Subcutaneous implantable devices

Blood Access Devices. An investigational device called the Osteoport system allows repeated access to the vascular system via an iatraosseous iafusion directiy iato the bone marrow. The port is implanted subcutaneously and secured iato a bone, such as the iUac crest. Medications are adrninistered as ia any conventional port, but are taken up by the venous sinusoids ia the marrow cavity, and from there enter the peripheral circulation (8). 

FIGURE 18 In vivo cumulative weight loss (o) and cumulative release of levonorgestrel (o) from a crossUnked polymer prepared from a 3,9-bis(ethylidene-2,4,8,10-tetraoxaspiro[5,5]undecane)/3-methyl-1,5-pentanediol prepolymer crossUnked with 1,2,6-hexane triol. Polymer rods, 2.4 X 20 mm, containing 30 wt% levonorgestrel and 7.1 mol% Mg(OH)2. Devices implanted subcutaneously in rabbits. (From Ref. 15.)... 

Implantable cardioverter-defibrillator (ICD) A device implanted into the heart transvenously with a generator implanted subcutaneously in the pectoral area that provides internal electrical cardioversion of ventricular tachycardia or defibriUation of ventricular fibrillation. 

Polymer 3.87 has been evaluated as a matrix for the controlled release of progesterone.197 It was first shown that the rate of release of this steroid and of bovine serum albumen can be controlled by variations in the ratio of aryloxy to imidazolyl side groups atached to the polyphosphazene chain. In vitro and in vivo studies were conducted to examine the release rate of labelled steroid from devices implanted subcutaneously in rats. Typical data are shown in Figure 3.22. The biocompatibility of this system, at least in rats, was found to be good. 

Blanco and Samadani37 obtained a patent for the construction of a microprosessor-based insulin pump that works in a similar fashion to the Biostator. The implantable infusion device consists of a catheter, an information-transmitting sensor located in the catheter, a microprocessor, a pump, the drug reservoir, and a power source. The pump, the sensor, and the valves are connected by appropriate leads to the microprocessor. The device is implanted in the subcutaneous tissue in the chest area, and the infusion catheter is tethered intravenously to a central location, such as the right atrium. The device is inserted with the inlet port facing outward so that it may be refilled periodically by a physician. 

Most recently, an in vivo investigation of a NIR Con A RET system was reported and the host response characterized.114 In this work, Cy-7-labeled Con A and Alexa Flour 647-labeled dextran system were entrapped with a hollow microdialysis fiber immobilized on the tip of an optical fiber. The device was characterized in vitro, the result of which indicated a response range of 36 150 mg/dL. More important, the device was implanted subcutaneously and evaluated in vivo for 16 days. Results were promising, as the implant readout retained a high degree of correlation with blood glucose fluctuations (as measured by blood-draw methods). An increase in response time was observed at the end of the experimental period, with fibrous encapsulation cited as the cause. 

Central venous catheters are reluctantly used as blood access for hemodialysis because of safety concerns and frequent complications, for example sepsis, thrombosis, and vessel stenosis. Nevertheless, 20% or more of all patients rely on atrial catheters for chronic dialysis because of lack of other access. Potentially fatal risks related to central venous catheters include air embolism (1), severe blood loss (2), and electric shock (3). These specific risks have been substantially eliminated by the inherent design and implantation of Dialock (Biolink Corporation, USA). Dialock is a subcutaneous device consisting of a titanium housing with two passages with integrated valves connected to two silicone catheters. The system is implanted subcutaneously below the clavicle. The tips of the catheters are placed in the right atrium. The port is accessed percutaneously with needle cannulas. 

A 67-year-old woman was provided with a totally implantable venous device in the right subclavian vein by the Seldinger technique with a peel-away sheath. The device was used for a course of chemotherapy. After about 1 month there was subcutaneous extravasation of the drug. A chest X-ray showed that the sihcone catheter had fractured below the clavicle and the distal portion of the catheter had embolized into the right atrium. The fragments were removed. 

Current research and development efforts have focused on the use of more biocompatible coatings to reduce the biological response of both intravascular and subcutaneous devices. These efforts are based on the expectation that such developments wfllbe critical to the ultimate success in developing implanted sensors that yield continuous analytical results that match closely with conventional in vitro test methods. One new approach in this direction employs novel nitric oxide (NO) release polymers to coat the surface of intravascular sensors.The potent antiplatelet activity of NO has been shown to greatly reduce the formation of thrombus on the surface of implantable electrochemical oxygen sensing catheters, and yield much more accurate continuous PO2 values in animal experiments. 

Fig. 27. Scanning electron micrograph of polymer prepared from 3,9-bis (ethylidene-2,4,8,10-tetraoxaspiro [5,5] undecane) and a 60/40 mol ratio of rrans-cyclo-hexane dimethanol and 1,6-hexanediol. Polymer rods 2.4 x 20 mm containing 30 wt% levonorgestrei. and 2 wt% calcium lactate. Device implanted subcutaneously in rabbit for 10 weeks, 30X [25], Reprinted with permission...

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