Meaning
Steroid acids found in the digestive fluid of mammals function as biological surfactants that aid in the emulsification of fats. Exposure to bile salts provides a standardized test for the chemical resistance of polyurethanes and other medical-grade elastomers. These compounds accelerate the environmental stress cracking of materials by lowering the surface tension of the fluid in contact with the part.
Surfactant Influence
Interaction between the plastic and the fluid reduces the energy required to create new surface area. While many polymers resist pure water, bile salts penetrate micro-voids and weaken the secondary bonds between polymer chains. This reduction in cohesive strength leads to the formation of crazes under lower stress than predicted by dry datasheets.
Degradation Mechanism
Hydrolysis occurs when the acidic components of the fluid react with the chemical linkages in the backbone of a biopolymer. Long chain molecules break into smaller fragments that are eventually solubilized or metabolized. Moulders of biocompatible parts must select resins where the rate of this reaction is slower than the intended life of the device.
Chemical Compatibility
Laboratory trials use concentrated solutions to simulate years of exposure in a matter of weeks. Results from these tests dictate whether a virgin resin or a specific additive package is suitable for long-term implantation. Failure to account for these salts results in premature part cracking and potential material failure inside a patient.
The testing environment must be strictly controlled because small fluctuations in pH or temperature can drastically alter the aggressive nature of the salt solution. Tracking the change in mass and tensile strength determines if the polymer remains stable enough for surgical use.
