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Materials for Prosthetic Devices and Implants Through the Ages

Through the ages, human medical needs have resulted from birth defects, trauma, age-related diseases, degenerative conditions, end-stage organ failure, etc. Several biological species have the capability for tissue/organ regeneration; for example, salamanders can regenerate lost limbs, tails, and even eyes [1]; deer regenerate antlers [2]; flatworms regenerate their bodies [3]; rabbits, domestic cats, and bats fill in ear holes [4]; zebrafish regenerate fins, etc. [5]. In contrast, humans have an extremely limited capacity to regenerate body tissues and organs.

In this respect, the Greek myth of Prometheus is pertinent. Long time ago, only the Gods of Olympus were privileged to have the comfort of fire. During those times, the mortal humans were doomed to live in a dark and cold world. Prometheus, a Titan and immortal, commiserated with the humans, stole the fire from Olympus, and, in direct opposition to the will of Zeus, the father of the Olympian Gods, gave it (and, thus, hope) to humans. This defiant act enraged Zeus, who severely punished Prometheus to be helplessly chained on the mountain Caucasus while a portion of his liver was eaten by a bird of prey every day. Because the liver regenerated overnight, this situation provided endless food supply to the bird of prey but subjected Prometheus to eternal torture. This Greek myth makes a great story but is also notable because it is part of the tradition of a civilization, which emphasized exclusively thinking approaches versus experimentation. Most remarkable is the "kernel" of truth, which the Prometheus myth contains. Organ and tissue regeneration in humans are extremely limited with the following very few exceptions: liver is the only human organ, which can regenerate completely from a minimum of 25% of its original mass [6]. As far as other tissues are concerned, bone fractures (but not nonunion bone defects) and small skin cuts heal through respective tissue regeneration processes.

1.1 Traditional External Prosthetic Devices

Through the millennia, medical responses to human loss of body parts (such as lower limbs and arms) were limited and involved exclusively external prostheses. Ancient treatments/solutions used "ordinary" materials such as wood, leather, string, etc., to assemble prostheses. An example is the replacement of the amputated right big toe of a 3000-year-old Egyptian mummy; this replaced appendage was composed of carved wood, attached to a piece of leather, and secured onto the foot of the recipient with string [7]. Undoubtedly, this is an anatomically correct and aesthetically acceptable prosthesis; its functional aspect, however, is questionable when one considers that the footwear of ancient Egyptians were sandals.

Prosthetic devices replacing lost parts of the body are also found in the literature. The legendary Captain Hook, in the children's fictional story of Peter Pan, was infamous for his bad deeds but also scary because he had a metal hook replacement for his lost left arm. Although anatomically incorrect, and aesthetically unacceptable, this prosthesis was most functional because it gave Captain Hook a reliable way to hold on (indeed for dear life) when his ship encountered bad weather in the open seas and, thus, prevented him from being swept away in the stormy waters.

As recently as the beginning of the 20th century, prosthetic limbs available to patients who needed them were composed of "ordinary" materials (such as wood and metals) and were anthropomorphic (with stiff and nonfunctional parts such as fingers) renditions of human arms and hands [8]. Synthetic materials (such as plastics and metals) that had been developed for various other industrial and engineering applications were used for making prosthetic devices. Attempts to incorporate functionality into prosthetic hands led to the inclusion of cylindrical tubes one each for the four fingers. Later on, incorporation of mechanical features enabled such prototypes of human hands to have motor control of the finger motions required for actions such as playing the piano [8]. Further advancements in design addressed the sensor needs of prosthetic hands. These devices used various materials and electronic parts and added a "thumb part", thus providing the hand prosthesis with the capability of grasp. During the 21st century, the needs of wounded soldiers promoted development of magnificent prosthetic arms and hands composed of "modern" materials (including various plastics, metals, and textiles); most importantly, these prostheses included sophisticated and advanced electronics attached to the muscles and neurons of the recipient amputee, thus allowing control of desirable movements and commands such as grasping, picking up objects, bringing a bottle to the mouth, and successfully drinking the contained water without any spills [9]. Another very important development of prosthetic hands was incorporation of mechanical and electronic parts that provided the recipient patients with the sensation of touch (i.e., recognizing hard and soft items) and pressure (e.g., while shaking hands) [10, 11].

During the aforementioned times, development of prosthetic legs paralleled that of arms and hands. Anthropomorphic but unyielding, one-piece (including monolithic wooden foot, calf, knee, and part of the thigh) legs were eventually replaced with devices which did not look like the human limbs but had unparalleled advantages. Such prostheses, composed of "modern" materials which are light and have the appropriate mechanical properties (such as strength in carrying mechanical loads during human locomotion, flexibility, etc.), provide needed support for several movements of the human body such as standing up and sitting down, walking (on various terrains including not only flat but also inclined surfaces and staircases) [12], jogging and running, swimming [13], and even surfing [14].

It should be noted that, in addition to humans, small (such as kittens) and big (such as elephants) animals need, and thus have received, prosthetic devices for missing or injured limbs [15]. Such prostheses are appropriately designed and scaled, either up or down, and are composed of similar materials as those used for applications to human patients. Veterinary care of pets, animals used in racing events, animals kept in zoos, and farm animals constitutes a sizable and major market for veterinary care and many biomedical applications including prosthetic devices.

1.2 Implantable Devices

External prosthetic devices have helped patients, but they neither addressed nor resolved medical needs requiring replacement, healing, and regeneration of mammalian tissues and organs; such needs require implantable devices. Both external and implantable prostheses are composed of materials and contain combinations of mechanical and electrical parts. In contrast to the temporary and intermittent use of the traditional, external prosthetic devices, implants are placed inside the bodies of patients (humans and/or animals) for various but mostly long-term time-periods, specifically the lifespan of the recipient.

Initially, synthetic materials developed for non-biomedical applications were chosen and used for biomedical implants; pertinent examples include elastic undergarment textiles to produce synthetic tubular substitutes for blood vessels, and titanium/titanium alloys (relatively light but strong materials) for load-carrying orthopedic implants (such as total hip prostheses). Because such cavalier, trial-and-error approaches resulted in dramatic failures (including implant malfunction and rejection), it became obvious that successful biomedical implant materials and devices require new specifications and must fulfill different criteria. Implant biomaterials (the components of implant devices) were eventually designed to be nontoxic, non-pyrogenic, nonallergenic, etc. Adherence to, and applications of, these requirements resulted in avoidance of implant rejection. For a while, attainment of a state of "tolerance" by the surrounding milieu inside the recipient patients' bodies became an acceptable clinical outcome, despite the fact that those implants did not produce desirable and controlled interactions with surrounding tissues and did not trigger the normal tissue-healing process at the implant-tissue interface.

1.3 Materials Used for Implants

By definition, a "biomaterial" is either a natural or synthetic material used for biomedical applications to support, enhance, or replace damaged tissue or a biological function [16].

A summary of the types of materials which have been used for implants is given in Table 1.1.

Table 1.1 Types of materials used for implants.

• Natural (e.g., collagen, fibrin, etc.)
• Products of organic chemistry synthesis
• Special formulation: Biodegradable polymers

• Natural (e.g., devitalized bone, coral, etc.)
• Products of inorganic chemistry synthesis

• Ores

• Products of two or more the above materials
• Products of various...