1. N. Weir
School of Mechanical & Manufacturing Engineering
Accelerated Degradation ofAccelerated Degradation of
Bioabsorbable Polymers Used inBioabsorbable Polymers Used in
Surgical ApplicationsSurgical Applications
David Farrar Visit – Smith + Nephew
15th
April 2003
2. Overview of the Project
• Accelerated In Vitro Studies at Increased Temperatures
• Accelerated In Vitro Studies Using Chemical Methods
• Two Polymers Investigated
Poly-ε-caprolactone
Poly-L-lactide
Characterised Through
Processing and Sterilisation
• In Vitro Degradation Studies Conducted at 37°C and In Vivo
Studies Using Rat as the Animal Model
Processing
of PCL
Characterising
PCL
In vitro & in vivo
PCL Degradation
Studies
Processing
of PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation
Studies on Both PCL &
PLLA
October 2000 Present
Timeline
3. Poly-ε-caprolactone
• Member of the Aliphatic Polyester Family
• Developed as a Copolymer in Suture
Materials & also in Drug Delivery Devices
In vitro & in vivo
PLLA Degradation
Studies
Processing
of PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
CAPA 6400 – Solvay Caprolactones
Melting Point 58-60°C
Glass Transition -60°C
Molecular Weight 37,000
% Crystallinity ≈50%
PCL Pellets
4. Extruded
2mm Diameter
Rod
Injection Moulded
Tensile
Flexural
Impact
Processing
of PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
Poly-ε-caprolactone - Processing
5. • Effects of Processing and Sterilisation on Properties
Crystallinity
Molecular
WeightMechanical Strength
EtO Sterilisation Conditions
Conditions Temperature
Time,
(Hours)
Relative
Humidity
Preconditioning 38-48°C 20-40 60-80%
Gas Dwell 36-46°C 7-7.5 NA
Degassing 38-48°C 16 NA
PCL Characterisation Tests and Degradation Studies
Conducted on ASTM D 638-99 Tensile Samples and
30mm Lenghts of 2mm Diameter Extruded Rod
Characterising PCL
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
6. 0
50
100
150
200
0 25 50 75 100 125
Extension, (mm)
Load,(N)
0
40
80
120
160
0.0 0.5 1.0 1.5 2.0
Extension, mm
Load,N
Characterising PCL
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
7. 40 50 60 70
Temperature, °C
HeatFlow
40 50 60 70
Temperature, °C
40 50 60 70
Temperature, °C
Before Sterilisation After Sterilisation
Pellets Extruded Injected
Sterilisation may have caused
some annealing of the polymer.
0
5
10
15
-80 -30 20 70
Temperature, o
C
HeatFlow,mW
Thermal Analysis - DSC
No Significant Change in Thermal
Properties After Processing
Characterising PCL
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
Conditions Temperature
Time,
(Hours)
Relative
Humidity
Preconditioning 38-48°C 20-40 60-80%
Gas Dwell 36-46°C 7-7.5 NA
Degassing 38-48°C 16 NA
8. Surgical
Adhesive
Forceps
Scalpel with No. 11
Blade
Material Feeder
10 Gauge Hypodermic
Needle
• In Vivo Degradation using Rat as the Animal Model
30mm lengths of 2mm diameter rod implanted subdermally
PCL Degradation
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
• In Vitro Degradation at 37°C in pH 7.4 Phosphate Buffered
Solution
• In Vitro Degradation at 50°C in pH 7.4 Phosphate Buffered
Solution
9. • No Change in Molecular Weight Detected After 82 Weeks
Mechanical Properties
Tensile Samples
14
16
18
20
22
24
0 20 40 60 80 100
Weeks
TensileStrength,MPa
37°C 50°C
10
14
18
22
26
30
0 20 40 60 80 100
Weeks
ShearStrength,MPa
37°C 50°C In vivo
Extruded Rod Samples
PCL Degradation - Results
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
10. Crystallinity of Extruded Rod
30
35
40
45
50
55
60
0 20 40 60 80 100
Weeks
%Crystallinity
37°C 50°C
40 50 60 70 80
Temperature °C
HeatFlow
Pellets Control Rod 82 Weeks at 50°C
Conclusions
• CAPA 6400 Degrades very Slowly
• Increased Temperature Does Not Accelerate Degradation Rate
PCL Degradation - Results
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
11. Melting point 192°C
Glass Transition Onset 67°C
Molecular Weight, Mv 133,131
% Crystallinity 61%
Properties of Supplied PLLA Pellets
Melting point 192°C
Glass Transition Onset 67°C
Molecular Weight, Mv 133,131
% Crystallinity 61%
Properties of Supplied PLLA Pellets
Poly-L-lactide
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
• Also a Member of Aliphatic Polyester Family
• Significant Research Focussed on Developing
PLLA for Orthopaedic Applications
12. Extruded PLLA
Extruded
Annealed PLLA
Extruded PLLA
Poly-L-lactide - Processing
PLLA Pellets
Compression Moulded
PLLA
Compression Moulded
Annealed PLLA
Compression Moulded PLLA
• PLLA Annealed at 120°C for 4 Hours
In vitro & in vivo
PLLA Degradation
Studies
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
13. Thermal Properties
40 90 140 190 240
Temperature, °C
HeatFlow
Before Annealing
40 90 140 190 240
Temperature, °C
HeatFlow
After Annealing
Pellets Compressed Extruded
61%
12%
20%
61%
43%
40%
• Sterilisation by EtO Gas Further Increased Crystallinity by Approximately 5%
Characterising PLLA
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
• Study Conducted to Determine the Effects of Processing
Annealing and Sterilisation on PLLA’s Properties
14. Molecular Weight
320
360
400
440
480
520
PLLA Pellets Compressed
PLLA
Annealed
PLLA
Annealed
Sterile PLLA
EquivalentMwtx103
Mechanical Properties
0
20
40
60
80
0 5 10 15 20
Percentage Strain
Stress,MPa
Compressed
Compressed
Annealed
Compressed
Annealed Sterile
• Similar Pattern Evident for the Extruded PLLA Tested in Shear
Characterising PLLA
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
15. 3004756508251000
Wa v e num be r, c m - 1
3004756508251000
Wa v e num ber,c m - 1
3004756508251000
Wa v e num be r, c m - 1
10 15 20 25 30 35
2 Theta
Intensity
15 20 25 30 35
2 Theta
10 15 20 25 30 35
2 Theta
Pellets Compressed Compressed Annealed
Characterising PLLA
• PLLA Was Further Characterised Using the Techniques of XRD and Raman
Spectroscopy
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
16. • Accelerated In Vitro Studies
50°C in pH 7.4 Phosphate Buffered Solution
70°C in pH 7.4 Phosphate Buffered Solution
In Accordance With Annex A of ISO 15814
At 37o
C in pH 7.4 Phosphate Buffered Solution
In Accordance With ISO 15814
• Control In Vitro Study
Mass loss, Mechanical Strength, Molecular Weight and
Thermal Properties
Degradation of PLLA
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo PLLA
Degradation Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
• In Vivo Study Using Rat as the Animal Model
17. 37°C Mass Change
-6
-4
-2
0
2
0 20 40 60 80 100 120
Days
%MassChange
Before Drying After Drying
50°C Mass Change
14 Days
18 Days
21 Days
23 Days
Mass Change
-0.4
0
0.4
0.8
1.2
1.6
0 10 20 30 40 50
Weeks
%MassChange
Before Drying After Drying
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo PLLA
Degradation Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
-12
-8
-4
0
4
0 5 10 15 20 25
Days
%MassChange
After Drying Before Drying
70°C Mass Change
18. -10
0
10
20
30
40
50
60
70
80
0 50 100 150 200 250
Days
TensileStrength,MPa
37°C 50°C 70°C
Zero Order Linear Model
Cktt +−=σ
70°C
256.652214.037
+−=°
tC
tσ
82.02
=R
37°C
375.61261.150
+−=°
tC
tσ
96.02
=R
50°C
202.61999.870
+−=°
tC
tσ
94.02
=R
Mechanical Properties
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo PLLA
Degradation Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
19. • Applying the Arrhenius Relationship
RT
Ea
AeK
−
=
ATREaK ln)1)((ln +−=
TvK 1)(ln
-2.0
-1.0
0.0
1.0
2.0
3.0
0.0029 0.003 0.0031 0.0032 0.0033
1/T °K
lnK
75.36ln)1(11799ln +−= TK
or;
TexK
11799
15
1013.9
−
=
Activation Energy for
PLLA of 98 KJ mol-1
Mechanical Properties
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo PLLA
Degradation Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
20. 0
40,000
80,000
120,000
160,000
200,000
0 50 100 150 200 250 300 350
Days
NumberAverageMolecular
Weight,Mn
37°C 50°C 70°C
0
40,000
80,000
120,000
160,000
200,000
0 10 20 30 40 50
Weeks
MolecularWeight,Mn
In Vitro 37°C In Vivo
OHRRCOOHOHRCOOR '
2
' +⇔+
Kinetics of the hydrolytic reaction can be expressed by the rate equation;
]][2][[][][ EOHCOOHkdtCOOHddtEd −=−=
Where, E = Ester concentration COOH = Acid Concentration OH2 = Water Concentration
Molecular Weight
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo PLLA
Degradation Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
21. Molecular Weight
TimevMn )(ln
y = -0.1101x + 11.569
R2
= 0.9076
8
9
10
11
12
13
0 5 10 15 20 25
Days
lnMn
70°C
y = -0.0364x + 12.064
R2
= 0.8533
9.00
10.00
11.00
12.00
13.00
0 10 20 30 40 50
Weeks
lnMn
37°C
y = -0.0219x + 11.942
R2
= 0.9828
8
9
10
11
12
13
0 50 100 150
Days
lnMn
50°C
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo PLLA
Degradation Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
22. 40
45
50
55
60
65
70
75
0 30 60 90 120 150
Days
%Crystallinity
50°C Crystallinity
50
60
70
80
90
100
0 5 10 15 20 25
Days
%Crystallinity
70°C Crystallinity
40 90 140 190
Temperature, °C
HeatFlow,mW
40 90 140 190
Temperature, °C
Control 23 Days at 70°C
Crystallinity
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo PLLA
Degradation Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
Similar Pattern
Evident at 37°C
23. • Study Focussed on the Secondary Amine Diethylamine
• Initial Objective to Accelerate the Degradation of PCL
Chemical Studies
• Amine Based Drugs Accelerated the Degradation of
Bioabsorbable Drug Delivery Systems
• Study Suggesting Secondary and Tertiary Amines Simply
Catalysed the Hydrolysis Reaction Without Taking Part in It
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
24. • PCL Pellets Degraded in 0.5M DEA & Methanol Solution
y = -1.6915x + 3.9108
R2
= 0.9822
y = -0.9553x + 7.354
R2
= 0.9903
-120
-100
-80
-60
-40
-20
0
20
0 20 40 60 80
Days
%MassChange
Before Drying After Drying
0
20,000
40,000
60,000
80,000
0 10 20 30 40 50
DaysMoleularWeight,Mn
Chemical Degradation of PCL
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
• Crystallinity Also Increased Significantly
• Study of PCL in Methanol Only Showed Limited Changes
to PCL’s Properties
25. Results Revealed PLLA Degraded by Surface Erosion
y = -3.0952x + 0.1011
R2
= 0.994
-40
-30
-20
-10
0
0 2 4 6 8 10 12
Weeks
%ChangeinArea
% Change in Area in DEA & H2O
Differentiation Between the
Materials Interior and Surface
Molecular Weight
Processing of
PCL
Characterising
PCL
In vitro & in vivo PCL
Degradation Studies
Processing of
PLLA
Characterising
PLLA
In vitro & in vivo
PLLA Degradation
Studies
Chemical Degradation Studies
on Both PCL & PLLA
October 2000 Present
Chemical Degradation of PLLA
• PLLA Degraded in 0.5M DEA & Methanol and 0.5M
DEA & Water
26. Conclusions
• Increased Temperature Appears to be a Powerful Means of
Accelerating Degradation of PLLA
• While Chemical Methods Appear to be More Appropriate
For Accelerating the Degradation of PCL
• PCL is Insensitive to Processing Although Anneals
Slightly Upon Sterilisation by EtO Gas
• PLLA is Sensitive to Processing and Sterilisation
27. Future Work
• Further Analysis of Experimental Results
• Writing Up of papers;
Processing, Annealing and Sterilisation of Poly-L-lactide
The Influence of Processing and Sterilisation on the Properties of
Poly-ε-caprolactone
Accelerated Degradation of PLLA at Increased Temperatures
In vitro and In Vivo Degradation of PCL
Accelerated Degradation of PCL and PLLA by the Secondary Amine
Diethylamine
• Thesis Write Up
Editor's Notes
Just give an overview of the project to start with. Basically there was 2 bioabsorbable polymers investigated, PCL and PLLA. For both an initial study was conducted into the effects of processing and sterilisation on there properties. It became apparent from the literature that altering the polymers critical properties of Mwt and crystallinity would have significant effects on degradation rate and I felt it was essential to have a good understanding of the polymer and its properties before commencing any degradation work.
Moving onto the degradation studies conducted on both polymers, I have basically split these into 3 sections, firstly really the control studies at 37°C in vitro in a phosphate buffered solution and also in vivo studies using rat as the animal model. Objective to see if both the in vivo and in vitro data match and evaluate any inflammatory response to the implant and provide a control for the accelerated studies to be measured against.
The accelerated studies basically 2 methods have been investigated for accelerating the degradation of PCL and PLLA firstly increased temp and chemical means using a secondary amine solution, which I’ll talk in more detail about later.
The rest of the presentation is really going to follow this timeline of events really. Looking at PCL first………..
First material investigated was PCL, mainly because it was cheap in comparison to the majority of bioabsorbable polymers and was readily available. It also is the slowest degrading of all the most common bioabsorbables therefore worth trying to develop accelerated testing procedures for it. Memebr of aliphatic polyester family currently the most significant family of bioabsorbable materials.
The grade of material investigated was supplied by Solvay Caprolactones, Table outlines its properties.
Firstly the material had to be processed into a useful form for testing. The pellets were processed by injection moulding in ASTM standard Tensile, Flexural and Impact test specimens and extruded into 2mm diameter lengths of rod.
The processed material was then characterised to determine the effects of processing and sterilisation on its properties of crystallinity, mwt and mechanical strength. Sterilisation was by ethylene oxide gas, under the conditions given. With mechanical strength of the material tested before and after sterilisation, on ASTM D 638-99 tensile samples, these were cut out of the injection moulded flexural specimen and 30mm lengths of the extruded rod, these were tested in shear.
Looking at the results of these tests. Graph shows the mwt distribution of the of the PCL pellets, injected and extruded material, see only one line as each of materials has exactly the same mwt distribution and mwts, indicating processing had no effect on mwt, similar story is apparent after sterilisation. Looking at mechanical properties of the small injection moulded tensile samples, see this rather peculiar curve, basically due to the high ductility of the material with extension not simply confined to the gauge length but reaching the thicker ends of the specimen. Although still no explanation for this odd oscillating behaviour. Results focussed on this portion up to the yield strength of the material. A similar pattern was observed after sterilisation, with no change in the materials properties of Young’s Modulus of tensile strength. The extruded rod tested in shear, this just shows the test setup and typical load v extension curve, again no change in shear strength after sterilisation observed.
Shows a typical DSC plot obtained indicating the main melting peak around 60°C. No significant change in PCLs properties was observed after processing by both injection moulding and extrusion. Although it appears sterilisation may have caused the polymer to anneal slighty. This is indicated by considering the graphs here comparing the melting peak of the pellets, extruded rod and injection moulded material both before and after sterilisation. The onset of melting becomes more defined after sterilisation and the peak melting temp seems to shift to higher temps, indicating possible increasing crystallite size. Not really surprising we see annealing considering the sterilisation conditions.
Now that had a good understanding of PCL’s properties move onto the degradation studies, firstly there was the control in vitro study at 37°C in PBS, secondly the in vivo study, with 30mm lengths of extruded rod implanted subdermally in the dorsum of sprague dawley rats and finally a study at 50°C in PBS solution pH 7.4.
Looking at the results of these studies, each study ran for a period of 82 weeks with material removed and tested at various time intervals. PCL did in fact prove to be very slow to degrade. No change in Mwt after 82 for all the studiesweeks. Looking at the mechanical properties, firstly of the tensile samples, again very little change in tensile strength was noted both at 37°C and 50°C, perhaps most significant, although difficult to see on this graph id the drop in tensile strength after 1 week followed by a recover again in subsequent weeks testing. As for the rod material tested in shear a similar pattern is evident with no significant change after 82 weeks at 37, 50°C and in vivo, although gain a drop in shear strength after 1 week is evident followed by recovery again.
Looking at the crystallinity of the extruded rod at 37 and 50°C an initial increase is evident after 1 week at 50°C and about 4 weeks at 37°C I would again attribute to annealing of the polymer. There also appears to be a significant increase in crystallinity between 60 weeks and 82 weeks. Although these changes are not reflected by a more brittle fracture mode observed in the shear test, I think this may be due to the water acting as a plasticizer on the shear test samples tested while yet. Just looking at the profile of the melting peak fof the pellets control rod and rod at 50°C for 82 weeks, we see at 82 weeks the melting temp has increased signifcantly.
Conclusions
Moving and now looking at PLLA, also a member of the aliphatic polyester family. The material used in this tudy was supplied by Boehringer Ingelheim, table gives its properties.
The PLLA was processed by compression moulding into sheets approximately 0.8mm thick with the small dumbbell samples cut from the plate and extruded into 2mm diameter rod. Unlike the PCL the PLLA was also annealed after processing at 120°C for 4 hours.
Again a study was conducted to characterise PLLA through processing sterilisation and annealing. Firstly looking at its thermal properties, looking at the pellets, compression moulded and extruded before annealing, see pellets 61% crystalline, 12% for Compressed and 20% for extruded, after annealing the recrystallisation peaks were removed and crystallinity increased with sterilisation further increasing crystallinity by about 5%.
Looking a the mwt of the extruded material a gradual decrease is evident through processing annealing and sterilisation. Mechanical properties of the tensile samples also displaying a behaviour as would be expected for a material whose crystallinity is increasing, with the amorphous compressed material having the greated extension on the more crystalline annealed sterilised material the least. Similar pattern also evident for the rod material tested in shear.
A few further techniques have also been investigated with the hope of developing them further for monitoring degradation. Firstly XRD and Raman spectroscopy, graphs just show the transition from the crystalline pellets to amorhous compression moulded material and back to a crystalline structure after annealing. Hoped Raman can be used to help monitor degradation by comparing the ratio of the COOH peak and CH2 peak, with further work being done to investigate this.
So now we have the PLLA samples to test and an understanding of the initial properties. I’d now like to move on and introduce the degradation studies. Firstly a control study was set up at 37°C in a phosphate buffered solution, in accordance with ISO15814. Increased temperature has been used as a means to accelerate the degradation of PLLA, this is known to have a significant effect on PLLA degradation, although is not suitable for all bioabsorbable polymers, with PCL also a member of the aliphatic polyester family exhibiting limited change in degradation rate at increased temperature. 2 accelerated studies were conducted utilising increased temperature at 50°C and also at 70°C in accordance with Annex A of ISO 15814.
Techniques have been used to characterise the properties of PLLA throughout degradation, including, mass loss, tensile testing for mechanical properties, GPC for molecular weight and DSC for thermal properties.
Looking a the results of these degradation studies beginning with changes in mass.
The graph presents the change in mass for the PLLA dumbbell specimens at 37°C, up to 44 weeks degradation. Depicting the % change in mass on removing the samples before drying and after drying. As you can see at 37°C there is an initial swelling of the polymer up to approximately 0.7% related to absorption of water, which remains constant up to 38 weeks before a sharp increase up to approximately 1 % at 44 week, although no significant mass loss detected after drying at present.
Results at 50°C degradation shows an initial swelling up to 1%, which seems to correspond with the onset of mass loss. With a maximum mass loss of approximately 4% observed at 115 days when the test was terminated.
A similar pattern appears to exist at 70°C again with the polymer swelling to approximately 1%, followed by mass loss up to approximately 8% after 23 days.
This is in agreement with the general model of degradation for bioabsorbable polymers, which suggests a lag in time before mass loss is observed, as the degradation products have to be broken down so they are small enough to leach out of the polymer. Although not presented here a decrease in the pH of the solutions was also observed with mass loss at 50°C and 70°C as the acidic degradation products were released.
Picture shows the degraded specimens at 70°C. The material became very brittle and difficult to handle, and in fact simply fell apart after 23 days.
Moving onto the mechanical properties of PLLA.
The mechanical properties of the dumbbell PLLA specimens were determined through tensile testing at a strain rate of 10mm/min.
I am going to focus on the loss of tensile strength for each of the degradation mediums.
Here the graph presents the loss of tensile strength for the samples at 37°C, 50°C and 70°C. The tensile strength had dropped to zero after 7 days at 70°C and 50 days at 50°C. Linear trendlines have been fitted for each temperature.
With a good linear fit obtained for each temperature a zero order linear model has been assumed to exist for loss of tensile strength.
Where sigma = Tensile strength, K = rate of tensile strength loss, t = the time in days and C the tensile strength at t=0.
The equation for each of the trendlines corresponding to the zero order linear model at each temperature are given as follows……
It can be seen from the relatively high R2 values a linear zero order model is not an unreasonable assumption.
Applying the Arrhenius relationship, which is commonly used when extrapolating results of higher temperatures to service temperatures, were reaction rate K is equal to a constant A upon the inverse ln of Activation Energy Ea divided by RT, where R = universal gas constant and T the temerature in degrees Kelvin. If the relationship holds true a linear relationship should exist between ln K and 1/T. Taking the ln of values of K determined in the previous slide and plotting against 1/T. Yields the following graph, as you can see it is obvious a linear relationship does in fact exist. Although it most also be noted this is a fairly simplified case with only 3 temperature points analysed.
Yielding the following linear equation and substituting in the Arrhenius relationship gives…. Providing an equation to determine the rate of reaction for loss of tensile strength at any given temperature, and giving an activation energy for the process of 98KJ/mol. Which fits in well with the work of Ye who found the activation energy to be 96.2KJ/mol. A similar relationship also holds true for Young’s modulus.
Moving on and looking at the Mwt for the dumbbell samples degraded at 37, 50 and 70°C, again evident that incresased temp significantly enhances PLLA degradation. Although difficult to see from this graph it I evident that the mwt drops of more rapidly in the early stages of degradation, most notably at 70°C. And comparing the degradation of the extruded rod material degraded at 37°C in vitro in in vivo we see a very similar profile.
Considering the ester hydrolysis of PLLA from a molecular viewpoint, gives the following equation. The kinetics for the hydrolytic degradation can be expressed by the rate equation. Upon further analysis of this equation if the theory holds true a linear relationship should exist for the ln Mn against time up until the point of mass loss..
As can be seen this relationship holds reasonably true with the results presented here for the compression moulded material.
Looking a the crystallinity of the material an increase is observed at both 50 and 70°C with a similar trend also evident at 37°C again no significant difference between the material degraded in vitro and that degraded in vivo. Graphs just show the change in profile from the control material to the highly crystalline material after 23 days degradation at 70°C, with the peak a lot sharper and the Tg region not evident anymore.
The reasons for looking at using chemical methods to accelerate the degradation of bioabsorbable polymers really came about due to the lack of success at accelerating PCL with increased temp. It was apparent that amine based drugs accelerated the degradation of bioabsorbable polymer drug delivery systems. With further study suggesting that secondary and tertiary amines simply catalysed the hydrolysis reaction without taking part in it. For these studies the secondary amine Diethylamine was used, basically because it was reasonably soluble and a liquid at room temperature. With the initial objective to accelerate the degradation of PCL.
Firstly looking at PCL a study was conducting mirroring that of previous researchers, with the PCL pellets degraded in a 0.5M DEA &methanol solution. With a further study also conducted with PCL pellets in methanol only. Just briefly run through some of the results. Firstly mass loss. Mass loss was detected after 1 day of the study with the pellets losing all there mass after approximately 60 days. Looking at the loss of Mn we see pretty good curve and what would be expected for the loss of mwt from bioaborbable polymers, with crystallinity also increasing. Although unfortunate there is no real degradation data to compare this to with limited success in the in vitro studies at in PBS at 37°C and 50°C. It does appear to follow the general pattern for hydrolysis of bioabsorbbale polymers.
Looking at PLLA, studies conducted in 0.5M DEA & methanol and 0.5M DEA and water. Results revealed PLLA degraded by surface erosion. Evident by this graph showing the loss of area of PLLA degraded in DEA and water with a very linear trend observed. There was also no real change in the mwt of the samples again indication of a surface erosion process taking place.
Further analysis of experimental results, including processing if in vivo material for histology. Some potential title for papers. And finally of course the thesis write up, probably follow reasonably similar structure to this presentation.
