Mechanical Design and Materials

Mechanical design begins with the medical context

Mechanical design is not merely the packaging of electronics. It can determine dose delivery, fluid containment, alignment, stability, protection, cleanability, user interaction and whether essential performance is maintained. A geometrically correct part can still be unsuitable when the loads, users, contact, environment or manufacturing process have been misunderstood.

Start with MTL-102 — Intended Purpose, Users and Use Environments and MTL-103 — User Needs and Design Inputs. They establish the product definition from which meaningful mechanical requirements and acceptance criteria can be derived.

Develop an explicit mechanical architecture

The mechanical architecture should explain how the product is supported, located, sealed, actuated, assembled and serviced. It should reveal load paths and critical interfaces rather than leave them hidden in a detailed CAD model.

Define reference datums, mechanical states, degrees of freedom, load transfer and interface ownership early. Record what aligns sensors and actuators, what maintains seals, what limits travel, what provides tactile or audible feedback and what happens when power is lost or a component fails.

Use MTL-112 — Systems Engineering, Architecture and Interfaces to connect the mechanical architecture to system functions, states and controlled interfaces. Mechanical and electrical teams should jointly resolve enclosure, connector, grounding, shielding, thermal and assembly constraints; see MTL-114 — Electrical and Electronic Design.

Design for loads, durability and loss of function

Identify the loads the product experiences from manufacture to retirement. Include normal use, foreseeable misuse, transport, installation, cleaning, service and fault conditions—not only the attractive nominal case used in early demonstrations.

• Define static, dynamic, impact, drop, vibration, pressure, torsion and actuation loads.
• Trace load paths through parts, joints, fasteners, interfaces and supports.
• Distinguish strength, stiffness, stability, deflection, resonance and energy absorption.
• Consider fatigue, creep, stress relaxation, wear, abrasion and repeated assembly.
• Account for stress concentrations, moulding features, threads, snap fits, welds and adhesive joints.
• Include temperature, humidity, fluids, sterilisation and ageing in material properties.
• Identify brittle, ductile, buckling, leakage, fracture and gradual-degradation failure modes.
• Define justified factors or margins and document the assumptions behind them.

A part can remain unbroken yet fail clinically because deflection changes sensor alignment, a seal loses compression, a dose mechanism slips or a latch no longer gives reliable feedback. Connect the load analysis to MTL-113 — Essential Performance and Safety Concepts and MTL-105 — Medical-device Risk Management.

Engineer dimensional variation

Nominal CAD geometry shows one ideal assembly. Production creates distributions of dimensions, form, orientation, surface condition and material behaviour. Tolerance analysis must show that permitted variation still allows safe assembly and performance.

Do not tighten every dimension to make an assembly work on paper. Redesign the architecture to reduce sensitivity, use locating features deliberately and place control on characteristics that matter. A drawing should express functional intent clearly enough that manufacture and inspection reach the same interpretation.

Select and control the complete material system

A material name such as stainless steel, silicone or polycarbonate is not a complete specification. Device performance can depend on grade, composition, additives, colourants, fillers, processing, heat treatment, surface preparation, coating, joining, cleaning, sterilisation, supplier and permitted regrind.

Record why the selected material system meets the requirements and what must remain controlled. If an alternative material, colour, moulding site, sterilisation cycle or cleaning process could change performance or biological safety, it requires a documented change assessment.

Connect material decisions to biological safety

Biological evaluation is a risk-management activity for the finished device. Begin with the nature and duration of body contact and with knowledge of the materials, manufacturing processes, residues, degradation, packaging and sterilisation. Testing is one possible source of evidence, not the definition of the process.

• Identify every direct and indirect patient-contacting material and pathway.
• Document composition as far as needed, including additives, colourants, coatings and processing aids.
• Consider manufacturing residues, cleaning agents, lubricants, adhesives and sterilisation residuals.
• Evaluate degradation, wear particles, corrosion products, leachables and material interactions.
• Use chemical characterisation, existing data and toxicological assessment where appropriate.
• Define representative or worst-case finished-device configurations for evaluation.
• Connect conclusions and remaining uncertainties to the device risk-management file.
• Reassess biological evidence when materials, suppliers, processes, contact or use change.

ISO 10993-1 places biological evaluation within the ISO 14971 risk-management process. It should therefore influence material selection and process control early, rather than become a late request for a generic “biocompatibility test”. A later dedicated topic, MTL-117, will explore biocompatibility and chemical safety in depth.

Design mechanisms for safe, understandable behaviour

Mechanisms translate user action or stored energy into product behaviour. Their safety depends on states, feedback, loads, variation, wear and misuse as much as nominal kinematics.

• Define permitted states, transitions, end stops, interlocks and recovery behaviour.
• Control stored energy in springs, pressure systems, rotating parts and suspended loads.
• Analyse pinch, crush, shear, entrapment, ejection, sharp-edge and instability hazards.
• Consider jam, slip, backlash, wear, rebound, incomplete engagement and unintended release.
• Provide clear tactile, audible or visual feedback where users need confirmation.
• Ensure protective guards and interlocks do not create new use or service hazards.
• Assess loss of power, control faults and manual override for electromechanical systems.
• Verify operation at load, tolerance, environmental and life-cycle boundaries.

Do not assume that a user will detect a partial latch or incorrect assembly. Apply usability engineering to critical physical interactions, including loading, connection, cleaning, adjustment and maintenance. IEC 62366-1 provides the safety-related usability process.

Design for environment, cleaning and ageing

The relevant environment is the complete journey of the device. Transport vibration, storage humidity, cold start, repeated disinfection or UV exposure may create damage that only becomes apparent during use.

Environmental tests should use justified conditioning and acceptance criteria derived from requirements and risk. An ingress rating or drop height copied from another product is not meaningful unless it represents the intended environment and failure consequences.

Design for manufacture, assembly and inspection

Production constraints are part of the design. Involve manufacturing engineers and suppliers before detailed outputs are frozen so the architecture, tolerances, tooling and inspection strategy are realistic.

• Select processes whose capability matches the functional requirements and expected volume.
• Design parts for stable moulding, machining, forming, finishing and handling.
• Reduce assembly ambiguity with orientation, poka-yoke, access and controlled sequences.
• Specify fasteners, torque, adhesives, welding and joining parameters where they affect performance.
• Identify critical characteristics and connect them to incoming, in-process or final controls.
• Plan fixtures, gauges and measurement-system capability before production release.
• Control cosmetic requirements only where they serve usability, cleaning, inspection or product claims.
• Define supplier deliverables, material certificates, traceability and change notification.
• Use prototypes from representative processes before relying on their verification results.

Rapid prototypes are excellent for learning but can conceal or create behaviour that differs from production tooling and materials. Record configuration and process whenever prototype evidence is used for a design conclusion.

Mechanical work across the lifecycle

Build verification and transfer evidence

Mechanical evidence should combine analysis, inspection and physical testing. Calculations and simulation can explore loads, tolerance and sensitivity efficiently, but their assumptions, boundary conditions and material properties must be justified and, where necessary, correlated with representative hardware.

Test the configurations and conditions most likely to challenge the requirement. Worst case may involve minimum wall thickness, maximum tolerance stack, aged polymer, repeated cleaning, highest load or a combination. Document the rationale rather than selecting samples by convenience.

Use MTL-106 — Verification and Validation for the wider evidence strategy and MTL-104 — Design Controls and Technical Documentation for maintaining the connected design record.

Common misconceptions