Article pubs.acs.org/Langmuir
Water Absorption of Poly(methyl methacrylate) Measured by Vertical Interference Microscopy Mambaye N’Diaye,† Florence Pascaretti-Grizon,† Philippe Massin,‡ Michel Felix Baslé,† and Daniel Chappard*,† †
GEROM Groupe Etudes Remodelage Osseux et bioMatériaux−LHEA, IRIS-IBS Institut de Biologie en Santé, LUNAM Université, CHU d’Angers, 49933 Angers Cedex, France ‡ Chirurgie Orthopédique et Traumatologie, Hôpital 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. 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 significant swelling depending on the composition of the polymer and its hardness. In the present
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 PMMAbased biomaterials have been extensively used for hard tissue © 2012 American Chemical Society
Received: June 3, 2012 Revised: July 12, 2012 Published: July 16, 2012 11609
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Figure 1. 2D images obtained by vertical interference microscopy of a block of PMMA with a graphite core (g) embedded in PMMA in the dry state (panel A) and after hydration (panel B). The roughness parameters are measured on a vector drawn at a distance from the graphite core in panel A and the section profile appears in panel C. In panel D, the height (hD) and distance (dD) between the graphite core surface (g) and the polymer surface are measured on the vector drawn on panel B. For each composition, three blocks were prepared. Embedment was made in Peel-a-Way embedding molds (Polyscience Inc., Warrington PA) in a cold environment (+4 °C in a refrigerator, with a water bath to limit the polymerization peak). Orthopedic bone cements were prepared at room temperature. Polymerization is not completed when the blocks have hardened since PMMA can release free radicals, monomers, or side polymerization products for several weeks.20,21 So, a 2 month period separated the preparation of the blocks from further analysis. Blocks were cut perpendicular to the longitudinal axis of the graphite core. Specimens were then ground on a Dap-V benchtop grinder (Struers, Copenhagen, Denmark), using ascending grades of paper polishing: 1200 then 2400 and 4000 during 2 min each. Final polishing was made with a MD-Nap paper spray with DiaPro 1 μm solution. Blocks were affixed onto plastic slides with a hand press (Struers) in order to have a perfectly horizontal surface parallel to the slide. 2.2. Nanoindentation. Characterization of the biomechanical properties of the different polymers was determined by nanoindentation on a NHT-TTX system (CSM, Peseux, Switzerland). A pyramidal Berkowitch diamond probe was applied onto the polished surface of the polymers with a Poisson’s coefficient of 0.35. This produces hardness impressions with depth varying according to the material’s properties. On each polymer, six indents were done at a set distance from the graphite core. The indents were run by applying a constant force of 25 mN for both loading and unloading at a speed of 50 mN/min. At maximum load, a 15-s holding period was applied. After the load was removed, the diagonal impression or width was measured by the system and the hardness was derived depending on the geometry of the diamond probe and the following parameters were determined according to Oliver and Pharr:22 the indentation modulus (EIT, expressed in GPa); the indentation hardness (HIT, expressed in MPa); the contact stiffness (S, expressed in mN/μm); and the elastic reverse deformation work of indentation (Welast, expressed in N·m) and the plastic deformation work of indentation (Wplast, expressed in N·m). 2.3. Vertical Interference Microscopy. Optical interferometric measurements were done with a Wyko NT 9100 optical profiling system (Bruker AXS, Champ sur Marne, France). The microscope is based on light interferometry and operates as a noncontact optical profiler in vertical scanning interferometry mode to produce 3D topography maps of the sample surface. Briefly, a white light source is emitted by a conventional light source and is split into two beams
study, the effect of water absorption was evaluated on PMMA blocks prepared with/without a plasticizer or a cross-linker and on two types of commercial orthopedic bone cements based on PMMA. Nanoindentation was used to measure the biomechanical properties of the polymers in the dry state and Vertical Interference Microscopy was used to visualize and measure the effect of water absorption and swelling on the same block before and after hydration.
2. EXPERIMENTAL SECTION 2.1. Preparation of PMMA Blocks. All reagents used were of laboratory grade and purchased from Merck Chimie (France) and Aldrich-Sigma (Saint Quentin Fallavier, France). Pure graphite cores were used as an internal standard that does not swell into water. For this purpose, HB pencil cores (0.9 mm in diameter) were embedded in five different polymers prepared from MMA (methyl methacrylate) purified according to previously published methods:18,19 pure MMA without plasticizer and initiated with 1% benzoyl peroxide accelerated by N−N dimethylaniline (NNDMA) (400 μL in 25 mL, prepared from a stock solution of pure NNDMA (2 mL) in 2-propanol (18 mL); pure MMA with dibutyl phtalate (DBP, 10%) as a plasticizer, initiated and accelerated as above; pure methylmethacrylate with 3% ethylene glycol dimethacrylate (EGDMA) as a cross-linker, initiated and accelerated as above; and two commercial orthopedic bone cements (a low viscosity cement (Cerafixgenta BV, ref 789, batch #108210, Ceraver Osteal, Roissy, France) and a high viscosity cement (DEPUY CMW Gentamicin, ref 3315040, batch #3166737, DePuy Int Ltd., Blackpool, UK). The low viscosity cement is obtained by mixing 21 mL of the liquid component (MMA and n-butyl methacrylate) with 48 g of the solid component comprised of PMMA beads, zirconium dioxide (4.3 g), gentamicin (0.8 g), and benzoyl peroxide as the polymerization accelerator, the w/w ratio monomer/PMMA beads being about 33%. The high viscosity cement is obtained by mixing 18.37 g of the liquid component (MMA) with 40 g of the solid component comprised of PMMA beads, barium sulfate (3.64 g), gentamicin (1.68 g), and benzoyl peroxide as the polymerization accelerator; the ratio w/w monomer/solid phase being about 35%. The mixtures were prepared by an orthopedic surgeon by mixing the different components (MMA and PMMA microbeads containing zirconium oxide particles) in the same way as in the surgical theater, according to the manufacturers’ recommendations. 11610
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which pass through a Mirau’s interferometric objective. The incident beams are reflected from the reference mirror and the sample surface, respectively. The light reflected from this mirror combines with the light reflected from the sample to produce interference fringes (known as an interferogram) where the best-contrast fringe occurs at best focus. The light and dark fringes are used in combination with the wavelength of the light to determine the height difference between each fringe. A piezoelectric stage moves the sample vertically with a nanometer precision, which produces phase shifts in the interferogram. Interferograms were digitized by using a CCD camera and data were analyzed to produce a topographic surface map. The software Vision (release 4.10, Wyco) was used to acquire the data. In the first step, acquisitions were made on dried specimens at a magnification of 100; the images always comprised a large surface of polymer (2.3 mm × 1.8 mm) and ∼1/2 of the surface area of the graphite core. In the second step, similar acquisitions were done after the specimens were hydrated for 24 h in distilled water and the same areas were imaged (Figure 1A,B). The arithmetical mean roughness of the profiles (Ra) was measured by drawing a vector on the polymer surface at a set distance from the graphite core; the fractal dimension of the roughness profile was determined with Matlab lab-made software (MathWorks, Natick, MA), using the box-counting routine. Heights of the different phases (graphite and polymer) were determined by tracing another vector starting from the graphite core to the polymer. Differences in height were derived on the dry vs hydrated block (repspectively hD and hH) and the mean distance between the side of the graphite core and the surface of the polymer was determined as the lateral distance (respectively dD and dH) (Figure 1C,D). Finally, the changes in height due to the swelling of the material was derived as h = hH − hD and the lateral distance change as d = dH − dD. For each block of polymer, four images were obtained and the values were averaged. In a third step, files of processed data were saved in the .asc format for Matlab imaging since it is impossible in the Vision software to compare and to standardize the two images in the dry and hydrated state. 2.4. Standardization of Heights between Two Images. In the Matlab software, the .asc files are converted into a xy matrix which can be viewed as an image by using a dedicated toolbox. Pixels corresponding to the graphite core were selected on both images D and H with an interactive procedure. The average value on the graphite levels M(g) was calculated for each image and the difference was calculated. This difference abs(M(gD) − M(gH)) was added to all pixels of the image with the lowest graphite value M(g). In this way, the pixels of the graphite surface now have the same value in the two images and this operation has helped to offset values on the surface of the polymer. After being standardized, the surfaces of the dry and hydrated states were imaged in 3D, using the surface rendering function of Matlab with Phong’s normal-vector interpolation technique for surface shading. 2.5. Statistical Analysis. Statistical analysis was performed with the Systat statistical software release 13.0 (Systat Inc. San José, CA). Data obtained for the different blocks were pooled before analysis. Differences between the groups of polymers were assessed with the Kruskall-Wallis nonparametric ANOVA followed by posthoc tests (Conover-Inman test) for pairwise comparisons. Relationships between parameters were searched by using linear regression analysis (Pearson’s r correlation coefficient). Differences were considered significant when p < 0.05.
Figure 2. Topographic images obtained on the nanoindenter showing the triangular indent left by the pyramidal Berkowitch diamond: (A) on a block of polished pure PMMA and (B) on a block of polished high viscosity bone cement. The round profiles correspond to the microbeads of PMMA partially melted with the liquid phase of the cement, which also contains the zirconium particles (arrows).
the tangent method is used to determine S. The last part of the curve is obtained during the removal of the load. The area under the curve in the first part of the curve represents the plastic mechanical work (Wplast) while the area under the last part of the curve represents the elastic work (Welast). The nanoindentation parameters of the different types of polymers appear in Table 1. The addition of DBP as a plasticizer had only minor and nonsignificant effects on the different biomechanical parameters. The addition of EGDMA as a cross-linker had no effect on EIT or HIT but significantly increased the contact stiffness S, Welast, and Wplast (Figure 3B,C). The orthopedic cements were characterized by a highly significant increase in hardness and indentation modulus and a significant reduction in Welast and Wplast. Taken all together, these results indicate that the orthopedic cements were harder and stiffer than blocks prepared with the MMA monomer only. 3.2. Vertical Interference Microscopy Results. In the three types of PMMA, a slight retraction was observed at the interface of the graphite rods and the polymer on the dry specimens, but hD did not differ among groups (data not shown). At the interface with the graphite rod, a shrinkage of the polymer occurred in all blocks but the distance dD was limited when EGDMA on DBP has been added; on the two orthopedic cements, the retraction was minimal. All values significantly differed from those of pure PMMA (Table 1). After hydration, the surface roughness of the blocks (at a set distance from the graphite rod) slightly increased similarly in
3. RESULTS 3.1. Nanoindentation Results. Images of a nanoindent on a pure PMMA block and on a high viscosity bone cement appear in Figure 2. The typical indentation curves for pure PMMA and high viscosity cement are illustrated in Figure 3A. The ascending part of the curve corresponds to the loading period, until the 25 mN value is reached. At the end of the load, the holding period appears as a plateau. This point is used to determine the maximum indentation depth obtained (dmax) and 11611
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Figure 3. Nanoindentation results: (A) Typical curves obtained by nanoindentation on pure PMMA (in gray) and high viscosity bone cement (in black), for description, see section 3.1. (B) Indentation hardness (HIT) measured on the different types of blocks. (C) Plastic deformation work of indentation (Wplast). The results are expressed as mean ± SEM (standard error of the mean).
Table 1. Nanoindentation Parameters and dD Measured on the Dry Blocks and the Correlation Coefficient r and Significance p Calculated for dD (measured on the dry state) with Each Biomechanical Parameter EIT, GPa pure MMA MMA DBP MMA EGDMA cement low visc cement high visc r p
3.10 ± 3.02 ± 3.41 ± 4.43 ± 4.93 ± −0.63 0.02
0.06 0.08 0.11 0.03 0.07
HIT, MPa 196.4 ± 189.3 ± 216.0 ± 272.4 ± 295.1 ± −0.60 0.03
6.7 10 8.6 9.5 9.3
S, mN/μm 46.67 ± 46.22 ± 48.76 ± 49.60 ± 53.00 ± −0.58 0.04
0.69 0.28 0.64 1.10 0.00
Welast, N·m 8786 8619 8303 8298 7793 0.67 0.01
± ± ± ± ±
132 26 123 177 76
Wplast, N·m 19129 19971 18122 14995 14505 0.54 NS
± ± ± ± ±
637 736 537 507 237
dD 0.677 0.256 0.241 0.039 0.018
± ± ± ± ±
0.041 0.015 0.165 0.010 0.006
Figure 4. Quantitative results obtained by vertical interference microscopy: (A) difference in roughness measured by the box-plot fractal dimension; (B) difference in h; and (C) difference in the lateral distance d of swelling. The results are expressed as mean ± SEM (standard error of the mean).
The swelling of the polymer induced a narrowing of the area between the graphite rod and the polymer evidenced by an increased d, which was pronounced with pure MMA and significantly less important other types of blocks (Figure 4C). On the standardized images of the different types of polymers, the swelling in height and in the lateral direction in the vicinity of the graphite rods is evidenced in pseudocolors in Figure 5. The vertical interference microscopy parameters correlated well
the pure PMMA blocks and in the two types of cements (Figure 4A). No significant change in roughness could be observed when DBP or EGDMA were added meaning that these polymers had a lower tendency to swell. The variation in h between the dry and hydrated states followed a similar evolution (Figure 4B) and here again, h did not significantly increase in PMMA containing either DBP or EGDMA. When considering the lateral swelling as evidenced by the variation in d the differences between the blocks were more pronounced. 11612
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retraction occurred because blocks were ground and polished under water and dried before vertical interference microscopy imaging. The lateral retraction appreciated by dD is decreased when a plasticizer or a cross-linker is added since it reduces the ∼20% shrinkage observed for pure MMA.33,34 Shrinkage is also reduced when PMMA beads are added in the composition of bone cements: their surface is partially melted by the monomer when preparing the cement and the new chains are incorporated onto the surface of the beads at the end of polymerization. The presence of beads (containing longer polymer chains) is also responsible for the increase in hardness evidenced by EIT, HIT, and S together with reduced works in the elastic and plastic domains. In a nanoindentation on bone embedded in PMMA, Ferguson reported very similar values for EIT on pure PMMA blocks (4.3 ± 0.2 GPa).35 It is likely that polymerization is not uniform throughout the blocks and the small area around the graphite rod probably contains a less well polymerized PMMA with low molecular weight chains. It was possible to measure the hardness by nanoindentation in this area only in the pure PMMA blocks because dD was sufficiently large to allow some indentations. EIT and HIT were markedly reduced (2.65 ± 0.3 vs 3.10 ± 0.06) confirming this hypothesis. However, this region was too narrow on the other blocks and its surface geometry impaired analysis. In the present study, vertical interference microscopy appeared as a very interesting method to evaluate the surface changes of a polymer after hydration. However, standardization of the images is necessary since the system does not provide the possibility to compare two images. It was possible to show the swelling after hydration in both directions: vertical, evidenced by h, and lateral, evidenced by d. Also, the roughness of the polymer was increased after hydration and characterized by the box-plot fractal dimension, a simple tool to measure the surface of polymer and hydrogels.36 The addition of a hydrophobic plasticizer (or the EGDMA cross-linker) considerably limits the possibility to swell. At the opposite end, the orthopedic cements which are prepared with about 50% of MMA monomer retain the possibility to absorb water on their surface leading to a similar swelling in the vertical direction. Inversely, the lateral swelling (which was also limited with EGDMA and DBP) was reduced in the cements due to the presence of the PMMA beads as mentioned above. The embedment of zirconium oxide particles in the cement are clearly evidenced by both nanoindentation and vertical interference microscopy. MMA cannot bind to these radio-opacifying particles which are visible at the dry state. Because these particles are hydrophobic, they do not swell nor increase roughness after hydration.
with nanoindentation data (Table 1): best results were observed between d and Wplast (r = 0.72, p = 0.005).
Figure 5. Vertical interference microscopy on the dry and hydrated polymer blocks. For each pair of blocks, the scale is expressed in pseudocolor in 10 nm. Note the changes in color which illustrate the swelling in the vertical direction and the reduced distance between the graphite rod and the polymer after swelling. On the cement, the zirconium particles and the bubbles created during mixing are evidenced.
4. DISCUSSION The main findings of this study are that PMMA can swell when hydrated and this modifies slightly the dimensions of the blocks in the vertical and lateral directions. PMMA is widely used in biomedicine and is classified as a biotolerated polymer. The polymer’s hardness obtained with orthopedic bone cements is known to depend on a high number of parameters. The ratio between the polymerization accelerator (benzoyl peroxide) and the initiator (NNDMA) is well-known to influence polymerization kinetics and mechanical properties.23 Other factors such as the duration of mixing, the thickness of the polymer mass, the room temperature, and the presence of fat and aqueous body fluids have also been reported to strongly influence the mechanical properties.7,24 On the other hand, several reports have shown the absence of influence of gentamicine incorporation.25,26 These properties strongly rely on the molecular weight distributions of the polymer chains which are heterogeneous for PMMA.27 The presence of oxygen is known to affect redox polymerization by producing smaller chains.28,29 We previously showed, with a microdurometer, that the hardness of methacrylic polymers was reduced at the top (exposed to O2) and the sides of the polymer blocks (in direct contact with the walls of the mold).30 This is due to the fact that the monomer does not cross-react with the mold surfaces. Similarly, it does not cross-react with other components (e.g., zirconium particles, prosthesis, bone, ...) at the time of polymerization.31,32 In the present study, the small peripheral zone in the form of a ring around the graphite rods noted on the dry specimens can be explained by such a mechanism. This
5. CONCLUSION PMMA and PMMA-containing plasticizers, cross-linkers or beads and zirconium oxide particles can absorb a small amount of water and the swelling can be evidenced in the vertical and lateral directions by vertical interference microscopy after standardization of the images. The addition other chemical compounds also strongly affects the biomechanical and hydration capacity of the PMMA. This may be of importance in the use of bone cements: the small swelling noted may contribute to cement aging and the development of aseptic loosening of the prostheses. Also, the radio-opaque particles (which are only encapsulated in the polymer without a firm attachment) may be released and contribute to the inflammatory reaction on the interface of the bone cements. 11613
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(16) Smith, K. E.; Trusty, P.; Wan, B.; Gall, K. Long-term toughness of photopolymerizable (meth)acrylate networks in aqueous environments. Acta Biomater. 2011, 7, 558−567. (17) Ayme, J. C.; Emery, J.; Lavielle, L.; Lischetti, G.; Schultz, W. Wettability of poly(methyl methacrylate) surfaces in dental use. J. Mater. Sci.: Mater. Med. 1989, 3, 387−390. (18) Chappard, D.; Palle, S.; Alexandre, C.; Vico, L.; Riffat, G. Bone embedding in pure methyl methacrylate at low temperature preserves enzyme activities. Acta Histochem. 1987, 81, 183−190. (19) Chappard, D. Technical aspects: How do we best prepare bone samples for proper histological analysis? In Bone cancer: progression and therapeutic approaches; Heymann, D., Ed.; Academic Press; Elsevier Inc.: London, UK, 2009; pp 203−210. (20) Moreau, M. F.; Chappard, D.; Lesourd, M.; Montheard, J. P.; Baslé, M. F. Free radicals and side products released during methylmethacrylate polymerization are cytotoxic for osteoblastic cells. J. Biomed. Mater. Res. 1998, 40, 124−131. (21) Gough, J. E.; Downes, S. Osteoblast cell death on methacrylate polymers involves apoptosis. J. Biomed. Mater. Res. 2001, 57, 497−505. (22) Oliver, W. C.; Pharr, G. M. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J. Mater. Res. 1992, 7, 1564−1583. (23) Milner, R. The development of theoretical relationships between some handling parameters (setting time and setting temperature), composition (relative amounts of initiator and activator) and ambient temperature for acrylic bone cement. J. Biomed. Mater. Res. Part B 2004, 68, 180−185. (24) Lee, A. J.; Ling, R. S.; Gheduzzi, S.; Simon, J. P.; Renfro, R. J. Factors affecting the mechanical and viscoelastic properties of acrylic bone cement. J. Mater. Sci.: Mater. Med. 2002, 13, 723−733. (25) Wright, T. M.; Sullivan, D. J.; Arnoczky, S. P. The effect of antibiotic additions on the fracture properties of bone cements. Acta Orthop. Scand. 1984, 55, 414−418. (26) Lewis, G.; Bhattaram, A. Influence of a pre-blended antibiotic (gentamicin sulfate powder) on various mechanical, thermal, and physical properties of three acrylic bone cements. J. Biomater. Appl. 2006, 20, 377−408. (27) Herington, E. F. G.; Robertson, A. Kinetics of polymerisation and molecular weight distributions in polymethyl methacrylates. Nature 1947, 159, 745. (28) Decker, D.; Jenkins, A. D. Kinetic Approach of O2 Inhibition in Ultraviolet- and Laser-Induced Polymerizations. Macromolecules 1985, 18, 1241−1244. (29) Tobolsky, A. V.; Mesrobian, R. B., Organic peroxides; Interscience Publ.: New York, 1954. (30) Chappard, D.; Vocanson, F.; Monthéard, J. P. Polymerization of methacrylates used as histological embedding mediums: local variations and time course of hardness of methylmethacrylate blocks. J. Histotechnol. 1993, 16, 65−68. (31) Vuorinen, A. M.; Dyer, S. R.; Lassila, L. V.; Vallittu, P. K. Effect of rigid rod polymer filler on mechanical properties of poly-methyl methacrylate denture base material. Dent. Mater. 2008, 24, 708−713. (32) Kühn, K. D. Bone cements. Up-to-date comparison of physical and chemical properties of commercial materials; Springer-Verlag: Berlin, Germany, 2000. (33) Gilbert, J. L.; Hasenwinkel, J. M.; Wixson, R. L.; Lautenschlager, E. P. A theoretical and experimental analysis of polymerization shrinkage of bone cement: A potential major source of porosity. J. Biomed. Mater. Res. 2000, 52, 210−218. (34) Nien, Y. H.; Chen, J. Studies of the mechanical and thermal properties of cross-linked poly(methylmethacrylate-acrylic acidallylmethacrylate)-modified bone cement. J. Appl. Polym. Sci. 2006, 100, 3727−3732. (35) Ferguson, V. L. Deformation partitioning provides insight into elastic, plastic, and viscous contributions to bone material behavior. J. Mech. Behav. Biomed. Mater. 2009, 2, 364−374. (36) Mabilleau, G.; Baslé, M. F.; Chappard, D. Evaluation of surface roughness of hydrogels by fractal texture analysis during swelling. Langmuir 2006, 22, 4843−4845.
AUTHOR INFORMATION
Corresponding Author
*Tel: (33) 244 68 83 49. Fax: (33) 244 68 83 50. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Authors thank Mrs. Laurence Lechat for secretarial assistance, Florence Mallard for help in preparing the bone cements, and Mr. Guillaume Mabilleau for reviewing the manuscript. This work was made possible by grants from Contrat Region Pays de la Loire: Bioregos2 program.
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