Implantable force sensor

These mechanical displacement and/or force sensors, when miniaturized appropriately, can be embedded in flexible stractures and can be used in biomedical appUcations such as implants and wearable sensing systems. 

Membranes and thin polymer films transport chemicals through channels that, owing to their molecular structure and electrical charge decoration, promote the facile transport of certain select species. Such phenomena are most completely described on the basis of electrochemical potentials and driving forces. Closely related to such phenomena are electrochemical sensors for health care and macromolecular electronic devices that respond directly to living systems in which they are implanted. 

The packaging of the pressure transducer is also a problem that needs to be addressed, especially when the transducer is in contact with blood for long periods of time. Not only must the package be biocompatible, but it also must allow the appropriate pressure to be transmitted from the biological fluid to the diaphragm. Thus, a material that is mechanically stable under corrosive and aqueous environments in the body is needed. Chronically implanted objects are usually coated with a fibrous capsule by the body as a part of the foreign body response, and this capsule can exert a force on the pressure sensor that will affect its baseline pressure. Thus, it is important to package pressure sensors with materials that will minimize this encapsulation. 

If neural signals are to be used to control prostheses (Fig. 9.27), additional encoding between the technical and the biological system has to be done to transfer the information in an appropriate code. The sensor has to be placed inside the body to record signals that are used as command variables in the control task (Fig. 9.25c). One example for this adaptronic system is an implantable neural stimulator for grasp in paralyzed people with feedback response from an implanted sensor to control grasp force [18]. 

This concept takes advantage of measured real-time feedback of AP and IE rotation from sensors on board the knee simulator. Based on those instantaneous AP and IE implant positions/motions, a real-time (or strictly, semi-real-time) computation is made of the influence/contribution of the desired soft tissue springs or any virtual mathematical restraint models. The computed AP force and IE torque contributions are added (vectorially, mathematically) to the electronic command signals for the externally actuated AP and IE torques of the simulator. 

Depending on the simulator, with suitable calibrated sensors (Figure 26.13), this force-control concept could therefore facilitate assessment of the dynamic performance of a TKR implant by measuring its kinematics in vitro during a simulated walking activity. 

---