In order to take advantage of the properties of poly(styreneisobutylenestyrene) PIBS and PIB based blends as lead insulation materials, they must be able to sufficiently bond to the various materials that make up the cardiac device. The bonded PIBS must be able to withstand the mechanical stress and corrosive environment of the human body due to the long term use of these devices. Based on the component requirements of lead insulation, the first objective of this study was to perform an initial screening of multiple PIBS stainless steel silicone adhesive combinations. The specific polymers of interest were PIBS, 10%55D polyurethane, 10%75D polyurethane, 10%PP, and a silicone control. Based on the bonding shear strength results of the initial screening, the best performing combinations were artificially aged to simulate their resistance to degradation in vivo. Each combination was subjected to both 3% hydrogen peroxide and Phosphate Buffered Saline solutions for a period of 8 weeks to test for oxidative and hydrolytic stability. Bonding shear strengths for all sample groups were tested at each 2week period. The 10%55D sample group had the highest mean bonding shear strength at .5602 MPa, but to observe the aging stability of all sample groups, all combinations were used in Phase II. The phosphate buffered saline solution in Phase II caused no significant decrease in bonding shear strength for all sample groups. Alternatively, oxidation caused by the 3% hydrogen peroxide solution did significantly affect the bonding shear strengths of all sample groups (minus the silicone control). Over the 8week period PIBS degraded 28% and 10%55D and 10%75D decreased 40.0% and 30.8%, respectively. 10%PP degraded 32.0% and the silicone control remained relatively unchanged.
SHEAR STRENGTH AND ARTIFICIAL AGING CHARACTERIZATION FOR SILICONE BONDING OF POLYISOBUTYLENE (PIBS) BLENDS IN RELATION TO THEIR USE AS LEAD INSULATION MATERIAL A Thesis Presented to the Faculty of California Polytechnic State University San Luis Obispo In Partial Fulfillment of the Requirements For the Degree Master of Science in Biomedical Engineering By Shawn Grening February 2009 ii © 2009 SHAWN GRENING ALL RIGHTS RESERVED iii COMMITTEE PAGE TITLE: SHEAR STRENGTH AND ARTIFICIAL AGING CHARACTERIZATION FOR SILICONE BONDING OF POLYISOBUTYLENE (PIBS) BLENDS IN RELATION TO THEIR USE AS LEAD INSULATION MATERIAL AUTHOR: Shawn Grening DATE SUBMITTED: May, 2009 COMMITTEE CHAIR: Robert Crocket, Associate Professor COMMITTEE MEMBER: Lily Hsu Laiho, Assistant Professor COMMITTEE MEMBER: Daniel W. Walsh, Associate Dean, College of Engineering iv ABSTRACT SHEAR STRENGTH AND ARTIFICIAL AGING CHARACTERIZATION FOR SILICONE BONDING OF PIBS BLENDS IN RELATION TO THEIR USE AS LEAD INSULATION MATERIAL Shawn Grening In order to take advantage of the properties of poly(styrene-isobutylene-styrene) PIBS and PIB based blends as lead insulation materials, they must be able to sufficiently bond to the various materials that make up the cardiac device. The bonded PIBS must be able to withstand the mechanical stress and corrosive environment of the human body due to the long term use of these devices. Based on the component requirements of lead insulation, the first objective of this study was to perform an initial screening of multiple PIBS / stainless steel / silicone adhesive combinations. The specific polymers of interest were PIBS, 10%55D polyurethane, 10%75D polyurethane, 10%PP, and a silicone control. Based on the bonding shear strength results of the initial screening, the best performing combinations were artificially aged to simulate their resistance to degradation in vivo. Each combination was subjected to both 3% hydrogen peroxide and Phosphate Buffered Saline solutions for a period of 8 weeks to test for oxidative and hydrolytic stability. Bonding shear strengths for all sample groups were tested at each 2-week period. The 10%55D sample group had the highest mean bonding shear strength at .5602 MPa, but to observe the aging stability of all sample groups, all combinations were used in Phase II. The phosphate buffered saline solution in Phase II caused no significant decrease in bonding shear strength for all sample groups. Alternatively, oxidation caused by the 3% hydrogen peroxide solution did significantly affect the bonding shear strengths of all sample groups (minus the silicone control). Over the 8-week period PIBS degraded 28% and 10%55D and 10%75D decreased 40.0% and 30.8%, respectively. 10%PP degraded 32.0% and the silicone control remained relatively unchanged. v TABLE OF CONTENTS LIST OF FIGURES VI LIST OF TABLES VII BACKGROUND 1 1.1. IMPLANTABLE CARDIAC RHYTHM MANAGEMENT DEVICES 1 1.1.1. Purpose 1 1.1.2. Components and Design 2 1.1.3. Failure Modes 3 1.2. LEAD INSULATION MATERIALS AND ADHESIVES 4 1.2.1. Silicone 4 1.2.2. Polyurethane 5 1.2.3. Poly (styrene-isobutylene-styrene) (PIBS) 6 1.2.4. PIBS Blends – Polypropylene (PP) 8 1.2.5. PIBS Blends – Polyurethane (PU) 8 1.2.6. Silicone Adhesive 9 1.2.7. Primer 10 1.3. DEGRADATION OF POLYMERS 12 1.3.1. Environment of the Human Body 12 1.3.2. Hydrolysis 12 1.3.3. Oxidation 13 2. PURPOSE AND EXECUTION OF STUDY 14 2.1. PHASE I: INITIAL SCREENING 14 2.1.1. Objective and Deliverables 14 2.1.2. Materials and Equipment 15 2.1.3. Procedure and Methodology 15 2.2. PHASE II: AGING STABILITY 18 2.2.1. Objectives and Deliverables 18 2.2.2. Materials and Equipment 18 2.2.3. Procedure and Methodology 19 3. RESULTS 21 3.1. PHASE I RESULTS 21 3.1.1. Statistical Summary 21 3.1.2. Statistical Analysis 22 3.1.3. Process Issues 25 3.2. PHASE II RESULTS 25 3.2.1. Statistical Summary 25 3.2.2. Statistical Analysis 34 3.2.3. Process Issues 35 4. DISCUSSION 36 4.1. PHASE I – INITIAL BONDING SHEAR STRENGTH 36 4.2. PHASE II – AGING STABILITY 39 5. CONCLUSION 42 6. REFERENCES 44 APPENDIX A 46 vi LIST OF FIGURES FIGURE 1. SCHEMATIC OF IMPLANTED PACEMAKER 5 2 FIGURE 2. SEGMENTS OF THE POLY(STYRENE-ISOBUTYLENE-STYRENE) TRI-BLOCK COPOLYMER 14 6 FIGURE 3. SILANE PRIMER ADHESION PROMOTION 11 FIGURE 4. ADHESIVE QUANTITY REFERENCE. 16 FIGURE 5. LLOYD LF PLUS TENSILE TESTER WITH GRIPPERS 17 FIGURE 6. LAB OVEN AND TEST TUBE SETUP 20 FIGURE 7. PHASE I INITIAL BONDING SHEAR STRENGTH BOX PLOTS 22 FIGURE 8. PHASE I PROBABILITY PLOT FOR EACH SAMPLE GROUP SHOWING NORMALITY 23 FIGURE 9. PHASE I TEST FOR EQUAL VARIANCES 24 FIGURE 10. PIBS/SS BONDING SHEAR STRENGTH (MPA) VS. AGING TIME (WEEKS) FOR PBS AND H 2 O 2 28 FIGURE 11. 10%PP/SS BONDING SHEAR STRENGTH (MPA) VS. AGING TIME (WEEKS) FOR PBS AND H 2 O 2 29 FIGURE 12. 10%55D/SS BONDING SHEAR STRENGTH (MPA) VS. AGING TIME (WEEKS) FOR PBS AND H 2 O 2 30 FIGURE 13. 10%75D/SS BONDING SHEAR STRENGTH (MPA) VS. AGING TIME (WEEKS) FOR PBS AND H 2 O 2 31 FIGURE 14. SILICONE/SS BONDING SHEAR STRENGTH (MPA) VS. AGING TIME (WEEKS) FOR PBS AND H 2 O 2 32 FIGURE 15. WEEK 8 BOX PLOTS OF BONDING SHEAR STRENGTHS 33 FIGURE 16. WETTING ANGLE OF ADHESIVE ON SUBSTRATE SHOWING BAD AND GOOD 27 36 FIGURE 17. INITIAL MEAN BONDING SHEAR STRENGTHS WITH RELATIVE SURFACE ENERGY VALUE 38 FIGURE 18. ADHESIVE ON STAINLESS STEEL NEEDLE FOLLOWING BOND FAILURE 39 vii LIST OF TABLES TABLE 1 - TYPICAL PROPERTIES OF SILASTIC BIOMEDICAL GRADE ETR ELASTOMERS 11 5 TABLE 2 - PROPERTIES COMPARISON OF SIBSTAR® GRADES. 18 8 TABLE 3 - TYPICAL PROPERTIES OF THE NUSIL MED-2000 SILICONE ADHESIVE 22 10 TABLE 4 - TYPICAL PROPERTIES OF THE NUSIL SP-135 SILANE PRIMER 24 11 TABLE 5 - PHASE I BONDING SHEAR STRENGTH STATISTICAL SUMMARY 21 TABLE 6 - TWO SAMPLE T-TEST FOR SIGNIFICANT DIFFERENCE BETWEEN PHASE I MEAN BONDING SHEAR STRENGTHS FOR EACH GROUP 25 TABLE 7 - PHASE II BONDING SHEAR STRENGTH DATA 26 TABLE 8 - TWO SAMPLE T-TEST RESULTS FOR DIFFERENCE IN MEAN BONDING SHEAR STRENGTH BETWEEN WEEK 0 AND WEEK 8 34 TABLE 9 - SURFACE ENERGIES OF RELEVANT MATERIALS 37 1 BACKGROUND 1.1. Implantable Cardiac Rhythm Management Devices 1.1.1. Purpose Patients with abnormal heart rhythms (cardiac arrhythmias) are often treated with implantable medical devices that deliver an electrical impulse to help restore their normal heart beat. The two most common devices are pacemakers and implantable cardioverter defibrillators (ICDs). The primary purpose of a pacemaker is to treat a condition called bradycardia, which is a heart rate that is too slow caused by a reduced rate of Sinoatrial Node (SA) firing. Long-term implantation is performed with minimally invasive surgery under local anesthesia and generally requires less than 45 minutes. The electrodes are placed in the heart through one of the large subclavian veins in the chest and after external testing the small generator is placed under the skin (Figure 1). Modern pacemakers are externally programmable and allow the physician to select optimum pacing modes for each patient. An ICD is a device implanted like a pacemaker that monitors the patient’s heart rhythm and waits for an arrhythmia. When it detects a tachycardia (a heart rate that is too fast), the ICD delivers a high-energy electric impulse (defibrillation) that restores normal heart rhythm. If a bradycardia is detected, it can also deliver a low-energy signal similar to a pacemaker. 1-4 2 Figure 1. Schematic of Implanted Pacemaker 5 1.1.2. Components and Design ICDs and pacemakers mainly consist of three main components: the generator, leads, and electrodes. All pulse generators include a power source, an output circuit, a sensing circuit, a timing circuit, and a header with a standardized connector to attach the leads. These generator components are typically hermetically sealed in a titanium casing termed “the can”. Lithium- iodide batteries now power most pulse generators and have an expected service life of 5-12 years depending on the pacing parameters. Most ICD designs use two capacitors in series to achieve maximum voltage for defibrillation. Electric impulse form the generator travels down one or more ICD leads, which use a coil structure to create the high density current required for defibrillation. At the distal tip of the lead an electrode is in direct contact with the myocardium and delivers the electric pulse for pacing, defibrillation, and/or sensing. These electrodes often possess a helix or screw at the tip to avoid dislodgement. A lead is covered with non-conductive polymer insulation except for at the distal end where the electrode makes contact with the heart 3 and the proximal end that connects to the generator. This lead insulation serves as a barrier to the electrical impulse supplied by the generator and the corrosive organic solvents in the body. 1,6,7 1.1.3. Failure Modes Generator breakdown most often occurs from the battery reaching end of life, which ceases the pacing and sensing capabilities of the device. Generator failure due to electronic or mechanical issues is extremely rare and according to most in the industry, the lead remains the “weakest link” of implantable pacing systems. 8 Problems begin with the connectors and sealing rings and become even more pronounced with insulation materials. The insulation used for the lead is a major design factor affecting lead reliability. The most frequently used insulation materials are silicone, polyurethane, and fluorine-polymers (PTFE, ETFE), but no pacemaker lead insulation has been proven to have complete reliability. Due to its softness, silicone can be prone to damage from abrasion once implanted and with the pursuit for smaller diameter pacing leads, some manufacturers have failed to consider the stresses placed on the insulation material during the manufacturing process. High levels of harmful organic solvents in vivo can change the chemical structure of polyurethane, destroying its elastic properties, subjecting it to built-in stresses, and increasing the potential for failure. Insulation fracture or erosion of any insulation material causes shunting of the electrical current away from the defibrillation electrode and into the body, decreasing the affect on the arrhythmia. Insulation breakdown always requires lead replacement. 8, 9 [...]... 2.1.1 Objective and Deliverables The Phase I objective is to get initial silicone bond shear strength data for multiple PIBS blends compared to a control (silicone tubing) Based on the collected data, samples will be selected for further investigation in Phase II of this study 14 Phase I deliverables include: • Raw data for silicone bond shear strength for PIBS blends to stainless steel and Silicone (control)... 0.448±0.022 0.022 Silicone H2O2 0 0.785±0.111 Silicone H2O2 2 0.860±0.112 -0.075 Silicone H2O2 4 0.878±0.101 -0.018 Silicone H2O2 6 0.781±0.059 0.097 Silicone H2O2 8 0.772±0.100 0.009 Silicone PBS 0 0.785±0.111 Silicone PBS 2 0.837±0.086 -0.052 Silicone PBS 4 0.835±0.105 0.002 Silicone PBS 6 0.801±0.077 0.034 Silicone PBS 8 0.814±0.095 -0.013 27 Artificial aging results for the PIBS/SS silicone bond degraded... PIBS/SS Silicone/ SS 0.0 0.1 0.2 0.3 0.4 95% Bonferroni Confidence Intervals for StDevs Figure 9 Phase I Test for Equal Variances Note: all tests not shown T-tests showed significant difference in initial bonding shear strength for the following sample groups: PIBS vs 10%55D, PIBS vs 10%75D, PIBS vs Silicone control, 10%PP vs 10%55D, 10%PP vs Silicone control, 10%55D vs Silicone control, and 10%75D vs Silicone. .. 0.5133 Silicone/ SS: Med2000 (CONTROL) Variable Silicone/ SS: Total Count 6 Mean 0.7853 Maximum 0.9700 Range 0.3400 For initial silicone bonding strength, the 10%55D sample group had the highest mean bonding shear strength at 0.560 MPa while PIBS had the lowest value of 0.407 MPa Figure 7 summarizes the Phase I data with boxplots of the four sample groups and the control group 21 Bonding shear strength. .. 0.293±0.067 MPa The H2O2 aging showed a relatively slow initial decrease, which dramatically increased from during the 4 to 8 week period 10%PP/SS vs Aging (weeks) Bonding Shear Strength (MPa) 0.44 Variable 10%PP H2O2 10%PP PBS 0.42 0.40 0.38 0.36 0.34 0.32 0.30 0 1 2 3 4 5 6 Aging Time (weeks) 7 8 9 Figure 11 10%PP/SS Bonding Shear Strength (MPa) vs Aging Time (weeks) for PBS and H2O2 29 ... rejected for the previously mentioned sample group comparisons For PIBS vs 10%PP, 10%PP vs 10%75D, and 10%55D vs 10%75D the data showed no significant difference and the null hypothesis was not rejected The 10%55D sample group showed the best initial bonding shear strength, but due to the interest in aging characteristics of each groups, it was determined to use all four plus the silicone control for Phase... 1 0.0 0.2 0.4 0.6 Data 0.8 1.0 1.2 Figure 8 Phase I Probability Plot for Each Sample Group Showing Normality A test for equal variances was also performed between each sample group All variances were assumed equal with 95% confidence except for 10%55D vs Silicone (control) (Figure 9) 23 Test for Equal Variances for Bonding Shear Strength Bartlett's Test 10%55D/SS Test Statistic P-Value 6.97 0.137 Material... solution (Figure 10) With an initial mean bonding shear strength of 0.407±0.048 MPa, PIBS/SS in H2O2 only showed a 28.0% decrease in bonding shear strength over the 8-week period to end at 0.293±0.027 MPa The PIBS/SS bonds in PBS over 8 weeks resulted in a 6.6% decrease in average bonding shear strength to 0.380±0.053 MPa PIBS/SS vs Aging (weeks) Bonding Shear Strength (MPa) 0.42 Variable PIBS H2O2 PIBS... Insulation Materials and Adhesives An important task for the biomedical industry is to move toward the design of thinner, more flexible, and less thrombogenic defibrillation lead with acceptable biostability and biocompatibility 1.2.1 Silicone During the 1960’s, silicone rubber became popular as an insulating material for pacemaker leads Silicone has excellent biocompatibility and biostability, but... oxidation, and other forms of degradation from the harsh organic solvents of the body.9 The specific silicone elastomer tubing used in this study was SILASTIC BioMedical Grade ETR Q7-4780, which is a two-part, enhanced-tear-resistant (ETR) silicone elastomer that consists of dimethyl and methylvinyl siloxane copolymers and reinforcing silica SILASTIC BioMedical Grade ETR Elastomers, when fully cured and washed, . (WEEKS) FOR PBS AND H 2 O 2 29 FIGURE 12. 10%55D/SS BONDING SHEAR STRENGTH (MPA) VS. AGING TIME (WEEKS) FOR PBS AND H 2 O 2 30 FIGURE 13. 10%75D/SS BONDING SHEAR STRENGTH (MPA) VS. AGING. (WEEKS) FOR PBS AND H 2 O 2 31 FIGURE 14. SILICONE/ SS BONDING SHEAR STRENGTH (MPA) VS. AGING TIME (WEEKS) FOR PBS AND H 2 O 2 32 FIGURE 15. WEEK 8 BOX PLOTS OF BONDING SHEAR STRENGTHS. PHASE I TEST FOR EQUAL VARIANCES 24 FIGURE 10. PIBS/SS BONDING SHEAR STRENGTH (MPA) VS. AGING TIME (WEEKS) FOR PBS AND H 2 O 2 28 FIGURE 11. 10%PP/SS BONDING SHEAR STRENGTH (MPA) VS. AGING TIME