## 2 General Characteristics of Implant Systems  
An implantable electrical system can perform a telemetry (sensing) function, whereby biological data is collected, a teleactuation (stimulation) function, or both in a form of closed loop control. Regardless of it intended function, an implantable system typically comprises of two fundamental components, an indwelling module which resides within the host body, and an external device located outside of the body. The external module is generally employed to transmit the information to and from the internal module and deliver power to the indwelling component of the device. The indwelling module may be fully electronic, or contain chemical, biological, or mechanical components. In sensing electronic implants, the indwelling sensors detect, collect and translate the desired biological and physiological parameters into electrical signals. These signals are then modulated by the interface electronics and transferred by means of an inductive coupling link to the external receiver component, where the data is recorded and analyzed. For example, a micro-accelerometer implanted directly on the surface of the heart of patients who have just undergone coronary artery bypass graft (CABG) surgery can be used to measure the heart wall motion as a means of early detection of surgery complications [9]. In stimulation implantable systems, the external component is used to wirelessly transmit commands to the indwelling component, where they are processed by the interface electronic circuitry to produce a range of electrical stimuli. The produced electrical currents are then delivered to tissues and nervous structure by means of electrodes. As an example, Enterra(r) therapy is used to treat diabetic gastroparesis by applying electrical stimulation to the antrum via two indwelling unipolar intramuscular leads and a neurostimulator, where the stimulation parameters are adjusted noninvasively by using Medtronic N'Vision clinician programmer [48]. A closed loop system encompasses both the indwelling sensing and the stimulation components, and all information transfer and processing takes place within the body of the patient. This type of an implantable electronics is commonly employed to maintain a certain level of function within the human body, e.g., cardiac resynchronization, to enable automated provision of medical care and prevention of critical incidents, e.g., sudden cardiac death [37]. Hemodynamic sensors have been integrated into implantable pacemakers to enable rate-responsive pacing; these closed loop systems function on both sensed and paced ventricular beats, thus surmounting the key constraint of the previous pacemaker systems, namely the need for permanent ventricular pacing [49]. Concomitant sensing and stimulation provided by closed loop neuromodulation devices provide a platform for enhancing therapies for neurological disease, while concurrently assessing the instantaneous response of the neural system to stimuli [50]. Within the host body, the individual modules of the electrical system can reside intracavity, e.g., within the intestinal, oral, or urinary systems _etc._, be implanted subcutaneously or deep within the tissues, or be located on the external surfaces of the body [50; 51; 52]. Hard shell packaging is often used to protect the electronic circuitry, whereas the remainder of the indwelling assembly may also have a soft encapsulation layer [53]. The role of the protective casing and the encapsulant is two-fold. For one, the hermetic protective casing ensures the _in vivo_ integrity and reliability of electronic performance of the devices over the life-time of the implant under the specific physiological conditions. This includes protecting the device elements from the highly corrosive environment and ensuring no leakage current flowing through the electrodes [53]. Secondly, the encapsulation layer performs a biocompatibility function, protecting the host tissues from potentially harmful elements of the device.