Electro-mechanical Factors Regulating Bone Architecture 79 



During recent years, the principle of negative feedback (closed loop) control 

 systems, commonly used in the electronics industry, have been applied in biology. In 

 this application, such a system requires an environmental signal, a sensor to detect 

 and convert the signal to a meaningful biologic response (a transducer), and a sensor 

 to translate this response to action which will stop or correct the original environ- 

 mental signal. McLean (1958) has suggested that the serum calcium is regulated by 

 such a five-part mechanism. A low serum calcium is the signal which triggers the 

 parathyroid gland (transducer # 1) to produce parathormone which, in turn, activates 

 osteoclasts (transducer # 2) to release calcium, thereby raising serum calcium and 

 cutting off the original signal. It now is apparent that Wolff's law probably is 

 another example of a negative feedback control system. In this case, the environ- 

 mental signal and final correcting response have been known for many years. They 

 are, respectively, a deforming force and a change in bone structure, appropriate to 

 resist the applied force. The mechanisms by which one lead to the other, however, 

 could not be known until the character of the transducers and their responses had been 

 identified. 



Since bone is a multicrystalline material, it was postulated in 1951 that it might 

 possess piezo-electric properties (Johnson, personal communication). If such were the 

 case, bone would convert a mechanical signal directly to an electrical signal. This 

 type of transducer response is analogous to that of a crystal in the tone arm of a 

 phonograph. In the mid-fifties, the postulate was confirmed independently in Japan 

 and America by the demonstration that electrical potentials were developed by bone 

 when it is deformed (Fukada and Yasuda, 1957; Bassett, in press). Although 

 additional evidence has been reported recently to substantiate the fact that mechanical 

 stress evokes an electrical response from bone (Bassett and Becker, 1962; Shamos 

 et al., 1963), the origin of this response remains obscure. It is evident, however, that 

 the stress-generated potentials are not dependent upon cell viability, are not related 

 directly to membrane potentials and arise, most probably, in the extracellular 

 osseous matrix. 



Potential differences are generated in certain types of crystal lattices when charges 

 are separated by pressure on the crystal (Mason, 1950). Similar activity results when 

 thin him p-n junctions^ are deformed (Huber, 1963). Furthermore, displacement 

 potentials are generated by bending rod-like polyelectrolytes, such as potassium 

 hyaluronate (Christiansen et al., 1961). If each of these charge separation phe- 

 nomena is classified as piezo-electric, there seems little doubt that the electric potentials 

 produced by stress in bone are also piezo-electric in nature. 



While it may be possible to classify pressure-induced charge generation in the 

 crystal lattices of diverse materials as piezo-electric, it is likely that signals generated 

 by different materials will have different characteristics. For example, the internal 

 resistance and capacitance of the crystal can determine the pattern of electrical pulse 

 for a given deforming force. Furthermore, if the signal is produced by a semicon- 

 ductor, p-n junction device, rectification may possible occur in it or In other devices 

 in the circuit, since current flow across the device is more efficient in forward than in 

 reverse bias. Therefore, it is important not only to demonstrate that electric potentials 



' A p-n junction is formed in a monocrystalline lattice, such as purified germanium, when impurity atoms 

 with an excess of holes, i.e., a deficit of electrons (p-type doping agents) occupy lattice positions on one side 

 of the crystal and atoms with an excess of electrons (n-type doping agents) the other. 



