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Study of the behaviour of irradiated polycaprolactam in the living organism
STUDY OF THE BEHAVIOUR OF IRRADIATED POLYCAPROLACTAM IN THE LIVING ORGANISM* A. N. ROGOVA, T. V. BAZHBEUK-MEL~OVA,T. T. DAUROVAand O. S. VOROI~KOVA A. V. Vishnevskii Institute of Surgery, Central Scientific Research Institute of the Cotton Industry (Receivezl 4 March 1977)
A study was made of the behaviour of surgical polycaprolactam suture in the biological medium of a living organism and the time of complete decomposition determined. By methods of electron raster microscopy and IR spectroscopy a relation was established between the type and degree of decomposition of structures and their structural state determined by the dose of radiation. THE extensive introduction in surgery of polymer products required reliable
methods of sterilization. The bacterial action of ionizing radiation explained the use of radiation as a sterilizing measure. However, the possibility of using the radiation method of sterilization of certain medical preparations is determined in concrete terms in each case since radiation causes structural change and a variation in the properties of polymer materials. Previously, it was found possible to use the radiation method of sterilization (r-sterilization) of surgical sutures prepared from polycaprolactam fibres [1, 2] and establish the lower limits of sterilizing doses for industrial sutures. I t was shown [1] t h a t by the action of sterilizing doses of ?-irradiation the ability of polycaprolactam sutures to "hold" surgical parts considerably improves. This means t h a t on becoming sterile surgical sutures lose one of the main shortcomings of synthetic sutures which is due to the self-undoing of parts. However, for the practical use of surgical threads it was necessary to observe the "destiny" of the polymer in the living organism. I n extensive literature concerning the use of materials prepared from nylon there is practically no information about the changes in the structure and properties of polycaprolactam previously subjected to radiation effects. This paper is concerned with the behaviour of p:,lycaprolactam surgical threads sterilized by ?-rays, in the living organism. The material examined--polyeaprolactam surgical thread No. 5 prepared from fibres of pentahedral cross-section was sterilized by 7 radiation using doses of 2.5; 5.0; 7.0 and 20.0 Mrad. The selection af these doses of radiation was due to medical and chemical stu* Vysokomol. soyed. A19: No. 9, 2120-2125. 1977. 2431
2432
A . N . ROGOVA e~ aZ,
dies previously carried out of all samples, which showed the absence of toxic effect of irradiated threads. Threads sterilized by a conventional method in an autoclave were the control samples. "To avoid various post-operative complications, the threads were freed from o~1before being used by methods previously proposed [3]. Chinchilla type rabbits were used as experimental .animals. Five suture samples, 1 control and four samples irradiated by the doses indicated, were placed simultaneously in the hypodermic tissue of the rabbit on both sides of the spinal column. After given time intervals--3, 7, 12, 30, 60, 150, 210 and 240 days--the animals were .slaughtered. Samples were cut and freed from biological tissues by alkaline hydrolysis of albumen residues with a 10% KOH solution heated to 40° in 10-15 rain. [3]. The following were determined for implanted samples: 1) breaking load and breaking elongation of fibres forming the thread; 2) the weight of samples before and after implantation after preliminary drying to constant values in an oven; 3) linear density (thickness). Apart from the indices listed, the morphological structure of implanted samples was -examined by electron raster microscopy. IR spectroscopy was used to study the variation ~f the chemical structure of polycaprolactam samples taking place during implantation. Kinetics of variation of the relative breaking load, breaking elongation a n d the weight of the implanted polycaprolactam fibres are shown in Fig. la-c, according to the duration of implantation, and the sterilizing dose. I t should be noted t h a t all curves are plotted from average results derived in two series of experiments. I t has previously been established [3] t h a t the time of decomposition in the living organism of an allotransplant in the form of a polycaprolactam prosthetic appliance, sterilized in an autoclave, is about 15 months. I t was indicated t h a t ~he time of disintegration will largely be determined by the structure and properties of the m a t e r i a l selected in each concrete case. In this case the threads became completely resolved (both irradiated and non-irradiated) 8 months after implantation. However, in practice, the fibres lose strength considerably earlier. I f the "working life" of threads is determined b y the time of implantation before the fibres loss strength completely, by extrapolating the curves to axis T we obtain the following "working lives" for irradiated samples: Dose of radiation, Mrad Working life, days
0 100-110
2.5 90-100
5.0 90-100
7.0 20.0 150-160 150-160
The difference in working lives for fibres irradiated by small doses (2.5-5.0 Mrad) and comparatively high (7.0-20.0 Mrad) doses is, evidently, due to the structural difference of fibres subjected to various doses. According to X-ray investigations [2], the most intensive changes in fibre structure which could result in a noticeable effect on hydrolysis in the living organism, occur with small doses (up to 5 Mrad) and fairly high doses (past 20.0 Mrad). Irradiation with doses of 7.0-20.0 Mrad has a conserving effect on the resultant structural change since crosslinklug begins to take place which evidently results in the formation of a structure which is more resistant to biological disintegration. This is confirmed when investigating micro-cracks formed in fibres by the action of the living organism.
2433
Behaviour of irradiated polycaprolactam in living organism
The rate of weight loss, as shown b y Fig. lc, is much lower than the rate at which strength is reduced. I f b y the 4-Sth month of implantation the strength of fibres is zero, weight loss represents 40-50%. This is, apparently, determined b y the t y p e of decomposition of polycaprolactam in the living organism. As indicated [3], hydrolysis of amide bonds b y the action of tissue fluid mainly P, g/~ex n I 2
2O
3# 5
0 l, ~/~
30 "=
90 Time, da//s
150
b
2o
0
3o
wf.,% 100
80 Time, daus
150
5O
0
60
120 Time, daus
180
2//0
FIG. 1. Kinetic curves of the variation o£ relative breaking load P (a), bre~J~ng elongation l (5) and weight (c) of non-iradlated, implant~l fibres (1) and fibres irradiatec~ with doses of 2.5 (g), 5 (3), 7 (4) and 20 (Mrad) (5).
$434
A . N . RoGovA et al.
effected from the fibre surface is one of the main processes causing a very marked change in the behaviour of implanted polyeaprolaetam. Since implanted fibres are in a highly stressed state in the organism, micro-cracks m a y be formed on the one hand hi the adsorption-active medium such as tissue fluid, which increase the specific surface of the sample and on the other, are evidently the main cause of a sudden reduction of strength of the material. Weight variation is due t0 the escape of hydrolytic products from the implanted material -- a fairly long process, which ends in complete resolution of the sample. An analysis of curves in Fig. lc shows that the effect of irradiation on weight loss sustained b y the fibres is only observed with short periods of implantation (up to 2 months): with an increase of the dose, weight loss also increases. However,
FIG. 2. Electron-microscope photographs of the surface of non-irradiated (a, b) and irradiated implanted fibres using a dose of 2.5 Mrad (c) with magnifications of 2500 (a) and 6000 (b, c). with long periods of implantation this difference is eliminated and weight loss sustained b y irradiated fibres does not differ in practice from that of non-irradiated samples. This is understandable if we deal with results of investigating the structure of irradiated fibres [1, 2]. Oxidation and partial disruption of the fibre surface in irradiation makes them more accessible to hydrolysis in the living organism and it has been estab-
Behaviour of irradiatedpolyeaprolactam in living organism
2435
lished that the degree of radiation damage to the surface under these conditions of irradiation shows a linear dependence on the dose. During hydrolysis of internal parts of fibres, less affected b y oxidizing processes, the changes due to irradiation are insufficient to influence the rate of hydrolytic decomposition. Figure 2 shows microphotographs of the most typical patterns of the quality of defect formation in fibres (time of implantation 30 days). Thus, for a control sample the main type of defect formed during implantation is the transverse crack arranged in an irregular manner along the entire length of fibres (Fig. 2a). The degree of increase of these cracks is slight they only extend to the surface layers of the fibre (Fig. 2b).
FIo. 3. Electron-microscope photographs of the surface of implanted fibres irradi-
ated with a dose of 7 (a) and 20 Mrad (b) with magnifications of 2500 (a) and 150 (b). .The qualitative pattern of defects found in fibres irradiated before implantation with doses of 2.5-5.0 Mrad is simple and is shown as follows: the transverse cracks described are formed and situated deep in the fibre and a qualitatively new type of defect is formed which involves the "etching" of very thin surface layers of fibres on fairly large areas of the fibre (Fig. 2c). •The formation of a defect, new from a quality point of view, no doubt involves a change in the fibre surface as a result of the radiation oxidation effect [1]. As far as the quantitative evaluation of defects is concerned, it should be noted that no increase was observed in the number of transverse cracks in irradiated fibres, compared with control ones. Defects were evaluated quantitatively b y counting defects along a given length of fibre. Fibre length in this case was limited b y the dimensions of the holder for the samples ( l : 1 cm). For a sample irradiated b y a dose of 7.0 Mrad, there was a new feature in the pattern of fibre decomposition in the living organism -- longitudinal cracks had formed (Fig. 3a). On further increasing the dose the number of longitudinal cracks and the degree of decomposition of the fibre suddenly increased in places of formation. On superimposing a longitudinal crack on a transverse one, the fibre separates into parts (Fig. 3b), which in the end breaks down the fibre. The formation of longitudinal cracks in implanted fibres is due to the changes
2436
A.N. ROOOVAet ed.
which have taken place in the fibre structure during irradiation using doses in the 7-20 Mrad region. As shown previously [2], these changes involve a changeover of the system of hydrogen bonds, the formation of cross-links and simultaneous breakdown, reduction of the long period of the supermolecular structure b y the amorphous component which in the end results in the formation of a non-equilibrium system. RELATIVE ABSORPTION BAND INTElffSITIES OF SAMPLES IMPLANTED FOR DIFFERENT LENOT]~ OF TIME
Relative band intensity DX670/1570 /)9960/1670 Daa4o/leTO Dsloo/l,7o
Time of implantation days 0
12
14
30
150
210
240
0"65 0"90 1 "00 0.21 0"10 0"07
0"56 0"79
0"58
0"50 0.67
1.09
1.08
1.13
0-15 0.06 0.012
0"17
0.10 0.07 0.13
0"51 0"64 1-08 0"14 0.03 0.05
0-37 0'57 0"93 0"07 0"02 0"04
0"45
0.79 0.08
0"06
0"48
1"17 0"07 0.02: 0"03
Since conditions exist in the organism for crack formation (constant dynamic. load, penetration of tissue fluid and elements of the connecting tissue inside the cracks), crack formation is inevitable in a similar non-equilibrium system and t h e longitudinal nature of the latter is determined by the fibrillar structure of fibres. At the same time as cracks are formed on the implanted fibres, their chemical structure undergoes considerable changes. These changes can be readily observed b y I R spectroscopy; Results obtained when studying I R spectra of fibres, implanted for different periods of time, are tabulated and shown in Fig. 4. As shown b y the Table, absorption band intensity decreases considerably for implanted fibres in the region of 1130, 1570, 2960 and 3100 cm -1. The band at 1570 cm -1 is attributed to vibrations of the - - C O N H - - amide group [4]. A reduction in intensity confirms evidently that par~ of amide bonds breaks down. A reduction in the relative intensity of bands at 2960 cm -1 of bondstretching vibrations of C---H of methylene groups corresponds to decomposition of these bonds. Intermolecular hydrogen bonds of the - - N H group also break down during implantation which is confirmed b y a reduction in the intensity of the band at 3100 cm -~ and the formation of fairly strong absorption in the 3400-3500 cm -1 regiol~, which corresponds to vibrations of free N H groups uncombined with hydr~,gen bonds [5]. The overall state of stress of hydrogen bonds decreases, whic], is confirmed b y a displacement to the high frequency region of the band at 3290 cm -1, which also corresponds to vibrations of - - N H groups combined b y hydrogen bonds.
Behaviour of irradiated polycaprolactam in living organism
2437
Breakdown in implanted polycaprolactam may also be evaluated by the reduction of band intensity at 1130 cm -1 which is attributed to skeletal vibrations of the polyamide chain [4] (Table). Significance is attached to the fact of a complex relation between the variation of absorption band intensity at 936 and 980 cm -1, these bands being structurally sensitive bands in the spectrum of polycaprolactam, and the length o f implantation. Absorption in these regions is determined by the interaction of
\
3q
82
30
2B
' 'lg
17
15
/3
11 .q "v,~lO~ZCm-!
Fro. 4. I R spectra of polyeaprolaetam fibres irradiated with a dose of 20 Mrad for 120
(2), 150 (3) and 240 days (4) before implantation (1) and after implantation.
vibrations in the most ordered (crystalline) ranges [6]. For implanted fibres, as shown by the Table, with brief periods of time (up to 30 days) band intensity at 980 cm -1 increases and then suddenly decreases. This effect may, evidently~ be explained by the fact that the decomposition of polycaprolactam in the living organism mainly begins in the amorphous ranges of the polymer. Owing to t h e increased mobility of macromolecules some increase in structural ordering i~ possible. With more prolonged retention of polycaprolactam in the living organism breakdown also covers the crystalline ranges, which in the end results in the. amorphization of the polymer, expressed in IR spectra in a sudden reduction of band intensity at 980 cm -1 and complete disappearance of the band at 936 cm-L Results obtained for implanted irradiated polycaprolactam fibres are similarto those in Fig. 4, i.e. the chemistry of decomposition of polycaprolactam in t h e living organism is the same for irradiated ~nd non-irradiated surgical suture&
2438
G.M. BARTENEV ~ a~.
, S o m e v a r i a t i o n o f r e s u l t s o b s e r v e d f o r i r r a d i a t e d s u t u r e s is e v i d e n t l y d u e t o a general structural non-equilibrium of fibres due to radiation, T h e a u t h o r s a r e g r a t e f u l to V. L . T s e t l i n t o r t h e u s e I u l c o m m e n t s m a d e
M U T U A L R E L A T I O N OF PROCESSES OF VISCO-ELASTICITY A N D BREAKDOWN * G. M. :BARTEI~EV, YU. A. SII~ICHKI~A and V. V. z~LEKSEYEV I n s t i t u t e of Physical Chemistry, U.S.S.R. A c a d e m y of Sciences
(Received 14 February 1977) Results obtained when studying temperature-time relations of the complex of m o s t i m p o r t a n t mechanical characteristics of crosslinked and non-crosslinked elastomers (stress relaxation viscous flow, breakdown processes related to durability a n d ~ensile stress) and analysis suggest t h a t above the glass temperature and below the temperature of chemical relaxation the temperature relation of visco-elastic processes and processes of breakdown are characterized b y the same activation energy. Since this activation energy is also typical of 2 processes of relaxation in the elastomer, the mechanism of processes of visco-elasticity and decomposition of the elastomer is determined b y the molecular mobility of low molecular weight structures -- micro-units. The difference between all processes is in non-temperature coefficients in viscosity equations, equations of stress relaxation and durability. Tensile stress is characterized b y a more complex temperature relation. The activation energy of these processes is unchanged in relation to stress (up to 100 kg/cm 2) and elongation (up to 300%) and is independen~ o f the fact whether the polymer is crosslinked or not. * Vysokomol. soyed. A19: No. 9, 2126-2131, 1977.
A study was made of the behaviour of surgical polycaprolactam suture in the biological medium of a living organism and the time of complete decomposition determined. By methods of electron raster microscopy and IR spectroscopy a relation was established between the type and degree of decomposition of structures and their structural state determined by the dose of radiation. THE extensive introduction in surgery of polymer products required reliable
methods of sterilization. The bacterial action of ionizing radiation explained the use of radiation as a sterilizing measure. However, the possibility of using the radiation method of sterilization of certain medical preparations is determined in concrete terms in each case since radiation causes structural change and a variation in the properties of polymer materials. Previously, it was found possible to use the radiation method of sterilization (r-sterilization) of surgical sutures prepared from polycaprolactam fibres [1, 2] and establish the lower limits of sterilizing doses for industrial sutures. I t was shown [1] t h a t by the action of sterilizing doses of ?-irradiation the ability of polycaprolactam sutures to "hold" surgical parts considerably improves. This means t h a t on becoming sterile surgical sutures lose one of the main shortcomings of synthetic sutures which is due to the self-undoing of parts. However, for the practical use of surgical threads it was necessary to observe the "destiny" of the polymer in the living organism. I n extensive literature concerning the use of materials prepared from nylon there is practically no information about the changes in the structure and properties of polycaprolactam previously subjected to radiation effects. This paper is concerned with the behaviour of p:,lycaprolactam surgical threads sterilized by ?-rays, in the living organism. The material examined--polyeaprolactam surgical thread No. 5 prepared from fibres of pentahedral cross-section was sterilized by 7 radiation using doses of 2.5; 5.0; 7.0 and 20.0 Mrad. The selection af these doses of radiation was due to medical and chemical stu* Vysokomol. soyed. A19: No. 9, 2120-2125. 1977. 2431
2432
A . N . ROGOVA e~ aZ,
dies previously carried out of all samples, which showed the absence of toxic effect of irradiated threads. Threads sterilized by a conventional method in an autoclave were the control samples. "To avoid various post-operative complications, the threads were freed from o~1before being used by methods previously proposed [3]. Chinchilla type rabbits were used as experimental .animals. Five suture samples, 1 control and four samples irradiated by the doses indicated, were placed simultaneously in the hypodermic tissue of the rabbit on both sides of the spinal column. After given time intervals--3, 7, 12, 30, 60, 150, 210 and 240 days--the animals were .slaughtered. Samples were cut and freed from biological tissues by alkaline hydrolysis of albumen residues with a 10% KOH solution heated to 40° in 10-15 rain. [3]. The following were determined for implanted samples: 1) breaking load and breaking elongation of fibres forming the thread; 2) the weight of samples before and after implantation after preliminary drying to constant values in an oven; 3) linear density (thickness). Apart from the indices listed, the morphological structure of implanted samples was -examined by electron raster microscopy. IR spectroscopy was used to study the variation ~f the chemical structure of polycaprolactam samples taking place during implantation. Kinetics of variation of the relative breaking load, breaking elongation a n d the weight of the implanted polycaprolactam fibres are shown in Fig. la-c, according to the duration of implantation, and the sterilizing dose. I t should be noted t h a t all curves are plotted from average results derived in two series of experiments. I t has previously been established [3] t h a t the time of decomposition in the living organism of an allotransplant in the form of a polycaprolactam prosthetic appliance, sterilized in an autoclave, is about 15 months. I t was indicated t h a t ~he time of disintegration will largely be determined by the structure and properties of the m a t e r i a l selected in each concrete case. In this case the threads became completely resolved (both irradiated and non-irradiated) 8 months after implantation. However, in practice, the fibres lose strength considerably earlier. I f the "working life" of threads is determined b y the time of implantation before the fibres loss strength completely, by extrapolating the curves to axis T we obtain the following "working lives" for irradiated samples: Dose of radiation, Mrad Working life, days
0 100-110
2.5 90-100
5.0 90-100
7.0 20.0 150-160 150-160
The difference in working lives for fibres irradiated by small doses (2.5-5.0 Mrad) and comparatively high (7.0-20.0 Mrad) doses is, evidently, due to the structural difference of fibres subjected to various doses. According to X-ray investigations [2], the most intensive changes in fibre structure which could result in a noticeable effect on hydrolysis in the living organism, occur with small doses (up to 5 Mrad) and fairly high doses (past 20.0 Mrad). Irradiation with doses of 7.0-20.0 Mrad has a conserving effect on the resultant structural change since crosslinklug begins to take place which evidently results in the formation of a structure which is more resistant to biological disintegration. This is confirmed when investigating micro-cracks formed in fibres by the action of the living organism.
2433
Behaviour of irradiated polycaprolactam in living organism
The rate of weight loss, as shown b y Fig. lc, is much lower than the rate at which strength is reduced. I f b y the 4-Sth month of implantation the strength of fibres is zero, weight loss represents 40-50%. This is, apparently, determined b y the t y p e of decomposition of polycaprolactam in the living organism. As indicated [3], hydrolysis of amide bonds b y the action of tissue fluid mainly P, g/~ex n I 2
2O
3# 5
0 l, ~/~
30 "=
90 Time, da//s
150
b
2o
0
3o
wf.,% 100
80 Time, daus
150
5O
0
60
120 Time, daus
180
2//0
FIG. 1. Kinetic curves of the variation o£ relative breaking load P (a), bre~J~ng elongation l (5) and weight (c) of non-iradlated, implant~l fibres (1) and fibres irradiatec~ with doses of 2.5 (g), 5 (3), 7 (4) and 20 (Mrad) (5).
$434
A . N . RoGovA et al.
effected from the fibre surface is one of the main processes causing a very marked change in the behaviour of implanted polyeaprolaetam. Since implanted fibres are in a highly stressed state in the organism, micro-cracks m a y be formed on the one hand hi the adsorption-active medium such as tissue fluid, which increase the specific surface of the sample and on the other, are evidently the main cause of a sudden reduction of strength of the material. Weight variation is due t0 the escape of hydrolytic products from the implanted material -- a fairly long process, which ends in complete resolution of the sample. An analysis of curves in Fig. lc shows that the effect of irradiation on weight loss sustained b y the fibres is only observed with short periods of implantation (up to 2 months): with an increase of the dose, weight loss also increases. However,
FIG. 2. Electron-microscope photographs of the surface of non-irradiated (a, b) and irradiated implanted fibres using a dose of 2.5 Mrad (c) with magnifications of 2500 (a) and 6000 (b, c). with long periods of implantation this difference is eliminated and weight loss sustained b y irradiated fibres does not differ in practice from that of non-irradiated samples. This is understandable if we deal with results of investigating the structure of irradiated fibres [1, 2]. Oxidation and partial disruption of the fibre surface in irradiation makes them more accessible to hydrolysis in the living organism and it has been estab-
Behaviour of irradiatedpolyeaprolactam in living organism
2435
lished that the degree of radiation damage to the surface under these conditions of irradiation shows a linear dependence on the dose. During hydrolysis of internal parts of fibres, less affected b y oxidizing processes, the changes due to irradiation are insufficient to influence the rate of hydrolytic decomposition. Figure 2 shows microphotographs of the most typical patterns of the quality of defect formation in fibres (time of implantation 30 days). Thus, for a control sample the main type of defect formed during implantation is the transverse crack arranged in an irregular manner along the entire length of fibres (Fig. 2a). The degree of increase of these cracks is slight they only extend to the surface layers of the fibre (Fig. 2b).
FIo. 3. Electron-microscope photographs of the surface of implanted fibres irradi-
ated with a dose of 7 (a) and 20 Mrad (b) with magnifications of 2500 (a) and 150 (b). .The qualitative pattern of defects found in fibres irradiated before implantation with doses of 2.5-5.0 Mrad is simple and is shown as follows: the transverse cracks described are formed and situated deep in the fibre and a qualitatively new type of defect is formed which involves the "etching" of very thin surface layers of fibres on fairly large areas of the fibre (Fig. 2c). •The formation of a defect, new from a quality point of view, no doubt involves a change in the fibre surface as a result of the radiation oxidation effect [1]. As far as the quantitative evaluation of defects is concerned, it should be noted that no increase was observed in the number of transverse cracks in irradiated fibres, compared with control ones. Defects were evaluated quantitatively b y counting defects along a given length of fibre. Fibre length in this case was limited b y the dimensions of the holder for the samples ( l : 1 cm). For a sample irradiated b y a dose of 7.0 Mrad, there was a new feature in the pattern of fibre decomposition in the living organism -- longitudinal cracks had formed (Fig. 3a). On further increasing the dose the number of longitudinal cracks and the degree of decomposition of the fibre suddenly increased in places of formation. On superimposing a longitudinal crack on a transverse one, the fibre separates into parts (Fig. 3b), which in the end breaks down the fibre. The formation of longitudinal cracks in implanted fibres is due to the changes
2436
A.N. ROOOVAet ed.
which have taken place in the fibre structure during irradiation using doses in the 7-20 Mrad region. As shown previously [2], these changes involve a changeover of the system of hydrogen bonds, the formation of cross-links and simultaneous breakdown, reduction of the long period of the supermolecular structure b y the amorphous component which in the end results in the formation of a non-equilibrium system. RELATIVE ABSORPTION BAND INTElffSITIES OF SAMPLES IMPLANTED FOR DIFFERENT LENOT]~ OF TIME
Relative band intensity DX670/1570 /)9960/1670 Daa4o/leTO Dsloo/l,7o
Time of implantation days 0
12
14
30
150
210
240
0"65 0"90 1 "00 0.21 0"10 0"07
0"56 0"79
0"58
0"50 0.67
1.09
1.08
1.13
0-15 0.06 0.012
0"17
0.10 0.07 0.13
0"51 0"64 1-08 0"14 0.03 0.05
0-37 0'57 0"93 0"07 0"02 0"04
0"45
0.79 0.08
0"06
0"48
1"17 0"07 0.02: 0"03
Since conditions exist in the organism for crack formation (constant dynamic. load, penetration of tissue fluid and elements of the connecting tissue inside the cracks), crack formation is inevitable in a similar non-equilibrium system and t h e longitudinal nature of the latter is determined by the fibrillar structure of fibres. At the same time as cracks are formed on the implanted fibres, their chemical structure undergoes considerable changes. These changes can be readily observed b y I R spectroscopy; Results obtained when studying I R spectra of fibres, implanted for different periods of time, are tabulated and shown in Fig. 4. As shown b y the Table, absorption band intensity decreases considerably for implanted fibres in the region of 1130, 1570, 2960 and 3100 cm -1. The band at 1570 cm -1 is attributed to vibrations of the - - C O N H - - amide group [4]. A reduction in intensity confirms evidently that par~ of amide bonds breaks down. A reduction in the relative intensity of bands at 2960 cm -1 of bondstretching vibrations of C---H of methylene groups corresponds to decomposition of these bonds. Intermolecular hydrogen bonds of the - - N H group also break down during implantation which is confirmed b y a reduction in the intensity of the band at 3100 cm -~ and the formation of fairly strong absorption in the 3400-3500 cm -1 regiol~, which corresponds to vibrations of free N H groups uncombined with hydr~,gen bonds [5]. The overall state of stress of hydrogen bonds decreases, whic], is confirmed b y a displacement to the high frequency region of the band at 3290 cm -1, which also corresponds to vibrations of - - N H groups combined b y hydrogen bonds.
Behaviour of irradiated polycaprolactam in living organism
2437
Breakdown in implanted polycaprolactam may also be evaluated by the reduction of band intensity at 1130 cm -1 which is attributed to skeletal vibrations of the polyamide chain [4] (Table). Significance is attached to the fact of a complex relation between the variation of absorption band intensity at 936 and 980 cm -1, these bands being structurally sensitive bands in the spectrum of polycaprolactam, and the length o f implantation. Absorption in these regions is determined by the interaction of
\
3q
82
30
2B
' 'lg
17
15
/3
11 .q "v,~lO~ZCm-!
Fro. 4. I R spectra of polyeaprolaetam fibres irradiated with a dose of 20 Mrad for 120
(2), 150 (3) and 240 days (4) before implantation (1) and after implantation.
vibrations in the most ordered (crystalline) ranges [6]. For implanted fibres, as shown by the Table, with brief periods of time (up to 30 days) band intensity at 980 cm -1 increases and then suddenly decreases. This effect may, evidently~ be explained by the fact that the decomposition of polycaprolactam in the living organism mainly begins in the amorphous ranges of the polymer. Owing to t h e increased mobility of macromolecules some increase in structural ordering i~ possible. With more prolonged retention of polycaprolactam in the living organism breakdown also covers the crystalline ranges, which in the end results in the. amorphization of the polymer, expressed in IR spectra in a sudden reduction of band intensity at 980 cm -1 and complete disappearance of the band at 936 cm-L Results obtained for implanted irradiated polycaprolactam fibres are similarto those in Fig. 4, i.e. the chemistry of decomposition of polycaprolactam in t h e living organism is the same for irradiated ~nd non-irradiated surgical suture&
2438
G.M. BARTENEV ~ a~.
, S o m e v a r i a t i o n o f r e s u l t s o b s e r v e d f o r i r r a d i a t e d s u t u r e s is e v i d e n t l y d u e t o a general structural non-equilibrium of fibres due to radiation, T h e a u t h o r s a r e g r a t e f u l to V. L . T s e t l i n t o r t h e u s e I u l c o m m e n t s m a d e
M U T U A L R E L A T I O N OF PROCESSES OF VISCO-ELASTICITY A N D BREAKDOWN * G. M. :BARTEI~EV, YU. A. SII~ICHKI~A and V. V. z~LEKSEYEV I n s t i t u t e of Physical Chemistry, U.S.S.R. A c a d e m y of Sciences
(Received 14 February 1977) Results obtained when studying temperature-time relations of the complex of m o s t i m p o r t a n t mechanical characteristics of crosslinked and non-crosslinked elastomers (stress relaxation viscous flow, breakdown processes related to durability a n d ~ensile stress) and analysis suggest t h a t above the glass temperature and below the temperature of chemical relaxation the temperature relation of visco-elastic processes and processes of breakdown are characterized b y the same activation energy. Since this activation energy is also typical of 2 processes of relaxation in the elastomer, the mechanism of processes of visco-elasticity and decomposition of the elastomer is determined b y the molecular mobility of low molecular weight structures -- micro-units. The difference between all processes is in non-temperature coefficients in viscosity equations, equations of stress relaxation and durability. Tensile stress is characterized b y a more complex temperature relation. The activation energy of these processes is unchanged in relation to stress (up to 100 kg/cm 2) and elongation (up to 300%) and is independen~ o f the fact whether the polymer is crosslinked or not. * Vysokomol. soyed. A19: No. 9, 2126-2131, 1977.