Electro-mechanical Factors Regulating Bone Architectur 



81 



fibrils are the most likely source of stress-induced potentials. On the other hand, 

 Bassett and Becker postulated in 1962 that bone has solid-state, semiconductor 

 properties, associated most probably with the collagen-apatite units (Becker et al., 

 1964) and that these units may form piezo-electric p-n junctions responsible for 

 electrogenesis in bone. The generator units are so small, however, that it is quite 

 impossible to determine directly their electrical properties. Therefore, both opinions 

 about their nature are based on indirect evidence derived from the behavior of 

 sizeable masses of whole bone. Although both views are necessarily speculative, it 

 does not follow that both are equally valid. For example, it should be noted that the 

 investigations of Fukada and Yasuda (1957), Fukada et al. (1959) and Shamos et al. 

 (1964) have been conducted with dry bone and dry tendon. Since collagen exists 

 in vivo in a hydrated, rather than dry, state, Bassett and Becker (1962) employed 

 wet tendon and wet collagen from decalcified bone as controls for their studies of the 

 electrical responses of bone to deformation. With this type of hydrated preparation, 

 no electrical activity was detected in response to bending. Recent studies in these 

 laboratories have yielded additional data that may be helpful in understanding the 

 apparent differences in the electrical be- 

 havior of dry and wet collagen, aside 

 from the obvious alterations in resistance, 

 possibly, capacitance. 



Electrical output from hydrated bone 

 strips, mounted and deformed as cantil- 

 evers, was measured and found to be 

 nearly a linear function of the amount of 

 deformation (Fig. 2) (Cochran et al., 

 unpublished). The plots resembled the 

 classic, stress-strain relationship only until 

 the plastic range was reached; thereafter, 

 there was a diminution in the rate of 

 increase of electric output. The roll off 

 was most marked, however, in thicker 

 specimens, since they reached the plas- 

 tic range with significantly less defor- 

 mation. 



Bone has been classed as a two-phase 

 material by Currey (1964). As he points 

 out, this class of materials, of which fiber- 

 glass is an excellent example, has a of elasticity intermediate between that of their 

 two components. In bone, collagen is the low modulus component and apatite, the 

 high. While each collagen fibril is encrusted in mineral, the individual units probably 

 are held together by a third material acting as a cement, so that in actuality, bone is 

 a three-phase material. On the basis of this concept and the known ultrastructural 

 features of osseous matrix, it seems reasonable that significant stress will develop 

 at the junctional zone between the flabby collagen and the rigid amalgam of encrust- 

 ing minearl-cement, whenever bone is subjected to any deforming force (Fig. 3). 

 Such a system is admirably fitted for electrogenesis if the generator unit is postulated 

 to be a collagen-apatite, p-n junction with piezo-electric properties. Its operation 



Fig. 1. Electrical output of strips of canine femur (0.6, 

 1.2, and 2.4 mm. in width) as a function of amount of 

 deformation. Broken line represents the region in which 

 plastic phenomena were observed, i.e., incomplete 

 elastic recoil after deforming force removed. 2.4 mm. 

 samples fractured routinely between 4 and 6 mm. (400 

 and 600"/ii) of a standard 1 mm. deformation 



3'''' Europ. Symp. on Cal. Tissues 



