Biomechanics and Ergonomics

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Make sure your implant is sound in structure. Every implant within the body is going to experience stress and biomechanical considerations are going to be crucial. The slightest structural defect can cause a cascade of horrible events that may become serious issues. To illustrate the importance of biomechanics, let us imagine this hypothetical device failure situation:

Let’s imagine that we have a small subcutaneous implant. It has a battery and electronic components which are housed in a case made from PVC material. The entire case is also enclosed in a sturdy silicone shell which serves as our primary solution for biocompatibility. Over time we may find that the PVC case begins to experience fatigue, simply by enduring normal load stresses within the body. After the device experiences an impact the consequences of that fatigue will begin to manifest. In our case we can imagine the impact causing minute chipping the size of a grain of sand. This grain of PVC is now lodged between our silicone layer and the rest of the PVC case. In time this grain will wear away at our silicone layer as well as our encasement. Our silicone layer might become compromised and fluids may fill in the breached space. Several events might occur from here, most of which will go undetected at first. Eventually your body will identify the loose PVC particulates as a foreign material. Macrophages will begin an enzymatic campaign to break down and metabolize this material. When it fails it will send out a signal to activate our body’s other methods of material disposal. Other outcomes may include inflammation or infection.Things get even more serious if our PVC enclosure is also breached. Electrocution from the battery will be an immediate concern as our electronics encounter body fluids. The body will rapidly break down various toxic materials inside our implant and carry them throughout our body. The eventual outcome could be as serious as death and it all started with a chip the size of a grain of sand. Each material will need to be analyzed by itself as well as how it might interact with other components in the device as well as the impact it may have in the body.

Here are a few things to consider in your build:

• Isotrophy/Anisotrophy
  
• Stiffness
  
• Bending stresses
  
• Contact stresses
  
• Multiaxial loading
  
• Plasticity
  
• Fatigue
  
• Fracture
  
• Wear
  
• Corrosion

Fatigue, corrosion, and wear are the most common modes of failure in medical implants, and the industry has many case studies detailing various structure related implant failures.