What is catheter pushability?

Pushability is a catheter's ability to transmit a longitudinal force (push) from its proximal end to its distal end without buckling. Mathematically, it is the longitudinal stiffness k_long = E·A/L, where E is Young's modulus, A is the annular cross-section, and L is the length.

Why does the flexural modulus vary so much across Pebax grades?

The Pebax range covers a very wide stiffness span — from 40 MPa (Pebax 3533, very soft) to 513 MPa (Pebax 7233, the stiffest). This variation enables building a variable-stiffness shaft: stiff proximal segment (pushability), progressive transition, soft distal segment (atraumaticity). This is the fundamental principle of modern catheter design.

Why use a logarithmic scale for flexural stiffness?

Flexural stiffness k_flex varies as OD⁴ (fourth power of the outer diameter). Over a realistic 0.5–6 mm OD range, this spans more than 4 decades of variation. A linear scale would visually crush all small diameters to zero. The logarithmic scale makes parameter sensitivity visible.

Why does a Pebax 72D catheter tested at 23 °C not have the same torque at 37 °C?

The elastic modulus E and shear modulus G of thermoplastic polymers drop significantly near their glass-transition temperature Tg. For Pebax 72D, whose polyamide-segment Tg is near 50 °C, going from 23 °C (bench) to 37 °C (human body) causes a roughly 30% modulus drop. As a result, for the same proximally applied torque, the torsion angle accumulated in the shaft is about 40% higher in-vivo than at the bench, degrading torque transmission fidelity. It is even more pronounced for Nylon 12 and Pellethane TPU. For PEEK, PTFE and 316L stainless, the effect is negligible. For nitinol, the martensite/austenite phase transition near Af can create a sharp discontinuity in behavior.

Which temperature should I use in the calculation: bench, body, or other?

To anticipate the actual clinical behavior of an intravascular catheter, use 37 °C (body temperature). To reproduce an ISO 25539 or ISO 10555 test performed at lab ambient, use 23 °C. For cryo applications (chilled saline, cryoablation), drop to 5–10 °C — the polymer becomes stiffer, shifting pushability and torqueability the other way. The difference between these temperature behaviors is often the source of surprises observed during the bench → clinical-trial transition.

What is catheter pushability?

Pushability is a catheter's ability to transmit a longitudinal force (push) from its proximal end to its distal end without buckling. Mathematically, it is the longitudinal stiffness k_long = E·A/L, where E is Young's modulus, A is the annular cross-section, and L is the length.

Why does the flexural modulus vary so much across Pebax grades?

The Pebax range covers a very wide stiffness span — from 40 MPa (Pebax 3533, very soft) to 513 MPa (Pebax 7233, the stiffest). This variation enables building a variable-stiffness shaft: stiff proximal segment (pushability), progressive transition, soft distal segment (atraumaticity). This is the fundamental principle of modern catheter design.

Why use a logarithmic scale for flexural stiffness?

Flexural stiffness k_flex varies as OD⁴ (fourth power of the outer diameter). Over a realistic 0.5–6 mm OD range, this spans more than 4 decades of variation. A linear scale would visually crush all small diameters to zero. The logarithmic scale makes parameter sensitivity visible.

Why does a Pebax 72D catheter tested at 23 °C not have the same torque at 37 °C?

The elastic modulus E and shear modulus G of thermoplastic polymers drop significantly near their glass-transition temperature Tg. For Pebax 72D, whose polyamide-segment Tg is near 50 °C, going from 23 °C (bench) to 37 °C (human body) causes a roughly 30% modulus drop. As a result, for the same proximally applied torque, the torsion angle accumulated in the shaft is about 40% higher in-vivo than at the bench, degrading torque transmission fidelity. It is even more pronounced for Nylon 12 and Pellethane TPU. For PEEK, PTFE and 316L stainless, the effect is negligible. For nitinol, the martensite/austenite phase transition near Af can create a sharp discontinuity in behavior.

Which temperature should I use in the calculation: bench, body, or other?

To anticipate the actual clinical behavior of an intravascular catheter, use 37 °C (body temperature). To reproduce an ISO 25539 or ISO 10555 test performed at lab ambient, use 23 °C. For cryo applications (chilled saline, cryoablation), drop to 5–10 °C — the polymer becomes stiffer, shifting pushability and torqueability the other way. The difference between these temperature behaviors is often the source of surprises observed during the bench → clinical-trial transition.

Catheter shaft stiffness — Pushability, torqueability, flexibility | Protobrix

Shaft mechanics

Catheter shaft mechanical behavior

Simultaneously computes the three physical responses of a shaft under loading: critical buckling load (push), distal whipping angle (twist), tip deflection (bend). Visualizes the trade-offs: stiffening for pushability degrades flexibility, and vice versa.

Push · Twist · Bend Simultaneous view + parametric trade-offs

Analytical calculation for a homogeneous circular tube, with temperature correction. The tool applies the canonical strength-of-materials formulas to a homogeneous polymer or metallic tube, without braid reinforcement. E and G are corrected for the use temperature entered: for polymers whose Tg is close to 37 °C (Pebax 72D/63D, Nylon 12, TPU), the in-vivo stiffness drop can reach 30% versus a 23 °C bench test. For braided composite shafts, the tool gives a lower bound on actual performance. Final validation always requires a physical test on a prototype.

Shaft configuration (geometry + boundary conditions + material)

Outer diameter OD External tube diameter (in mm)

Inner diameter ID Inner lumen diameter (in mm)

Length L Shaft length considered for the calculation (in mm)

Boundary conditions K Shaft restraint at the ends — affects F_crit (×4 between extremes)

Material Published elastic modulus E and shear modulus G

Use temperature T E and G drop near the polymer's Tg. Human body 37°C by default; bench test 23°C; chilled saline ~10°C (cryo). [PA12, Amstutz 2021]

°C

Applied forces (the three loadings in parallel)

Push force F_push Typical ranges: 0.1–0.8 N (tip-vessel contact [1]), 2–6.5 N (clinician force [2]), up to 9 N (advanced catheter [3])

Distal friction T_f Estimated friction torque at the tip (sliding catheter 0.5 mN·m, moderate bend 2–5 mN·m, constrained in tight bend 10+ mN·m). Determines when the tip breaks free.

mN·m

Lateral force F_flex Lateral force applied at the shaft tip: 0.1–0.5 N for navigation [1], up to 1 N in high-resistance zones.

Simultaneous mechanical response

Parametric trade-offs

Pick a parameter to vary. The three quantities update live. Reading the trade-off: if you stiffen the shaft (OD↑, stiffer material), F_crit goes up (push improves) but y goes down (the shaft becomes stiff in bending) and the whipping angle changes with inertia.

Vary:

F_crit (push) Whipping θ (twist) Deflection y (bend) — · — Current position

A / B configuration comparison

Capture two different configurations to compare them side by side on the mechanical profile. The radar normalizes against configuration A (reference at 100% on each axis).

Notation

F_crit — critical buckling load (max compression force before lateral buckling)

F_push — push force applied by the clinician (N)

T_f — estimated distal friction torque (mN·m). As long as T_proximal < T_f, the tip does not rotate and the shaft accumulates torsion.

θ_whip — whipping angle: instantaneous tip rotation when it breaks free from friction T_f.

y — deflection: lateral tip displacement under F_flex (mm)

E, G — elastic (Young) and shear (Coulomb) moduli of the material (MPa)

I — second moment of area in bending (mm⁴); hollow tube: I = π/64 · (OD⁴ − ID⁴)

J — polar second moment of area (mm⁴); hollow tube: J = π/32 · (OD⁴ − ID⁴)

A — cross-sectional area (mm²); A = π/4 · (OD² − ID²)

K — boundary-condition coefficient for buckling

About this tool

The tool applies three classic analytical strength-of-materials calculations to a hollow homogeneous circular tube, within the limits of small linear-elastic deformations: Euler buckling for pushability, Saint-Venant torsion for torqueability (with distal-friction and whipping model), Euler-Bernoulli cantilever beam for flexibility.

Buckling boundary conditions are configurable (K=0.5 to 2.0) to match the real setup — fixed in the handle, guided by an introducer sheath, mandrel-supported test bench. The default fixed-free (K=2.0) is the most representative of a catheter held in hand and free at the distal end.

Model limitations. The tool does not model braided reinforcements (stainless or nitinol braid), multi-lumen sections, stiffness gradients along the shaft (multi-durometer), combined loads (simultaneous bending + torsion), or vascular friction (capstan effect in tortuous anatomies). A physical test on a prototype always remains necessary to validate a design — Protobrix can perform it for you (see CTA at the bottom of the page).

Temperature correction. The E and G moduli drop near the polymer's glass-transition temperature Tg. For Pebax 72D/63D, Nylon 12 and TPU, the drop between 23 °C (bench) and 37 °C (body) is typically 25 to 35%. The k(T) coefficients tabulated in the tool are order-of-magnitude estimates based on DMA curves published by manufacturers and the open literature (PA12: Amstutz et al. 2021, EMPA Bern, conditioned medical tubing). For an exact grade, cross-check with the supplier's TDS. Special case for nitinol: the martensite ↔ austenite phase transition (around Af, typically 0–40 °C depending on grade) creates a discontinuity in modulus — check the Af grade used.

References

Formulas: Strength of materials (Euler 1757, Saint-Venant 1855, Euler-Bernoulli). Medtech industry reference: TE Connectivity — Medical Device Design Formulas.

Clinical ranges: [1] Tip-vessel contact force 0.1–0.8 N (0.12 N safety threshold, ScienceDirect 2023). [2] Proximal critical buckling force 2–6.5 N (USPTO 11331137). [3] 5F · 1000 mm catheter withstanding 9 N thanks to advanced design (PMC 9010673, 2022).

Temperature dependence: [4] Amstutz et al. 2021, BioMed Eng OnLine — PA12 mechanical properties measured at 23 / 37 / 50 / 80 / 100 °C on wet-conditioned medical tubing (DOI 10.1186/s12938-021-00947-8). For Pebax: Arkema TDS DMA curves and Sadeghi et al. 2022 Confluent Medical (multilayer Pebax/HDPE).

Material datasheets: Arkema (Pebax MED), Lubrizol (Pellethane), Evonik (Vestamid), Victrex (PEEK), Fort Wayne Metals (nitinol, 316L stainless), MatWeb.

Upstream orientation tool — to be validated by testing. The returned values are order-of-magnitude estimates computed on a polymer or metallic shaft, homogeneous and unreinforced, with temperature correction for the use case. The k(T) coefficients are indicative and based on published DMA curves — for an exact grade, cross-check with the supplier's TDS. Real catheter performance depends on many factors not modeled here (braid, durometer gradient, vascular friction, real-use conditions). Any design choice must be validated by the developer in compliance with applicable regulatory and normative requirements.

Going further — validated data

The values from this tool are computed order-of-magnitude estimates. For a technical file, two validation paths:

Path 1 · Validation by physical testing

Protobrix catheter prototype

480+ test-ready configurations (6 to 24 Fr) in under 7 days, from €600. Verify your pushability, torque and flexibility calculations on a real shaft, with or without braid, multi-segment geometry, durometer gradient.

Configure my catheter →

Path 2 · Analytical validation & report

Protomed mechanical study

For a signed regulatory deliverable: advanced analytical characterization (FEM modeling), bench testing, torque measurement, load-displacement curves. Report compliant with the requirements applicable to your device, ready to integrate into the technical file.

Protobrix · R&D Tools 2026 Catheter shaft mechanical behavior V5

Before using this tool

Catheter shaft mechanical behavior

Decision-support tool

This tool applies canonical strength-of-materials formulas (Euler buckling, Saint-Venant torsion with distal friction, cantilever beam) to a homogeneous circular tube, within the limits of small linear-elastic deformations.

The returned values are indicative order-of-magnitude estimates, with no account taken of braided reinforcements, stiffness gradients, or combined loads. It is up to each user to verify their relevance for their project and to validate any critical decision by physical testing on a prototype.

The tool does not replace regulatory expertise or experimental validation. For a deliverable compliant with the requirements applicable to your device, contact us for a study service.

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Catheter shaft mechanical behavior

The values from this tool are computed order-of-magnitude estimates. For a technical file, two validation paths:

Decision-support tool

Catheter shaft mechanical behavior

The values from this tool are computed order-of-magnitude estimates. For a technical file, two validation paths:

Decision-support tool

Follow us