PEGylation of UHMWPE to Reduce Mechanical Adhesion

Sheryl R. Kane* and Lisa A. Pruitt*^

*UC San Francisco and UC Berkeley Joint Graduate Group in Bioengineering, San Francisco and Berkeley, CA^Department of Mechanical Engineering, UC Berkeley, Berkeley, CA

AcknowledgementsThanks to Stephen Kaplan and 4th State Inc. for use of the plasma equipment, and to Paul Ashby and Shikha Gupta for AFM assistance. AFM work at the Molecular Foundry was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Science and Engineering, of the U.S. Department of Energy under Contract #DE-AC02-05CH11231. XPS was performed at the University of Washing-ton NESAC/BIO facility, supported by NIH grant number EB-002027. In addition, the authors would like to acknowledge support from the American Association of University Women and the NSF (a Graduate Research Fellowship and grant number CMS 0505272 at UC Berkeley).

References1. Wang, M.L. et al. J. Arthroplasty 19(8): 1028-1038, 2004. 2. http://www.integran.com/applications/hip%20replacement%201.jpg3. Sato, T. et al. CLA Journal 28(4): 181-185, 2002.4. Johnston, E.E. et al. Langmuir 21: 870-881, 2005.5. Luk, Y.-Y. et al. Langmuir 16: 9604-9608, 2000.

Introduction Wear-mediated osteolysis, or bone loss, is the primary cause of late-stage im-plant failure in total hip replacements (THRs) [1]. Articulation between the metal or ceramic femoral head and the ultrahigh molecular weight polyethyl-ene (UHMWPE) acetabular cup (Figure 1 [2]) generates wear particles from the surfaces of the components. As particles accumulate around the implant, they elicit an immune response that results in chronic inflammation, pain, and oste-olysis, which can necessitate implant removal. The hip joint is lubricated by aqueous synovial fluid, but the UHMWPE surface (the main source of wear particles) is hydrophobic and poorly lubricated. This study uses a hydrophilic coating on UHMWPE to improve lubrication at the ar-ticulating interface. The coating is plasma-polymerized tetraglyme, a cross-linked network with a structure similar to polyethylene glycol (PEG). Grafted PEG has also been used to lubricate contact lenses [3]. PEG-derivative surfaces also resist protein and cell deposition [4,5], minimizing the immune response.

Results and Discussion The 10 and 20 minute time points in Figure 4 show the results of three depositions performed under identical plasma conditions on different days. Both the XPS and ATR-FTIR data indicate that the process generates reproducible surfaces. Figure 3 highlights the difference in sampling depth of ATR-FTIR and XPS, which ex-plains the different trends shown in Figure 4. The XPS results indicate that the deposi-tion reaches equilibrium after 5-10 minutes, such that longer processes yield the same surface chemistry. However, the E/H ratio increases linearly with deposition time. For 10-30 minute treatments, the change in E/H must reflect changes in the chemical structure from 5 nm to 1 µm. Increasing thickness of the PEG-like layer results in more ether and less hydrocarbon in the sampling volume and is consistent with longer plasma exposure. This suggests that ATR-FTIR is measuring relative layer thickness. Surface adhesion, a metric for lubricity, was evaluated by AFM. The results (see Figure 5) strongly suggest that these treatments reduce adhesion, and the effect ap-pears to be independent of the exact chemical composition of the PEG-like layer.

Materials and Methods UHMWPE (McMaster-Carr) was sonicated, then plasma treat-ed (see Figure 2) using a Plasma Science PS0500 instrument with a 550 W generator. The surfaces were analyzed using a Surface Science Instruments S-probe XPS spectrometer with a monochromatic Al Kα x-ray source with a 55˚ take-off angle. ATR-FTIR was performed on a Nicolet Avatar 360 with Omni-Sampler. The E/H ratio was defined as the ratio of the heights of the ether peak at ~1100 cm-1 and the hydrocarbon peak at 1472 cm-1. AFM was performed on an Asylum Research MFP-3D instrument using a Novascan Si cantilever (nominal stiffness 0.05 N/m) with a 10 µm borosilicate sphere.

Figure 1: a total hip replacement. The acetabular cup is usually UHMWPE.

acetabular cup

acetabular shell

femoral head

femoral shaft

Conclusions The tetraglyme deposition process generates a reproducible, well-defined PEG-like surface whose thickness can be controlled by changing the deposition time. These surfaces exhibit lower adhesion than untreated UHMWPE, but the measured effect is confounded by the topographical dependence of the adhesive force. Lower adhesive force should reduce the mechanical adhesion between articulating surfaces, thereby reducing wear particle formation from treated surfaces.

XPS (~5 nm)

UHMWPE substrate(shorter deposition)

PEG-like layer

UHMWPE substrate(longer deposition)

PEG-like layerATR-FTIR(~1 µm)

EH

EH

Figure 3: XPS probes the top ~5 nm of the surface, while ATR-FTIR probes ~1 µm deep, beyond the PEG-like layer.

UHMWPE UHMWPE

1. Ar cleaning (400 W) 2. Ar/tetraglyme deposition (50 W)

3. Ar flow (0 W, 3x)

OO

4

Figure 2: the tetraglyme plasma deposition process. The chemical structure of tetraglyme is highlighted in white.

Figure 4: a comparison of surface composition met-rics. The ether percent becomes constant after ~10 minutes of deposition (solid line), while the E/H ratio increases linearly with deposition time (dashed line).

Figure 5: the adhesive force of untreated UHMWPE (4% ether) and four modified sur-faces. The large standard deviations reflect the effect of topography on adhesion.