, measurement timing, and failure criterion.

7. Three stack architectures for wearable electrodes

The substrate is only one layer. These simplified diagrams expose where PET, TPU, or both can carry the mechanical job.

PET-centered stack: stable circuit, bending only

patient / environment side
──────────────────────────
interface layer defined by the device design
printed electrode + dielectric on treated PET
pressure-sensitive adhesive / spacer as specified
release liner or enclosure bond
──────────────────────────
connector tail kept outside tensile-motion zone

This stack suits a circuit that bends to fit a surface but does not stretch with it. The designer should keep the trace away from a crease, abrupt adhesive edge, or unsupported connector transition.

TPU-centered stack: compliant active area

patient / textile side
──────────────────────────
validated interface / adhesive layer
stretch-compatible conductor and dielectric
printable TPU substrate
optional TPU encapsulation or heat-bond layer
carrier retained through selected converting steps
──────────────────────────
strain-relieved transition to rigid electronics

This version treats the ink, dielectric, and connector transition as stretch-critical. TPU's film elongation does not excuse a rigid silver path across the peak-strain zone.

Hybrid stack: stable island plus compliant field

stable connector island                    compliant sensing field
PET support + contact pads ── overlap/transition ── TPU + stretchable traces
          │                                              │
   strain relief / stiffener                    skin or textile motion

A hybrid medical sensor patch substrate can place PET beneath the connector or high-registration region and TPU across the moving field. The overlap is not a neutral zone: adhesive shear, thickness steps, conductor termination, and encapsulant edge can concentrate strain there. It needs its own coupons and inspection criteria.

8. Failure chains and a prototype test matrix

A useful validation plan traces each requirement to a mechanism, a measurable symptom, and a decision. “Passed flex test” is too vague for a material release.

The following matrix is a project template, not a claim that JASPER performs every method. The OEM and manufacturing partner should assign ownership before samples are built. A testing and sample-approval plan should retain raw data, specimen orientation, lot identity, and failure location.

For medical use, substrate and ink selection remains component engineering. Biological evaluation considers the whole device in final finished form, including relevant processing, manufacturing aids, residues, and sterilization. ISO 14971:2019 frames life-cycle medical-device risk management, while ISO 10993-1:2025 places biological safety evaluation within that risk process.^12 None of these sources makes a raw PET film, TPU film, printed electrode, or supplier automatically “approved” or biocompatible for a finished device.

9. Decision matrix: which construction should the project choose?

PET is not “better” for every printed electrode, and TPU is not “more advanced.” Choose PET when deformation can be designed as bending. Choose TPU when extension is unavoidable and the conductive stack is engineered to share it. Choose a hybrid when one product contains both mechanical regimes.

Neither construction is the best choice if the required cure, chemical exposure, dynamic fatigue, or high-temperature operation falls outside validated PET/TPU behavior. Polyimide, PEN, silicone, SEBS, textile, or a rigid-flex architecture may deserve screening. That is a new comparison, not permission to substitute a material family without testing.

10. Inputs required before substrate and sample approval

To compare quotations and prototype results, send the same input set to every manufacturing candidate. JASPER can use this package to discuss a printed component, but the article does not claim a universal process limit or finished-device approval.

Drawing and process checklist

• Electrode geometry, active areas, trace widths, spacing, tail, connector pads, and cut outline
• Datum scheme tying print, dielectric opening, adhesive, connector, and die cut together
• Film candidate by supplier, grade, thickness, treatment side, color/clarity, and roll direction
• Conductive and dielectric ink candidates, dried thickness targets, and approved cure ceiling
• Installed bend radius, bend axis, tensile strain map, peak strain, strain rate, dwell, recovery time, and cycle profile
• Lamination adhesive, encapsulant, interface material, release liner, backing, and rigid components
• Connector or lead attachment method, stiffener, transition geometry, and assembly sequence
• Environmental profile: temperature, humidity, sweat/fluid simulant, cleaning agent, storage, packaging, and sterilization if applicable
• Initial resistance and permitted change during and after each exposure
• Inspection method, sample size, failure definition, traceability, and change-control requirements

Sample-approval closeout

1. Confirm material and ink identities against purchase and lot records.
2. Measure print-to-cut registration after the final thermal and converting step.
3. Map resistance before, during, and after the specified mechanical exposure.
4. Record adhesion failure locus rather than only “pass/fail.”
5. Inspect transition zones, cut edges, dielectric terminations, and connector islands.
6. Repeat on production-intent tooling and stack materials.
7. Keep component approval separate from finished-device verification and regulatory submission.

Projects that need several sensing sites or routing variants can also prepare a drawing for custom printed electrode arrays. Where intended use is medical, the OEM should connect component testing to its wider medical-device application risk file. For early process learning, define what a production-intent prototype must prove before ordering samples.

Next step: share the bend, stretch, and process requirements—not just “PET or TPU.” Include the strain map, cure ceiling, full layer stack, connector transition, and electrical acceptance limits.

11. Frequently asked questions

Is PET or TPU better for printed electrodes?

PET is usually better for stable registration and circuits that mainly bend. TPU is usually better when the printed electrode must extend and conform to skin or textiles. The result still depends on film grade, thickness, ink binder, trace geometry, cure, adhesive stack, interconnect, and the measured strain profile.

What is the main PET vs TPU electrode substrate difference?

PET behaves as a flexible but dimensionally stable film; TPU behaves as a flexible elastomer that can stretch. In practice, PET simplifies printing and converting, while TPU can follow tensile motion but makes tension control, recovery, registration, and ink-mechanics matching more important.

Can conventional silver ink stretch on TPU?

Not reliably by assumption. Cahn et al. Showed that a flexible acrylic silver ink localized strain and changed resistance more severely than a polyurethane-binder stretchable ink under the reported 35% strain test. The exact TPU, ink, printed thickness, geometry, cure, encapsulant, and cycle profile require validation.

Does a thin PET flexible printed electrode substrate work on skin?

It can work where the patch conforms mainly by bending and where the complete adhesive stack controls local strain. PET is less suitable when the circuit must elongate with skin motion. Skin contact also introduces adhesive, moisture, edge, comfort, and finished-device biological-evaluation requirements beyond the film itself.

Does TPU break strain define the allowable stretch of an electrode?

No. Covestro reports 500% tensile strain at break for 50 µm Platilon U 4201 AU reference film, but a printed conductor may exceed its resistance limit far earlier. Use the application's strain map and an electrical acceptance criterion during cyclic testing; do not derive circuit allowable strain from bare-film rupture.

Can PET and TPU be used in the same medical sensor patch substrate?

Yes. PET can stabilize a connector or fine-registration island while TPU carries a compliant sensing field. The overlap then becomes a critical design zone. Adhesive shear, thickness steps, conductor termination, encapsulation edge, and carrier removal must be tested under production-intent assembly and motion.

How should ink adhesion be compared on PET and TPU?

Use the same printed-layer geometry, thickness, conditioning, and defined interface method where the comparison allows it. IEC 62899-202-7:2021 provides a 90° peel framework for printed layers on flexible substrates. Record the failure locus and repeat the measurement after cure, lamination, mechanical cycling, and relevant environmental exposure.

What must be validated for a wearable medical electrode?

Validate the complete construction: dimensions, resistance, adhesion, bending/stretch response, converting, connector transition, environment, packaging, and intended-use function. The finished-device manufacturer must also address ISO 14971 risk management and applicable biological/regulatory evaluation. A supplier film datasheet or component sample cannot approve the finished device.

Technical References

• Source: FDA Use of ISO 10993-1 Biological Evaluation Guidance. Accessed 2026.
• Source: ISO 10993-1:2025 Biological Evaluation of Medical Devices. Accessed 2026.
• Source: ISO 14971:2019 Medical Device Risk Management. Accessed 2026.
• Source: FDA Design Control Guidance for Medical Device Manufacturers. Accessed 2026.
• Source: DuPont Melinex ST505 Polyester Film Technical Data. Accessed 2026.
• Source: Covestro Platilon Thermoplastic Polyurethane Film Guidance. Accessed 2026.
• Source: DuPont PE873 Stretchable Silver Conductor Technical Data. Accessed 2026.
• Source: Henkel LOCTITE Printed Electronics Ink Selection Guidance. Accessed 2026.
• Source: ASTM D3359 Coating Adhesion Test Methods. Accessed 2026.
• Source: ISO 527-3 Tensile Properties of Plastic Films and Sheets. Accessed 2026.
• Source: IPC-9204 Flexible and Stretchable Printed Electronics Guidelines. Accessed 2026.
• Source: IPC-6013 Qualification and Performance for Flexible Printed Boards. Accessed 2026.
• Source: ASTM D882 Tensile Properties of Thin Plastic Sheeting. Accessed 2026.
• Source: ASTM D903 Peel or Stripping Strength of Adhesive Bonds. Accessed 2026.
• Source: ISO 6721 Plastics Dynamic Mechanical Properties. Accessed 2026.
• Source: DuPont Mylar Polyester Film Technical Information. Accessed 2026.
• Source: Covestro TPU Film Processing and Lamination Guidance. Accessed 2026.