Nanomedicine, Volume IIA: Biocompatibility
© 2003 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003
15.4.1 Large Particle Movement
Large immotile macroscopic particles can migrate through soft tissue on their own if they possess a certain degree of asymmetry . A sphere, such as a buckshot pellet, tends to stay in place for an indefinite time. But an asymmetric object such as a sewing needle or porcupine quill can move point first and may travel for long distances due to the action of natural tissue movements and the muscular forces acting upon it . Various implants used for internal fixation of fractures and for adjunctive tissue immobilization during placement of permanent implants may also become dislodged and migrate . Smooth pins and wires [2600-2622] are more likely to be reported as migrating than threaded objects such as screws  or staples [2600, 2601], although screws [2623-2625], threaded pins , and staples  are occasionally reported as having migrated. For instance, of 47 occurrences reported in one survey , eight patients died, six suddenly, due to damage to the heart or to the blood vessels near the heart by the migrating implants, with migration typically occurring within 8 months of implantation. Migrating device or wire fragments have entered the lungs [2602-2604], heart [2601, 2625], aorta [2610-2613], pulmonary artery [2615-2618], small bowel , abdominal wall , spinal canal , and knee joint . Migrating fragments have moved as far as from hand to elbow ; from pelvis to heart ; from hip bone to ureter  or bladder ; from cervical vertebra to lungs ; from right hip to left lung ; and from the neck or shoulder down to the heart , aorta [2610, 2611], thorax [2606, 2609], lung , pulmonary artery , diaphragm , spleen [2605, 2607] (in one case, reaching the spleen in only 12 hours ), liver , and lower abdomen . Incompletely absorbed dissolvable subdermal sutures can work their way back to the surface of the skin  (Section 7.3.3) – suture [2638-2640] and ICD patch [3936, 6080] migration is well known.
Large material particles can also become involved in blood circulation, as illustrated by four occurrences of shell fragments transported into the cerebral circulation , Kirschner wire migration through the great vessels into the heart , intrapelvic migration of a Knowles pin through the external iliac vein , and catheter fragments removed from the central circulation in children . Smaller wear particles from vascular prostheses will move downstream until trapped by reduced vessel diameters on the arterial side of capillary beds, or in the lungs on the venous side of the circulation . Bloodborne cholesterol crystal emboli typically occlude 50- to 500-micron diameter arteries  when cholesterol crystals flake off from the proximal arterial wall during medical procedures (e.g., angioplasty) or in the natural course. Such pathological cholesterol crystals [2647-2653] are usually found in the vasculature of kidney [2652-2658], gastrointestinal tract [2645-2647], muscle [2655, 2663], skin [2654, 2655], eye , penis [2660, 2661], brain , or the extremities [2663-2665]. More common is the presence in animals [2666, 2667] and in human patients [2670-2673] of extracellular particles that are too large to be phagocytosed. These particles include wear debris, precipitated corrosion products, mineral dusts, fibrillar fragments from tendon prostheses, or even 4- to 100-micron Teflon paste particles (Section 18.104.22.168). They ultimately appear in the lymphatic drainage, in regional lymph nodes, or in remote medullary locations or organs.
Relatively large nanorobots and nanoorgans lacking powered locomotive capability (Chapter 9) but having a proper shape or dynamic surface geometry (Chapter 5) could exploit the natural propulsive forces in the tissues to achieve a slow, biologically-assisted histomigration throughout the body. However, nanomedicine usually demands more rapid, precise, and controlled movement. Appropriate tissue anchoring normally will be the paramount concern for nanorobotic organs, as in the case of implanted macroscale communication (Section 7.3.4), navigation (Section 8.3.6), or computational (Section 10.2.5) nodes.
Last updated on 30 April 2004