How Flex-Life Testing Helps Medical Cable Assemblies Maintain Performance After Repeated Bending

Medical devices are put into use in clinical practice and repeatedly exercised in a variety of different ways. Robotic surgical arms undergo thousands of bends and twists, while ultrasound probes are repeatedly re-angled during lengthy procedures and endoscopes navigate tortuous anatomical pathways. If not properly managed, these mechanical stresses can degrade electrical performance over time, potentially leading to equipment failure, signal artifacts, and premature cable replacement. At Hotten, we take every measure to ensure that our medical cable assemblies deliver reliable, long-term performance in clinical environments – this is why we use a full flex-life test as an absolute design verification test. This article examines four key aspects of flex-life testing that help ensure reliable performance under real-world clinical conditions.

What Does Flex-Life Testing Actually Verify in Medical Cable Assemblies?

Direct Answer

Flex-life testing verifies more than whether a medical cable can survive a specified number of bends.

A meaningful flex-life qualification evaluates whether the complete cable assembly continues to meet its mechanical and electrical requirements after repeated motion under representative operating conditions.

Important factors can include:

Bending → Torsion → Tensile Movement → Strain Relief → Electrical Performance → Environmental Conditions → Failure Mode → Service-Life Requirement

If → the cable only needs to survive occasional static movement

Then → define the mechanical test according to the actual installation and movement profile

Why → a static bend requirement is different from continuous dynamic flexing.

If → the cable is continuously moved during normal device operation

Then → define bending direction, radius, speed, cycle count and motion pattern

Why → the number of cycles alone does not describe the actual mechanical load.

If → electrical performance is important after repeated movement

Then → monitor relevant electrical parameters during or after flex testing

Why → a cable can remain physically intact while its electrical performance changes.

If → the device operates under temperature, humidity, cleaning or sterilization conditions

Then → evaluate mechanical and environmental stresses together where appropriate

Why → real-world use may expose the cable to combined stresses rather than isolated laboratory conditions.

The current HOTTEN article already uses customized motion profiles and electrical measurements during flex-life testing, including DC resistance, capacitance, TDR-based impedance and high-frequency attenuation.

For customized Medical Cable Assembly, flex requirements can be incorporated into the overall cable design and validation process.

Simulating Real-World Clinical Motion Profiles

This simple bend test does not replicate the multi-axis motion required in healthcare environments and typically involves repeated bending around a fixed mandrel. Hotten's flex-life testing procedures are customised to simulate the use of each type of device. We have developed test rigs for our robotic wire harnesses that simulate the compound motions—including torsion, bending, and axial translation—experienced at the articulation points of surgical robotic arms. In endoscopic systems, gimbal mechanisms help stabilize the camera cable while allowing the camera to rotate through 360° around a pulley. The tests performed on these types of cables are known as "roll-on" cable flex tests. We conduct flex cycles to the required qualification level. For example, when qualifying a cable for 50,000 cycles, we compare the test parameters with data from actual clinical use.This approach provides a realistic representation of clinical use and helps verify long-term reliability in the field.

Real-Time Electrical Performance Monitoring During Flexing

A cable may withstand 1,000,000 mechanical cycles without visible mechanical damage while still experiencing degradation in electrical performance (such as an increase in insertion loss, drift in impedance or crosstalk) without any mechanical damage. Electrical performance can be evaluated at defined intervals during the flex-life test to identify changes in cable performance. At the same time, we measure the DC resistance, capacitance, characteristic impedance (TDR) and high-frequency attenuation within the required frequency range of a test sample bent several times. Special monitoring of phase stability and skew between differential pairs is performed for our high frequency transmitting cables and harnesses (ICE – intracardiac echo; IVUS – intravascular ultrasound) This real-time data enables us to identify the onset of performance degradation rather than detecting failure only after it occurs,and to set end-of-life thresholds conservatively to help ensure that the signal integrity of the device is maintained during its service life.

Multi environment flex conditioning – temperature & sterilizations

In clinical environments, cables are rarely flexed under controlled room-temperature conditions. Operating rooms may be cooled for cardiac surgery or warmed for long oncological surgeries and cables may also be flexed immediately after high-temperature autoclave sterilization or chemical disinfection, when residual moisture may still be present. At Hotten, flex-life testing can be conducted across a range of temperatures (0°C – 60°C) and humidity levels in our environmental chambers with controlled conditions. Only cable designs qualified to withstand these combined mechanical and environmental stresses are offered for clinical applications, minimizing failure in the field under these combination environmental stresses.

How Should OEM Engineers Define and Validate Flex-Life Requirements?

For an OEM medical device project, “flexible” is not a sufficient engineering specification.

A useful flex-life requirement should define:

Where the Cable Moves → How It Moves → How Often It Moves → Under What Conditions → What Must Remain Within Specification

The test should therefore connect the actual device motion profile with measurable mechanical and electrical acceptance criteria.

1. Start With the Actual Motion Profile

Different medical devices impose different types of mechanical movement.

Typical motion modes may include:

• repeated bending
• torsion
• axial translation
• rolling
• combined bending and twisting
• localized movement near a connector
• repeated movement through a defined bend radius

If → the cable follows a simple repeated bend

Then → define the bend radius, direction, speed and cycle count

Why → these variables determine the mechanical loading experienced by the cable.

If → the cable follows a multi-axis mechanism

Then → reproduce the combined movement as closely as practical

Why → a single-axis mandrel test may not represent the actual stress applied to the cable.

The current HOTTEN article gives examples of compound motion for robotic systems and roll-on testing for endoscopic cable applications.

This principle can also be applied to ultrasound probes, catheters and other devices where cable movement is tied to the mechanical architecture.

2. Cycle Count Alone Does Not Define Flex-Life

A specification such as:

50,000 cycles

is useful, but it does not completely define a flex-life test.

Other parameters may include:

If → two suppliers both claim the same cycle count

Then → compare the complete test conditions rather than comparing cycle numbers alone

Why → 50,000 cycles under one bend radius and loading condition may not be equivalent to 50,000 cycles under another.

This is an important point for OEM supplier evaluation.

3. Define What Counts as a Test Failure

A flex-life test should have clear acceptance criteria before testing begins.

Possible failure indicators can include:

If → the conductor remains continuous but attenuation increases beyond the allowed limit

Then → treat the electrical change as a potential qualification issue

Why → continuity alone does not demonstrate signal-integrity performance.

If → there is visible jacket abrasion but all specified electrical parameters remain within the acceptance criteria

Then → evaluate whether the physical damage itself is acceptable for the intended application

Why → mechanical appearance and electrical function are related but may have different acceptance criteria.

4. Monitor Electrical Performance During Flex-Life Testing

This is one of the most valuable parts of the current HOTTEN article and should be expanded rather than simply repeated.

A useful validation sequence is:

Initial Electrical Baseline

→ Mechanical Cycling

→ Intermediate Electrical Measurements

→ Final Mechanical Inspection

→ Final Electrical Verification

Depending on the application, relevant parameters may include:

• DC resistance
• capacitance
• characteristic impedance
• high-frequency attenuation
• insertion loss
• crosstalk
• phase stability
• differential-pair skew
• continuity

The current article specifically states that HOTTEN can monitor DC resistance, capacitance, TDR-based characteristic impedance and high-frequency attenuation, with phase stability and skew monitored for certain high-frequency transmitting cables and harnesses.

If → electrical degradation begins before visible mechanical failure

Then → use intermediate measurements to identify the onset of performance change

Why → the first measurable electrical degradation may occur before a cable becomes visibly damaged.

5. Test the Critical Areas, Not Just the Cable Center

Mechanical failures are not necessarily distributed evenly along the cable.

Potential high-stress locations may include:

• connector exits
• strain relief transitions
• overmold edges
• bend points
• cable fixation points
• shield termination areas
• locations where cable diameter changes

If → repeated failures occur near a connector

Then → inspect strain relief geometry, termination structure and local bend radius

Why → local stress concentration may be contributing to the failure.

If → failures occur consistently at a cable transition

Then → consider changing the local geometry rather than only increasing the global cycle count

Why → increasing test cycles does not correct a design-driven stress concentration.

The current HOTTEN article already describes changing strain-relief geometry after observing failures near the connector overmold and then repeating the test.

6. Combine Flex Testing With Environmental Conditioning When Required

Medical cables may experience mechanical movement together with environmental exposure.

Relevant conditions can include:

• temperature
• humidity
• cleaning
• chemical disinfection
• sterilization
• moisture exposure

If → the cable is bent immediately after an environmental process

Then → reproduce the relevant sequence during qualification

Why → material and mechanical properties may differ under environmental exposure compared with room-temperature dry conditions.

The current HOTTEN article describes environmental-chamber testing across 0°C to 60°C and controlled humidity conditions as part of its flex-life validation approach.

For an OEM project, however, the exact environmental profile should be based on the actual device specification rather than applying one universal test condition to every cable.

7. Ultrasound Probe Cables Require a Different Flex Profile

Ultrasound probes can experience repeated re-angling and handling during normal operation.

Therefore, the cable may need to balance:

Flexibility

Small Bend Radius

Signal Integrity

Mechanical Durability

If → the cable is integrated into an ultrasound probe

Then → define movement around the probe handle, cable exit and strain-relief region

Why → these areas may experience repeated localized movement during use.

HOTTEN's current ultrasound cable portfolio includes customized cable structures, high-density configurations, cable diameter, bend radius and connector/termination requirements.

See Ultrasound Probe Cable.

8. Endoscope and Robotic Cables May Require Compound Motion

Some systems combine bending, rotation and translation.

For example:

Endoscope

→ rotation / rolling

→ repeated bending

→ constrained routing

Robotic System

→ bending

→ torsion

→ axial movement

→ repeated articulation

If → the actual device motion contains multiple axes

Then → design the test to reproduce the important combined movements

Why → a single fixed-axis test may not represent the mechanical load applied during operation.

The current HOTTEN article already describes compound motion testing for robotic harnesses and roll-on testing for endoscopic cable systems.

9. Flex-Life Testing Should Feed Back Into Cable Design

Flex testing should not end with:

PASS / FAIL

The more useful engineering output is:

What caused the failure, and what should change in the next design iteration?

A typical loop is:

Test

→ Failure Observation

→ Root-Cause Analysis

→ Design Change

→ Prototype

→ Retest

→ Qualification

Potential design changes may involve:

The current HOTTEN article describes microscopy, cross-sectional analysis and impedance profiling as part of failure analysis, followed by design changes and additional testing.

10. Separate Design Qualification From Production Monitoring

An OEM should distinguish between:

Design Verification

and:

Production Quality Control

The first question is:

Can the design survive the required service profile?

The second question is:

Can production repeatedly manufacture assemblies with the same characteristics?

A practical process is:

Prototype

→ Design Verification

→ Flex-Life Qualification

→ Process Validation

→ Pilot Production

→ Production Inspection

The current HOTTEN article positions flex-life testing as a design-verification activity, while its broader manufacturing workflow also supports prototype-to-production development.

This distinction is important because a qualified design and a stable production process are related but different engineering requirements.

11. What Should OEM Engineers Put in a Flex-Life Test Specification?

For a customized medical cable assembly, a useful test specification should include:

If → the RFQ only says “high-flex medical cable”

Then → request the actual motion profile and expected service conditions

Why → “high-flex” does not define bend radius, cycle count, torsion, speed or acceptance criteria.

Practical Flex-Life Qualification Logic

A practical OEM qualification sequence can be structured as:

Actual Device Motion

→ Identify Critical Stress Areas

→ Define Bend / Torsion / Translation

→ Define Cycle Count

→ Define Environmental Conditions

→ Establish Electrical Baseline

→ Perform Mechanical Cycling

→ Monitor Electrical Performance

→ Inspect Failure Modes

→ Compare With Acceptance Criteria

→ Modify Design if Required

→ Retest

→ Qualify Production Process

For an ultrasound probe:

Repeated Probe Re-Angling

→ define bend radius

→ identify connector / strain-relief stress area

→ establish electrical baseline

→ perform repeated flex cycles

→ measure electrical performance at defined intervals

→ inspect conductor / shield / insulation

→ compare with acceptance criteria

→ modify design if required

→ repeat qualification.

For an endoscope:

Rotation + Bending

→ define roll-on or equivalent motion

→ establish cable fixation and routing

→ define cycle count

→ test under representative environmental conditions

→ monitor signal performance

→ inspect mechanical wear

→ analyze failure mode

→ validate the revised design.

For a robotic medical system:

Bending + Torsion + Axial Translation

→ reproduce the combined motion

→ monitor electrical parameters

→ inspect high-stress transition areas

→ identify the dominant failure mechanism

→ adjust cable structure or strain relief

→ repeat testing

→ validate production consistency.

The important engineering principle is:

Flex-life qualification should reproduce the way the cable actually moves and measure the performance that the medical device actually depends on.

Related HOTTEN Engineering Resources

How Micro Coaxial Cable Design Ensures Signal Integrity in Medical Devices

Design Factors in Medical Cable Assemblies That Protect Signal Integrity in Clinical Devices

How RF Coaxial Cable Assemblies Control Impedance and Loss for Reliable High-Frequency Signals

How Impedance Control Helps High-Frequency Coaxial Cable Maintain Stable Signal Transmission

How Material Selection and Strain Relief Extend RF Coaxial Cable Assembly Service Life

How to Specify a Custom Cable Assembly for OEM Projects

How Custom Medical Cables Are Processed and Inspected Before Delivery

Medical Cable Assembly

Ultrasound Probe Cable

Failure Mode Effects Analysis (FMEA); Continuous Design Improvement

Flex-life testing is not simply a pass-or-fail verification method; it is also a tool for identifying design improvements. If a test sample fails due to conductor fracture, shield damage, or insulation abrasion, our engineering team conducts a detailed failure analysis of the test sample, using microscopy, cross-sectional analysis, and impedance profiling techniques. If, for example, a cable breaks at the end of the strain relief at the connector overmold, we change the geometry of the strain relief and test once again. For example, a dental sensing cable may be replaced if it exhibits increased attenuation after 30,000 cycles, or the conductor material or construction may be reviewed to improve flexibility and fatigue resistance. Through this continuous improvement process, Hotten develops more than 300 new cable specifications each year and incorporates the resulting knowledge into future designs. All changes will undergo additional flex-life testing and each design iteration further improves product performance and reliability.

Conclusion

Ultimately, flex-life testing is more than a quality assurance tool; it helps bridge the gap between laboratory testing and real-world clinical use. Whether it's for use in an ultrasound probe or a robotic harness, our medical cable assemblies at Hotten are designed to maintain reliable performance through the repeated bends, twists, and tensile stresses encountered in daily hospital use. Supported by a 10,000 m² facility, more than 40 production units, and annual production exceeding 144 million meters, Hotten translates flex-life testing data into durable, reliable cable solutions, so that healthcare providers can focus on their patients and not the failure of the cable.