Design for Manufacturing (DFM) ensures that there is effective communication between the designer and the manufacturer of a medical device, say a hip implant. Therefore, the designer needs to mention the GD&T control symbols in his drawings. Here is the process flow for achieving the tolerance stack analysis for an implant or any other medical device.
Types of Tolerances
Here are the widely used tolerances:
- Form Tolerance: Straightness, Flatness
- Profile Tolerance: Profile of a line or profile of a surface
- Orientation: Perpecdicularity, Parallelism, angularity
- Location: Position and tolerance
Tolerance Stack Analysis
Tolerance stack-up analysis is required to ensure the the geometerical fit of different mechanical parts.
Tolerance analysis help us to determine the Worst case minimum dimension, Worst case maximum dimension, Root of Sum of Squares (RSS) Minimum dimension, RSS maximum dimension and sensitivity analysis of the dimension.
The following video is a great explanation of the tolerance analysis.
Why is there a variation between the designed part and manufactured part?
Sources of error:
- Tooling: Variation in locating surfaces using a fixture, tool wear
- Materials: Non-uniformity in material properties and internal stress relaxation.
- Operator: Inconsistent setups and other manual errors
- Environment: Temperature, humidity etc.
Design Process of an Implant or Medical Device
The following flow chart is an exhaustive process highlighting the various stages of the design like design input (stakeholder analysis, user requirements), followed by concept design, design verification, manufacture, validation and design transfer.
Risk Management Steps
According to ISO 14971, risk management of medical devices involves 4 steps:
- Risk Analysis
- Risk Evaluation
- Risk Control
- Residual Risk Evaluation
Risk Assessment = Risk Analysis + Risk Evaluation
ISO 14971 dictates the responsibility of the management for implementing the risk management process. According to ISO 14971, the management should provide the adequate resources and assign the qualified personnel for risk management.
Moreover, the top management is responsible for defining and determining the criteria for risk acceptability.
I will solve a case study involving the design of an exoskeleton for lower limb rehabilitation and document the risk management steps for this.
1. Risk Analysis
These are the three tools typically used in Risk Analysis:
- Hazard analysis
- Fault Tree Analysis (FTA)
1.1. Hazard Analysis
Hazards are potential damage to the user or the environment because of the use of medical devices. ISO 14971 Annex C contains examples of hazards associated with use of medical devices.
This is a tutorial to help you with FMEA and CAPA in medical device design.
Using only FMEA doesn’t meet the quality requirements as per ISO 14971. This is because FMEA assumes that risks are involved when the device fails. However, there are potential risks of the medical device even without failures.
|Part||Function of part||Potential Failure Mode||Severity||Cause of Failure Mode||Occurrence||Detection Methods or Design Controls||Detection||RPN||CAPA|
|Series Elastic Actuator (SEA)||Transmit force to the joints||Inconsistent torque||7||Lack of planarity between the slots||5||Peer review, literature data for torque required||2||70||Increase the moment arms|
|Moment arm of joint||Provide moment to the rotating parts||Impact loading, yielding, fracture||8||Repeated loading, stress concentration||7||Finite Element Analysis (FEA)||5||280||Increase the part thickness, optimize the design|
According to ISO 14971, manufacturers have the freedom to decide upon the threshold for risk acceptability. Only non-acceptable risks have to be integrated into the overall risk-benefit analysis.
If you are working on a group project, just the way I was working in my internship, it is up to the design team to assign the scores. The relative RPN scores are important. The part with the highest RPN score gets the most attention.
3. Risk Control
The risk assessment step is followed by the risk control step. Here is an example of the risk control measures as per ISO 14971.