Water absorption of poly(methyl methacrylate) measured by vertical interference microscopy

Area of Science:

• Biomaterials Science
• Polymer Science
• Orthopedic Engineering

Background:

• Poly(methyl methacrylate) (PMMA) is a key component in orthopedic cements, interacting directly with biological environments.
• Despite its hydrophobic nature, PMMA can absorb approximately 2% water by weight.
• Understanding water absorption and subsequent swelling is crucial for orthopedic implant longevity.

Purpose of the Study:

• To quantify the swelling behavior of various PMMA formulations upon water absorption.
• To investigate the influence of plasticizers, cross-linkers, and commercial bone cement compositions on PMMA swelling.
• To correlate swelling characteristics with mechanical properties determined by nanoindentation.

Main Methods:

• Vertical interference microscopy was employed to image PMMA blocks in dry and hydrated states.
• Nanoindentation was used to measure hardness, indentation modulus, and work parameters under a fixed force.
• Surface roughness was assessed using fractal dimension, and swelling was measured in vertical and lateral directions.
• Graphite rods served as an internal standard for image standardization and 3D visualization.

Main Results:

• Pure PMMA and commercial bone cements exhibited significant vertical swelling after hydration.
• The presence of polymer beads in bone cements restricted lateral swelling.
• Swelling parameters showed a correlation with nanoindentation-derived mechanical properties.
• A minor retraction and retraction ring were observed around the graphite standard after grinding.

Conclusions:

• PMMA-based orthopedic materials can undergo notable swelling due to small water absorption.
• The degree of swelling is dependent on the specific polymer composition and the inclusion of particles.
• These findings have implications for the long-term stability and performance of PMMA in orthopedic applications.