W7 Mechanical Aspects of Implantable Stimulators Flashcards
What do the mechanics of a Neurostimulator provide?
Physical framework for integration of other components e.g. electrodes
Enclosure containing electronics:
- to protect electronics from the human body
- to protect the human body from the electronics
- almost always hermetic by design
Enclosure interfacing with the human body
- harm-free interface
List the standard requirements of implantable stimulators (~5)
User needs
- must function as intended for design life and not cause harm
- useability /aesthetics
- compatible with medical procedures e.g. MRI
Hermeticity
Biocompatibility and Biostability
Design for life
Business (profitability)
Discuss the Requirement and Constraint behind Implant Size
- implant recipients don’t want to know their implant is there
- there is limited space in the body
- in the head thickness takes priority over plan area for monolithic designs
- challenge also for modular systems
Discuss the Requirements behind Hermeticity
- Hermeticity - being sealed or gas tight
- Ion transfer is a two-way street
- outward flow of ions may be toxic (e.g. lead, copper, tantalum, etc.)
- inward flow of moisture may eventually lead to free water molecules which can provide a medium for ion movement and initiate corrosion or dendritic growth ultimately leading to failure of the electronics
- hermeticity is the most critical deliverable for the mechanical package of a neurostimulator
- no enclosure is absolutely hermetic
- all materials are permeable to some degree
- all joints are permeable to some degree
Hermeticity of the device must be capable of being tested on an individual basis. What are the typical methods?
- Helium ‘sniffing’ - apply vacuum to one side of the device and attempt to pull Helium through the leak, which is then detected
-
Mechanical deflection - apply vacuum to the device and monitor deflection. Lack of deflection indicates equilibrium = leak
- (not as accurate)
Discuss the Requirements surrounding Biocompatibility and Biostability
- Material Biocompatibility: “the ability of a material to perform with an appropriate host response in a specific application”
- i.e. a material which causes no harm to the body
-
Biostability: not changing the body over time
- appropriate material longevity
- appropriate duration of efficacy
- Biocompatibility is used by many to infer both biocompatibility and biostability
- improtant to differentiate when it comes to testing as most biocompatibility tests (ISO 10993) are relatively short duration (<6 months)
- biostability testing typically requires acceleration factors, e.g. increased temperature
- Materials must be biocompatible and biostable individually and in combination
What are some typical materials used for implantable medical device packaging?
- titanium and its alloys
- nobel metals and their alloys
- biodegradable stainless steels
- some cobalt-based alloys
- tantalum
- niobium
- titanium-niobium alloys
- nitinol
- MP35N (nickel-cobalt-chromium-molybdenum alloy)
- alumina
- zirconia
- biocompatible glass
- polymers
Discuss the Requirements surrounding Design for Life
Design for life
- 75+ years for a cochlear implant
- no opportunity of maintenance
- challenge due to
- harsh implanted environment
- mechanical/thermal loading during manufacture, storage, transport and use
The life requirement generates multiple lower level requirements, including:
- impact strength
- fluid ingress protection
- fatigue of conductors
- robustness to environmental loads (including storage and transport)
- low pressure, high presure, high temperature, low temperature, random vibration, drop, shock, thermal cycling, thermal shock
How do we ensure design meets requirements?
Good mechanical design practice
Good choice of materials
Feedthrough design (critical sub-assembly for Stimulators)
Testing
Discuss the processes involved with good mechanical design practice
- Includes developing robust requirements
- Using best practice product risk analysis - Hazards Analysis (HA), Failure Modes and Effects Analysis (FMEA)
- identification of failure modes
- each risk quantified by a risk priority number (RPN) which is a function of:
- Severity - effect on recipient if risk is realised
- Occurrence - likelihood that risk will be realised
- Detection - chances of detecting problem before risk is realised
- RPNs drive risk mitigations e.g. design changes, improved detection, increased testing
What are the Important Mechanical Design Features of the Cochlear Implant?
- titanium enclosure created from laser welded pressed shells
- silicone overmould
- electrical assembly and insulators
- wound coil - transfer of power and data
- titanium encased magnet
- inter and extra cochlear electrodes
- feedthrough assembly
In the image provided, label the materials used in each part of the Cochlear Implant and the brief reason why this choice of material was used in mechanical design.
Describe the general reasons for choice of materials.
See Image for labels.
General reasons for good choice of materials:
- chemically stable, low activation energy - biostability
- biologically inert - no leaching and biocompatible
- stable joining and interfaces between materials
Describe 3 tests (and purpose) for Design Verification of the Cochlear Implant.
- Biocompatibility Validation - Establish that the materials and processes used result in a biocompatable device.
- Severe Impact Validation - To verify that the implant stimulator has the required resistance to severe impacts.
- MRI Compatability Validation - To verify that patients with a Cochlear Implant, with magnet removed, are able to safely undergo MRI.
(see table for more)
Describe the Design Verification tests involved for the requirement of withstanding Severe Impact.
- simulates impact to the head
- 5 kg mass represents maximum head mass
- implant mounted to ‘head mass’
- 3 mm sheet of silicone over the implant simulates a thin skin flap
- 50 mm diameter steel ball on .058 kg hammer is lifted and dropped
- EN45502-2-3 requires cochlear implants to survive 2.5J impact
Describe the interactions between an MRI scanner and Cochlear Implants
- Unintended Stimulation - induced voltages due to Radio Frequency (RF) fields
- Implant Function - potential for induced voltages due to RF fields to damage implant
-
Implant Heating
- eddy currents due to RF fields
- Requirement: temperature rise at implant surface to be less than 2 degrees celsius (EN45502-2-3:2010)
-
Implant Force and Torque
- action of magnetic fields on magnetic and ferromagnetic components of implant
- potential patient harm
- magnet may become dislodged
- Demagnetisation (of magnet)
- Image Artefact - requirement for minimum artefact radius
- Issue as MRIs become more powerful for higher resolution
- may require removal of magnet
Using Circuits as an example, how may Failure Analysis be carried out and what may be revealed?
Scanning Eelctron Microscopy (SEM) can be used to reveal the nature of the field issue resulting in an e.g. open circuit (see image, left to right in following order):
- necking surface - indicative of blunt impact
- shear plane - indicative of narrow shearing force (sharp impact)
- rounded appearance - indicative of mechanical fatigue
Whilst also potentially evident visually (naked eye), SEM may further reveal contamination from moisture ingress.
Numerous general improvements have been made to the Cochlear Implant since CI22M as the foundation, with initial features of:
- titanium case to withstand impact
- flexible, malleable antenna coil to mould to small heads
- receiver/stimulator to transmit power and data
- 22 channel electrode array to offer best hearing
Many enhancements were seen through the CI24M and CI24RE, to the CI500. i) Describe the superiority of the CI500 in terms of design features. ii) In the attached image, list the improved benefit of each component (e.g. ‘improved robustness’)
i) CI500 design features:
- emphasis on ‘Design for Manufacturability’ (DFM)
- reduction in part count
- reduction in manual processes
- reduction in part handling
- support scalability
- thinnest implant ever <4mm (child firendly thickness)
- facilitates alternative surgical techniques
- sterile silicone dummy supplied
- enhanced strength >2.5J (child resistant strength)
- design principles used for improved resistance to biofilm adhesion
ii) see image attached
