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Formation of bone electrets and their charge decay characteristics
Formationof bone electrets and their charge decay characteristics W.H.Andrabi and J. Behari* School of Environmenral Sciences, Jawaharlal (Received 12 May 1980; revised 3 September
Nehru 19801
University,
New Delhi -
110067,
India.
Therm0 electret, electro electret and magneto electret phenomena have been examined for bone, collagen and apatite. It is found that they acquired a saturation charge of about 10-6C/cm2-10-8C/cm2 when subjected to moderate temperature, electric and high magnetic field treatments. A significant fraction of charge developed is found to be retained in these materials for a period of several months. A possible explanation for the mechanism at work is suggested. A relationship between energy storage and development of bone is perceived.
Electrets, the electrostatic analogue of magnets first prepared by Eguchi’, are polarized dielectrics. The striking characteristic of electrets is the retention of electrostatic charge for a long time, if kept properly. The study of electret effects in various types of dielectric has elucidated the mechanism of polarization and absorption. The properties of electrets depend upon the nature of the material and the conditions under which they are processed. Common types of electrets are ‘electro electret’; ‘therm0 electret’ and ‘magneto electret’*. The most widely accepted theory attributes the electret effect to different field producing mechanisms. They are superposed in such a way that first one and then the other predominates3. Interest in the electret state in certain biomaterials has increased recently. The presence of charge storage has been reported for bone, blood vessel wall materials, keratin, cellulose, DNA and many other biopolymers4. The induction of callus formation in bone as a result of contact with a Teflon@ electret indicates that the latter might affect the normal physiological processes in bone in vivo5. This gives rise to a renewed search for energy storage capability - the result of application of various physical factors - of biomaterials such as bone, which is an important parameter operating at the physiological and structural level of this tissue. The wide ranging bio-electrical phenomena prevalent in biosystems suggest that electrical stimulation is one of the parameters affecting them. The possibility of charge retention in bone suggeststhe feasibility of a simultaneous application of magnetic field for purposes of locating discrete effects, if any, produced by it on the conduction phenomena of the material. Further, in case of thermoelectrets, the effect of heating the sample under different electric fields would elucidate the mechanism operating in carrier transport. The fact that bone possessesvarious other solid state properties’, which can be perturbed in a variety of ways, provides additional impetus to such investigations. *Address
all correspondence
to Dr. J. Behari.
0142-9612/81/020120-04$02.00 120
Biomaterials
1981,
0 Vol2
April
1981
IPC Business
Press
Mascarenha8’ studied the electret effect of bone qualitatively and pointed out that its mineral component does not show electret behaviour owing to the absence of piezoelectric phenomenon. The data on bone electret reported so far has largely been obtained under varied physical and physiological conditions’**. In the present investigation an attempt was made to study the energy storage capacity and charge decay characteristics of bone and its two major components i.e. collagen and apatite, after subjecting them to different treatments. An aspect of interest is the charge storage property of apatite and the long term existence of an electret state in all three materials. Recently’ the present authors observed a Hall effect in bone and its two components and reported the anomalous behaviour of apatite”. While the Hall voltage for apatite varied with the magnetic field, it did not do so.for bone and collagen under identical conditions. The electret phenomenon has also been observed” in a large number of organic compounds under given sets of conditions. A similar methodology was adopted in the present work in order to seek some comparison with these materials. This approach was also expected to be helpful in eliciting some information regarding the mechanism involved in such processes. In the case of bone thermoelectret, studies were carried out in the region of moderately high electric field and a temperature range where some other data on bone behaviour are already available’2*‘3. Electroelectrets are magneto electrets for bone and were prepared in a way similar to that used for other organic materials, and their characteristic patterns are reported.
MATERIALS
AND METHODS
Bone samples of tibiae were obtained from freshly sacrificed rabbits and goats. The details regarding the treatment of bone and recovery of collagen and apatite have already been described elsewhereg.
Bone electrek:
The specimens of rectangular shape were sandwiched by pasting colloidal silver paste in amyl acetate and mounted on an electret holder. The connections were taken with 40 gauge enamelled copper wire placed on the two sides of the specimen. The battery connections are shown in Figure 7. All the connections were performed with grounded shielded wires and the various voltage and currents were measured with a high input impedance 616 Keithley electrometer. A measurement of total polarization (charge/cm2) was obtained by calculating the area under the current versus time plot in the usual manner. About twenty samples of each of the three materials bone, collagen and apatite were studied. The repeat observations on the samples were performed only after they lost the electret effect.
Therm0
electret
W. H. Andrabi
and J. Behari
The data for electro electrets and magneto electrets presented here correspond to room temperature and normal humidity levels, and their variation was within five percent during the course of experiments.
RESULTS
AND DISCUSSION
For therm0 electrets the fall of charge/cm2 with time, in the case of bone, collagen and apatite is plotted (Figure 2) at indicated temperatures and electric fields. Figures 3 and 4 represent the fall of charge/cm2 with time in the case of bone, collagen and apatite for electro electrets and magneto electrets respectively at indicated electric and magnetic field. The rate of fall of charge under identical conditions did not show any marked difference for the samples obtained from goat and rabbit. Further, the electret effects were found reproducible in the same sample after it had lost
This type of electret was prepared by placing the electret holder in a temperature-controlled chamber. The temperature of this chamber was allowed to stabilize for at least one hour before taking the readings. The specimens were subjected to a particular field strength at the required temperature for thirty minutes before the saturation currents were recorded. Field was then put off and the fall of current with time was recorded till it became almost constant. In the second set of experiments the temperature was varied and the entire procedure repeated. In this way the samples were polarized at three temperatures viz. 35’C, 45OC and 65’C at different field strengths namely 1 Kv/cm, 3 Kvlcm, 4 Kv/cm and 5 Kvlcm. From these observations a charge/cm2 vs time graph was obtained.
1
--_-_---_-____ - --=----___-_--__-_r______-_-~~
--_
---_------------
Electro electret J
This type of electret was prepared by applying a high electric field across the specimen sandwich for about thirty minutes and then putting off the field. The charge/cm2 vs time graph was obtained for different electric fields as stated above.
I
2
3
4
5
6
20
60
Time (m:n ) Figure 2a Charge/area vs. Time plot for bone{-. -. -. -J collagen (-J and apatite I---J at 35oC (therm0 electretl. The curves correspond to applied electric field strength of 7 K V/cm, (OJ; 3 K V/cm, (Al; 4 K V/cm (*I and 5 K V/cm, (Al.
Magneto electret In this case the polarization was carried out by placing the sample between the two pole pieces of an electromagnet. A weak electric field and a high magnetic field of - 16 K Gauss was applied across the specimen sandwich. At the end of ten minutes both electric and magnetic fields were put off and the charge/cm’ vs time graph was plotted. In all the three cases a similar method of observation was adopted for collagen and apatite.
10-e
“E ,”
B
E
-0
2
10-7
2
F
2
P
u
168
Sample
I
-f
Figure
1
Experimental
I
I
I
I
I
1
I
2
3
4
5
Time
T
set up for studies on bone
(min
6
20
60
1
Figure 26 Charge/area vs. Timeplot for. bone (-. -. -I collagen I-J and apatite I- - -J at 45DC (therm0 electret). The curves correspond to applied electric field strengths of 1 KV/cm, (oJ;
electret.
3 KV/cm
@J and 5 K V/cm.
kJ.
Biomaterials
198 1, Vol2
April
12 1
Bone electrets:
W.H. Andrabi
and J. Behari
I
2
3
4
Time
Figure 3 Charge/area vs. Time plot for bone I-. -. l-j and apa tite I- --I at different electric field. of applied field were for bone, 5 KV/cm, 10 K V/cm for collagen, 5 K V/cm, 7 K V/cm and 10 K V/cm, for 12 KV/cm and 18 KV/cm.
5
4 Tune
5
6
20
1981,
Vol2
April
60
-I collagen The magnitude and 16 KV/cm; apatite, 10 KV/cm.
60
the polarization induced earlier. Since statistical data revealed P > 0.1, therefore, variation is of less significance. It is evident that the rate of fall of charge in the first ten seconds in the case of therm0 electrets is faster compared to the other two cases, when the respective treatments are withdrawn. The charge then continues to decrease rather slowly for another ten minutes. There is a further decline in the rate of fall and it almost ceases after about thirty minutes. All the three materials (viz., bone, collagen and apatite) show the electret state and are able to retain large amounts of polarization (10’6col/cm2-10-8col/cm2) in the region of field under study. This value is comparable to the polarization storage obtained with good electrets”. A comparison of Figures 2a-c with 3-4 brings out the point that electro electrets are more efficient in terms of storage of charge. A comparison with magneto electret suggeststhat a high magnetic field (- 16 K Gauss) produces an electret effect which is significantly less in magnitude for a sample of almost identical dimensions. This emphasizes the well known fact that the magnetic field by itself is less important in controlling and affecting the biological processes. Figure 2 further demonstrates that keeping the temperature constant, the charge storage capacity increases sharply for a moderate increase in electric field. The electrets prepared in this way retain the charge for as many as seven days without any significant decrease in its magnitude. However, monthly measurements of charge in the same samples revealed that charge retention diminishes by one or two orders of magnitude within a period of five months. The efficiency of retaining charge for apatite is comparable with those of bone and collagen when preserved and tested under identical conditions. In therm0 electrets a manifold increase in the charge accompanies an increase in temperature. The bone electret when placed in position for about twenty four hours at freezing temperature loses the electret effect but recovers it immediately after the temperature is raised. Such a behaviour suggeststhat at lower temperature the mobile charges are in a state of ‘freeze’ in an ordered arrangement, so that they produce no net electric field. At higher temperatures, however, the packed arrangement is broken, resulting in a net polarization charge. This
Biomaterials
20
( mm)
Figure 2c Charge/area vs. Time plot for bone (- . - . -I collagen &--I and apatite I---) at temperature of Ss”C (therm0 electret). The curves correspond to applied electric field strengths of lKV/cm, lol;3KV/cm. iAI;and 5KV/cm, (*I.
122
5
(mm)
-----II---------
$8 b
-----_____-.
--__ -----__ \\ \ ----- _____ -____________ \;===_- _- _- _- _- _- _-_-_y_-_ -------- _=:z ____------------=-
.z
__z---L-l
,(yO
- -
______
5
t ,;
2
-5723 4
1
Time (mln) Figure 4 Charge/area vs. Time plot for bone (-. -. -J collagen (--I and apatite (---I at magnetic field strength of 16 K Gauss. The measurements were taken at electric field strengths of lo,20 and 40 V/cm for all the three samples.
confirms the important role of bound water in such processes. It may be suggested that the sources of electric fields are dipoles4, ionic space charge and the phenomenon of protonic conduction’3 (H*-vacancy migration). On switching off the electric field, change in alignment takes place and this accounts for the decay of charge with time. A similar mechanism may be assumed to prevail in collagen and apatite. The behaviour observed here is similar to Cat reported for other dielectrics’4*‘5. Our results on collagen are in conformity with the findings of Athenstaedt16 and Sawyer et al.” who found the existence of permanent electrostatic field in it. However, they are at variance with earlier reports4 in so far as existence of electret state in apatite is concerned. Hall effect9 measurement have revealed some similarity in the behaviour of all the three materials. The electret behaviour of apatite can therefore be attributed to asymmetry in crystal structure produced during its extraction from whole bone and some inherent lattice defects. Since electrical activity is an inherent property of bone in vivo, it is interesting to compare them with our findings. Quite recently the role of electret effect in bone in viva has been substantiated’. It has been suggested that the generation and reorientation of the permanent dipoles as a result of external stimulus seem to be in a direction which controls the potential variation for enhancement of bone growth. Thus the areas of active growth have higher electrical activity. The reports that Teflon electrets are able to induce callus formation 5*‘8 suggestsa clear relationship between the charge storage property of bone and callus formation. It
Bone electrets:
may be concluded that the bone electrets be employed instead of other electrets for laying the callus to avoid the hazards of repeated surgery. Work in this direction is in progress in our laboratory. Further, the body temperature does not change significantly and the self electric field generated by the body must be inducing charge storage capacity in the bone which can, in turn, stimulate the electrical activities taking place in an around the bone. The effect so induced may be important in the control and growth processes in bone.
6 7 8 9 10
11 12 13 14
REFERENCES 1 2 3 4 5
Eguchi. M., Phil. Mag. 1925,49,1?8 Bhatnagar, C.S., Indian J. Pure and Appl. Phys. 1964.2, Natarajan, S.. J. Appl. Phys. 1972,43, 11,4535 Mascarenhas, S., Ann. N. Y. Acad. Sci. 1974,238,36 Yasuda, I., Clin. Orthop. Ref. Res. 1977, 124, 53
331
15 16 17 18
W.H. Andrabi
Cope, F.W., J. Biol. Phys. 1965,3,1 Mascarenhas, S., in: Ektrets (Ed. M.
and J. Behari
Perlman),The
Electro-
chemistry Society, Inc., New York, N.Y. 1973,657 Behari, J. and Singh S., Med. Biol. Engng. Comput. (England) in Press, 1980 Behari, J. and Andrabi, W.H., Connect. Tiss. Res. 1978, 6,181 Behari, J. and Andrabi, W.H., Proc. 7th New England (North East) Bioengineering Conference, New York (Ed. Lee E. Ostrander), Pergamon Press, USA, 1979,420 Jain, V.K., Ph.D. thesis submitted to I.I.T., Delhi (1970) Behari, J., Guha, S.K. and Agarwal, P.N., Connect. Tiss. Res. 1974,2,325 Behari, J., Guha, S.K. and Agarwal. P.N.. Calcif. Tiss. Res. 1975,19,223 Jain, V.K. and Pillai, P.K.C., J. Appl. Phys. 1969,40, (91 3487 Nair, C.K., Ph.D. thesis submitted to I.I.T., Delhi (1976) Athenstaedt, H., Nature 1970,228 (28) 830 Sawyer, P.N. and Pate, J.W., Amer. J. Physiol. 1953,175, 113 Shiro. I., Tosh Iroohashio, Yasuda, I. and Fukada, Z., C/in. Orthop. Ref. Res. 1977. 124, 57
Bioma terials 198 1, Vol 2 April
123
Nehru 19801
University,
New Delhi -
110067,
India.
Therm0 electret, electro electret and magneto electret phenomena have been examined for bone, collagen and apatite. It is found that they acquired a saturation charge of about 10-6C/cm2-10-8C/cm2 when subjected to moderate temperature, electric and high magnetic field treatments. A significant fraction of charge developed is found to be retained in these materials for a period of several months. A possible explanation for the mechanism at work is suggested. A relationship between energy storage and development of bone is perceived.
Electrets, the electrostatic analogue of magnets first prepared by Eguchi’, are polarized dielectrics. The striking characteristic of electrets is the retention of electrostatic charge for a long time, if kept properly. The study of electret effects in various types of dielectric has elucidated the mechanism of polarization and absorption. The properties of electrets depend upon the nature of the material and the conditions under which they are processed. Common types of electrets are ‘electro electret’; ‘therm0 electret’ and ‘magneto electret’*. The most widely accepted theory attributes the electret effect to different field producing mechanisms. They are superposed in such a way that first one and then the other predominates3. Interest in the electret state in certain biomaterials has increased recently. The presence of charge storage has been reported for bone, blood vessel wall materials, keratin, cellulose, DNA and many other biopolymers4. The induction of callus formation in bone as a result of contact with a Teflon@ electret indicates that the latter might affect the normal physiological processes in bone in vivo5. This gives rise to a renewed search for energy storage capability - the result of application of various physical factors - of biomaterials such as bone, which is an important parameter operating at the physiological and structural level of this tissue. The wide ranging bio-electrical phenomena prevalent in biosystems suggest that electrical stimulation is one of the parameters affecting them. The possibility of charge retention in bone suggeststhe feasibility of a simultaneous application of magnetic field for purposes of locating discrete effects, if any, produced by it on the conduction phenomena of the material. Further, in case of thermoelectrets, the effect of heating the sample under different electric fields would elucidate the mechanism operating in carrier transport. The fact that bone possessesvarious other solid state properties’, which can be perturbed in a variety of ways, provides additional impetus to such investigations. *Address
all correspondence
to Dr. J. Behari.
0142-9612/81/020120-04$02.00 120
Biomaterials
1981,
0 Vol2
April
1981
IPC Business
Press
Mascarenha8’ studied the electret effect of bone qualitatively and pointed out that its mineral component does not show electret behaviour owing to the absence of piezoelectric phenomenon. The data on bone electret reported so far has largely been obtained under varied physical and physiological conditions’**. In the present investigation an attempt was made to study the energy storage capacity and charge decay characteristics of bone and its two major components i.e. collagen and apatite, after subjecting them to different treatments. An aspect of interest is the charge storage property of apatite and the long term existence of an electret state in all three materials. Recently’ the present authors observed a Hall effect in bone and its two components and reported the anomalous behaviour of apatite”. While the Hall voltage for apatite varied with the magnetic field, it did not do so.for bone and collagen under identical conditions. The electret phenomenon has also been observed” in a large number of organic compounds under given sets of conditions. A similar methodology was adopted in the present work in order to seek some comparison with these materials. This approach was also expected to be helpful in eliciting some information regarding the mechanism involved in such processes. In the case of bone thermoelectret, studies were carried out in the region of moderately high electric field and a temperature range where some other data on bone behaviour are already available’2*‘3. Electroelectrets are magneto electrets for bone and were prepared in a way similar to that used for other organic materials, and their characteristic patterns are reported.
MATERIALS
AND METHODS
Bone samples of tibiae were obtained from freshly sacrificed rabbits and goats. The details regarding the treatment of bone and recovery of collagen and apatite have already been described elsewhereg.
Bone electrek:
The specimens of rectangular shape were sandwiched by pasting colloidal silver paste in amyl acetate and mounted on an electret holder. The connections were taken with 40 gauge enamelled copper wire placed on the two sides of the specimen. The battery connections are shown in Figure 7. All the connections were performed with grounded shielded wires and the various voltage and currents were measured with a high input impedance 616 Keithley electrometer. A measurement of total polarization (charge/cm2) was obtained by calculating the area under the current versus time plot in the usual manner. About twenty samples of each of the three materials bone, collagen and apatite were studied. The repeat observations on the samples were performed only after they lost the electret effect.
Therm0
electret
W. H. Andrabi
and J. Behari
The data for electro electrets and magneto electrets presented here correspond to room temperature and normal humidity levels, and their variation was within five percent during the course of experiments.
RESULTS
AND DISCUSSION
For therm0 electrets the fall of charge/cm2 with time, in the case of bone, collagen and apatite is plotted (Figure 2) at indicated temperatures and electric fields. Figures 3 and 4 represent the fall of charge/cm2 with time in the case of bone, collagen and apatite for electro electrets and magneto electrets respectively at indicated electric and magnetic field. The rate of fall of charge under identical conditions did not show any marked difference for the samples obtained from goat and rabbit. Further, the electret effects were found reproducible in the same sample after it had lost
This type of electret was prepared by placing the electret holder in a temperature-controlled chamber. The temperature of this chamber was allowed to stabilize for at least one hour before taking the readings. The specimens were subjected to a particular field strength at the required temperature for thirty minutes before the saturation currents were recorded. Field was then put off and the fall of current with time was recorded till it became almost constant. In the second set of experiments the temperature was varied and the entire procedure repeated. In this way the samples were polarized at three temperatures viz. 35’C, 45OC and 65’C at different field strengths namely 1 Kv/cm, 3 Kvlcm, 4 Kv/cm and 5 Kvlcm. From these observations a charge/cm2 vs time graph was obtained.
1
--_-_---_-____ - --=----___-_--__-_r______-_-~~
--_
---_------------
Electro electret J
This type of electret was prepared by applying a high electric field across the specimen sandwich for about thirty minutes and then putting off the field. The charge/cm2 vs time graph was obtained for different electric fields as stated above.
I
2
3
4
5
6
20
60
Time (m:n ) Figure 2a Charge/area vs. Time plot for bone{-. -. -. -J collagen (-J and apatite I---J at 35oC (therm0 electretl. The curves correspond to applied electric field strength of 7 K V/cm, (OJ; 3 K V/cm, (Al; 4 K V/cm (*I and 5 K V/cm, (Al.
Magneto electret In this case the polarization was carried out by placing the sample between the two pole pieces of an electromagnet. A weak electric field and a high magnetic field of - 16 K Gauss was applied across the specimen sandwich. At the end of ten minutes both electric and magnetic fields were put off and the charge/cm’ vs time graph was plotted. In all the three cases a similar method of observation was adopted for collagen and apatite.
10-e
“E ,”
B
E
-0
2
10-7
2
F
2
P
u
168
Sample
I
-f
Figure
1
Experimental
I
I
I
I
I
1
I
2
3
4
5
Time
T
set up for studies on bone
(min
6
20
60
1
Figure 26 Charge/area vs. Timeplot for. bone (-. -. -I collagen I-J and apatite I- - -J at 45DC (therm0 electret). The curves correspond to applied electric field strengths of 1 KV/cm, (oJ;
electret.
3 KV/cm
@J and 5 K V/cm.
kJ.
Biomaterials
198 1, Vol2
April
12 1
Bone electrets:
W.H. Andrabi
and J. Behari
I
2
3
4
Time
Figure 3 Charge/area vs. Time plot for bone I-. -. l-j and apa tite I- --I at different electric field. of applied field were for bone, 5 KV/cm, 10 K V/cm for collagen, 5 K V/cm, 7 K V/cm and 10 K V/cm, for 12 KV/cm and 18 KV/cm.
5
4 Tune
5
6
20
1981,
Vol2
April
60
-I collagen The magnitude and 16 KV/cm; apatite, 10 KV/cm.
60
the polarization induced earlier. Since statistical data revealed P > 0.1, therefore, variation is of less significance. It is evident that the rate of fall of charge in the first ten seconds in the case of therm0 electrets is faster compared to the other two cases, when the respective treatments are withdrawn. The charge then continues to decrease rather slowly for another ten minutes. There is a further decline in the rate of fall and it almost ceases after about thirty minutes. All the three materials (viz., bone, collagen and apatite) show the electret state and are able to retain large amounts of polarization (10’6col/cm2-10-8col/cm2) in the region of field under study. This value is comparable to the polarization storage obtained with good electrets”. A comparison of Figures 2a-c with 3-4 brings out the point that electro electrets are more efficient in terms of storage of charge. A comparison with magneto electret suggeststhat a high magnetic field (- 16 K Gauss) produces an electret effect which is significantly less in magnitude for a sample of almost identical dimensions. This emphasizes the well known fact that the magnetic field by itself is less important in controlling and affecting the biological processes. Figure 2 further demonstrates that keeping the temperature constant, the charge storage capacity increases sharply for a moderate increase in electric field. The electrets prepared in this way retain the charge for as many as seven days without any significant decrease in its magnitude. However, monthly measurements of charge in the same samples revealed that charge retention diminishes by one or two orders of magnitude within a period of five months. The efficiency of retaining charge for apatite is comparable with those of bone and collagen when preserved and tested under identical conditions. In therm0 electrets a manifold increase in the charge accompanies an increase in temperature. The bone electret when placed in position for about twenty four hours at freezing temperature loses the electret effect but recovers it immediately after the temperature is raised. Such a behaviour suggeststhat at lower temperature the mobile charges are in a state of ‘freeze’ in an ordered arrangement, so that they produce no net electric field. At higher temperatures, however, the packed arrangement is broken, resulting in a net polarization charge. This
Biomaterials
20
( mm)
Figure 2c Charge/area vs. Time plot for bone (- . - . -I collagen &--I and apatite I---) at temperature of Ss”C (therm0 electret). The curves correspond to applied electric field strengths of lKV/cm, lol;3KV/cm. iAI;and 5KV/cm, (*I.
122
5
(mm)
-----II---------
$8 b
-----_____-.
--__ -----__ \\ \ ----- _____ -____________ \;===_- _- _- _- _- _- _-_-_y_-_ -------- _=:z ____------------=-
.z
__z---L-l
,(yO
- -
______
5
t ,;
2
-5723 4
1
Time (mln) Figure 4 Charge/area vs. Time plot for bone (-. -. -J collagen (--I and apatite (---I at magnetic field strength of 16 K Gauss. The measurements were taken at electric field strengths of lo,20 and 40 V/cm for all the three samples.
confirms the important role of bound water in such processes. It may be suggested that the sources of electric fields are dipoles4, ionic space charge and the phenomenon of protonic conduction’3 (H*-vacancy migration). On switching off the electric field, change in alignment takes place and this accounts for the decay of charge with time. A similar mechanism may be assumed to prevail in collagen and apatite. The behaviour observed here is similar to Cat reported for other dielectrics’4*‘5. Our results on collagen are in conformity with the findings of Athenstaedt16 and Sawyer et al.” who found the existence of permanent electrostatic field in it. However, they are at variance with earlier reports4 in so far as existence of electret state in apatite is concerned. Hall effect9 measurement have revealed some similarity in the behaviour of all the three materials. The electret behaviour of apatite can therefore be attributed to asymmetry in crystal structure produced during its extraction from whole bone and some inherent lattice defects. Since electrical activity is an inherent property of bone in vivo, it is interesting to compare them with our findings. Quite recently the role of electret effect in bone in viva has been substantiated’. It has been suggested that the generation and reorientation of the permanent dipoles as a result of external stimulus seem to be in a direction which controls the potential variation for enhancement of bone growth. Thus the areas of active growth have higher electrical activity. The reports that Teflon electrets are able to induce callus formation 5*‘8 suggestsa clear relationship between the charge storage property of bone and callus formation. It
Bone electrets:
may be concluded that the bone electrets be employed instead of other electrets for laying the callus to avoid the hazards of repeated surgery. Work in this direction is in progress in our laboratory. Further, the body temperature does not change significantly and the self electric field generated by the body must be inducing charge storage capacity in the bone which can, in turn, stimulate the electrical activities taking place in an around the bone. The effect so induced may be important in the control and growth processes in bone.
6 7 8 9 10
11 12 13 14
REFERENCES 1 2 3 4 5
Eguchi. M., Phil. Mag. 1925,49,1?8 Bhatnagar, C.S., Indian J. Pure and Appl. Phys. 1964.2, Natarajan, S.. J. Appl. Phys. 1972,43, 11,4535 Mascarenhas, S., Ann. N. Y. Acad. Sci. 1974,238,36 Yasuda, I., Clin. Orthop. Ref. Res. 1977, 124, 53
331
15 16 17 18
W.H. Andrabi
Cope, F.W., J. Biol. Phys. 1965,3,1 Mascarenhas, S., in: Ektrets (Ed. M.
and J. Behari
Perlman),The
Electro-
chemistry Society, Inc., New York, N.Y. 1973,657 Behari, J. and Singh S., Med. Biol. Engng. Comput. (England) in Press, 1980 Behari, J. and Andrabi, W.H., Connect. Tiss. Res. 1978, 6,181 Behari, J. and Andrabi, W.H., Proc. 7th New England (North East) Bioengineering Conference, New York (Ed. Lee E. Ostrander), Pergamon Press, USA, 1979,420 Jain, V.K., Ph.D. thesis submitted to I.I.T., Delhi (1970) Behari, J., Guha, S.K. and Agarwal, P.N., Connect. Tiss. Res. 1974,2,325 Behari, J., Guha, S.K. and Agarwal. P.N.. Calcif. Tiss. Res. 1975,19,223 Jain, V.K. and Pillai, P.K.C., J. Appl. Phys. 1969,40, (91 3487 Nair, C.K., Ph.D. thesis submitted to I.I.T., Delhi (1976) Athenstaedt, H., Nature 1970,228 (28) 830 Sawyer, P.N. and Pate, J.W., Amer. J. Physiol. 1953,175, 113 Shiro. I., Tosh Iroohashio, Yasuda, I. and Fukada, Z., C/in. Orthop. Ref. Res. 1977. 124, 57
Bioma terials 198 1, Vol 2 April
123