Water Absorption of Poly(methyl methacrylate) Measured by Vertical Interference Microscopy Mambaye N’Diaye, † Florence Pascaretti-Grizon, † Philippe Massin, ‡ Michel Felix Basle ́ , † and Daniel Chappard* ,† † GEROM Groupe Etudes Remodelage Osseux et bioMate ́ riaux-LHEA, IRIS-IBS Institut de Biologie en Sante ́ , LUNAM Universite ́ , CHU d’Angers, 49933 Angers Cedex, France ‡ Chirurgie Orthope ́ dique et Traumatologie, Hô pital Bichat, 75018 Paris, France ABSTRACT: PMMA (poly(methyl methacrylate)) is widely used to prepare orthopedic cements. They are in direct contact with cells and body fluids. PMMA, despite its hydrophobic nature, can absorb ∼2% w/w water. We have evaluated by vertical interference microscopy if water absorption can produce a significant swelling in different types of PMMA blocks: pure, with a plasticizer, with a cross-linker, and in two types of commercial bone cements. Graphite rods which do not swell in water were used as internal standard. Hardness, indentation modulus, plastic, and elastic works were determined by nanoindentation under a 25mN fixed force. Vertical interference microscopy was used to image the polymer in the dry state and hydrated states (after 24 h in distilled water). On the surface of the polished polymers (before and after hydration), we measured roughness by the fractal dimension, the swelling in the vertical and the lateral directions. For each polymer block, four images were obtained and values were averaged. Comparison and standardization of the images in the dry and hydrated states were done with Matlab software. The average value measured on the graphite rod between the two images (dried and hydrated) was used for standardization of the images which were visualized in 3D. After grinding, a small retraction was noticeable between the surface of the rod and the polymers. A retraction ring was also visible around the graphite rod. After hydration, only the pure PMMA and bone cements had a significant swelling in the vertical direction. The presence of polymer beads in the cements limited the swelling in the lateral direction. Swelling parameters correlated with the nanoindentation data. PMMA can swell by absorbing a small amount of water and this induces a swelling that varies with the polymer composition and particle inclusions. 1. INTRODUCTION Biomaterials are either synthetic or natural products used after implantation in the body, to augment or replace a tissue function that has been lost through disease or injury. Biomaterials are often studied in the dry state by microscopic methods such as light, scanning electron or atomic force microscopy, etc. to characterize the surface morphology that will come in direct contact with cells. One important characteristic is surface roughness that is known to influence cell adherence and spreading. 1-4 Surface roughness controls protein adsorption and can modify the shape of cells anchored at the surface of the biomaterial; this has been particularly evidenced for osteoblasts (bone forming cells) which can adapt and spread according to roughness geometry. 1,5 However, a biomaterial’s surface can strongly be modified by hydration in body fluids which can alter surface geometry and interact with protein adsorption. 6 To date, little is known about the aspect and behavior of the hydrated surface of biomaterials and their relationships with cells and tissues due to the limited available techniques to study them without dehydration. Acrylic polymers constitute a large family of biomaterials that can be used to repair a variety of tissues. 7,8 During the last decades, poly(methyl methacrylate) (PMMA) and PMMA- based biomaterials have been extensively used for hard tissue repair (bone and teeth). 9,10 PMMA was first used as a biomaterial for preparing hip prostheses, 11 but was popularized by Charnley in the 1960s as a bone cement for sealing metallic hip prostheses in the femoral shaft. 12 PMMA is a hydrophobic polymer that can be used in weight-bearing sites due to its ability to withstand important mechanical loads and elastic deformation for prolonged time periods. However, wear debris can be generated and can induce a giant cell inflammatory reaction leading to aseptic loosening of the prosthesis. 13 In odontology, PMMA has been used to prepare dentures since the 1950s and more recently reliner materials and composite restoration. It has been noted that PMMA, although a hydrophobic polymer, can take up ∼2% w/w of water. 14-17 Water absorption is known to affect the mechanical properties and can induce a significant susceptibility to mechanical damages. Swelling of PMMA-based materials has been seldom studied. We hypothesized that this small amount of water absorption can produce a signi ficant swelling depending on the composition of the polymer and its hardness. In the present Received: June 3, 2012 Revised: July 12, 2012 Published: July 16, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 11609 dx.doi.org/10.1021/la302260a | Langmuir 2012, 28, 11609-11614