Partially Crosslinked Polyurethane for Medical Implants
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Solution Overview
Problem
Polyurethanes used in medical applications face challenges such as chemical degradation and excessive strain relaxation due to hydrolytic and oxidative instability, particularly in cardiovascular prostheses, where cyclic stress is incurred, necessitating the development of chemically and mechanically stable linear and crosslinked polymers.
Innovation Solution
The development of partially crosslinked polyurethane polymers comprising methylene diphenyl diisocyanate (MDI) and hydrogenated polybutadiene diol (HPBD) soft segments, with a functionality of 1.9 to 2.2 and a number average molecular weight of 2000 to 3000, and a crosslinker selected from triethanolamine (TEOA) or trimethylol propane (TMP) in the range of 3% to 15% of the polymer, which enhances mechanical properties and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If polyester macroglycols are used as soft segments in polyurethanes, then the polymers can be synthesized with good mechanical properties, but they exhibit hydrolytic instability and chemical degradation in vivo
Solution Approach 1:
The patent changes the chemical composition parameters of the soft segment from polyester macroglycol to polyether macroglycol, specifically using polymers with ether linkages that resist hydrolysis. This parameter change maintains mechanical properties while dramatically improving chemical stability and biocompatibility in physiological environments.
Solution Approach 2:
The patent creates a composite segmented polyurethane structure combining polyether soft segments with hard segments containing aromatic or aliphatic diisocyanates and chain extenders. This composite structure at the molecular level provides both the mechanical strength from hard segments and the chemical stability from polyether soft segments.
2Reliability
If polyether segments are used to replace polyester segments, then hydrolytic stability is improved, but the polymers become susceptible to oxidative degradation in the presence of metal ions and strong oxidants
Solution Approach 1:
The patent introduces antioxidant additives locally into the polyurethane formulation to protect specific vulnerable sites. The antioxidant concentration and type are optimized to provide localized protection against oxidative degradation at metal ion sites and other vulnerable regions without compromising the overall polyether structure.
Solution Approach 2:
The patent uses antioxidant additives as intermediary substances that mediate between the polyether polyurethane and oxidative environmental factors. These antioxidants act as sacrificial agents that preferentially react with oxidants and metal ions, protecting the polyether backbone from degradation.
3Reliability
If polycarbonate soft segments are used, then chemical stability is increased, but the polymers remain susceptible to hydrolytic and oxidative degradation
Solution Approach 1:
The patent changes the soft segment chemistry from polycarbonate to polyether, fundamentally altering the chemical bonds from carbonate linkages (susceptible to hydrolysis) to ether linkages (resistant to hydrolysis). This parameter change provides comprehensive resistance to both hydrolytic and oxidative degradation while maintaining chemical stability.
4Reliability
If siloxane based polyurethane materials are used, then hydrolytic and oxidative stability is achieved, but the polymers have lower toughness than polyether counterparts
Solution Approach 1:
The patent changes the soft segment from siloxane to polyether, altering the molecular chain flexibility and intermolecular interactions. The polyether structure with its flexible C-O-C linkages provides both the required chemical stability and superior toughness compared to the more rigid siloxane structures.
5Reliability
If uncrosslinked hydrocarbon-based polyurethanes are used, then biostability is enhanced, but the polymers are prone to significant plastic deformation
Solution Approach 1:
The patent incorporates crosslinking agents and antioxidants into the polyurethane formulation during synthesis, creating a pre-crosslinked network structure before the product is deployed. This preliminary crosslinking action prevents plastic deformation and maintains dimensional stability while preserving the biostability of the hydrocarbon-based polyurethane.
Solution Approach 2:
The patent creates a composite network structure combining the hydrocarbon-based polyurethane matrix with crosslinking agents that form additional bonds between chains. This composite structure at the molecular level provides both the biostability of the hydrocarbon polymer and the dimensional stability of the crosslinked network.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The resulting polyurethane polymers exhibit improved mechanical properties, including a Young's modulus of 4 to 15 MPa, ultimate tensile strength of 2 to 25 MPa, and maximum strain of 350 to 600%, along with enhanced toughness and resistance to oxidative degradation, making them suitable for medical implant devices.
Implementation Method 1
having a crosslinker selected from one or both of triethanolamine (TEOA) and trimethylol propane (TMP) in the range of 3% to 15% of the polymer
Data Source
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Figure 3~4
Figure 5
AI summary
Partially crosslinked polyurethane polymers comprising diisocyanates and aliphatic hydrocarbon soft segments with a short-chain diol chain extender and a multifunctional amine and/or alcohol crosslinker to provide a polyurethane polymer with useful properties for the production of medical implant devices such as heart valves, are described. The polymers have an unexpected linear elastic region in a range from 5 - 100% and preferably between 10 - 35%. In some embodiments the polyurethanes are a thermally convertible gel formulation which may be converted to a liquid formulation by extended heating to render the polymer suitable for solvent processing techniques such as casting, spraying, spinning, etc. The invention also provides for living hinge polyurethane polymers which are thermally modifiable from a gel to a liquid and reaction injection moulded (RIM) polyurethanes with an enhanced flex life.