Reactive species inhibited by

Richard Suzuki,1,3 Julie Muyco,2,3 Joanna McKittrick,2,3 John A. Frangos1,3 1Department of Bioengineering, University of California, San Diego, La Jolla, California 92037 2Department of Mechanical and Aerospace Engineering, Material Science, and Engineering Program, University of California, San Diego, La Jolla, California 92037 3La Jolla Bioengineering Institute, 505 Coast Boulevard South, La Jolla, California 92037

Received 3 June 2002; revised 25 September 2002; accepted 30 September 2002

Abstract: Titanium is a successful biomaterial that pos- stimulated to produce and interleukin-6. Super- sesses good biocompatibility. It is covered by a surface layer oxide production was measured by the chemiluminescent of titanium dioxide, and this oxide may play a critical role in reaction with 2-methyl-6-[p-methoxyphenyl]-3,7-dihydro- inhibiting reactive oxygen species, such as peroxynitrite, imidazo[1,2-a]pyrazin-3-one (MCLA). Titanium dioxide- produced during the inflammatory response. In the present coated exhibited a 55% decrease in superoxide com- study, titanium dioxide was coated onto silicone substrates pared to uncoated silicone and a 165% decrease in by radio-frequency sputtering. Silicone with tita- superoxide compared to uncoated polystyrene. Titanium nium dioxide enhanced the breakdown of peroxynitrite by dioxide-coated silicone inhibited IL-6 production by 77% 79%. At physiologic pH, the peroxynitrite donor 3-morpho- compared to uncoated silicone. These results show that the linosydnonimine-N-ethylcarbamide (SIN-1) was used to ni- anti-inflammatory properties of titanium dioxide can be trate 4-hydroxyphenylacetic acid (4-HPA) to form 4-hy- transferred to the surfaces of silicone substrates. © 2003 droxy-3-nitrophenyl (NHPA). Titanium dioxide- Wiley Periodicals, Inc. J Biomed Mater Res 66A: 396–402, coated silicone inhibited the nitration of 4-HPA by 61% 2003 compared to aluminum oxide-coated silicone and 55% com- pared to uncoated silicone. J774A.1 mouse macrophages Key words: titanium dioxide; silicone; anti-inflammatory; were plated on oxide-coated silicone and polystyrene and oxide coating; peroxynitrite

INTRODUCTION duces colonic inflammation in rats7 and has been dem- onstrated to be present in the inflamed guinea pig 8 Inflammatory response is part of a general pattern ileum. Peroxynitrite was found to be produced by of recovery and wound healing that to eventual acute inflammation from edema induced in the hind 9 acceptance of a foreign material placed in the body.1 paws of rats. This pattern of events typically leads to fibrotic encap- Clinical studies also provide evidence that per- sulation of the implant. Prolonged inflammatory re- oxynitrite is produced during inflammation. The sponses can have the consequence of more intense blood serum and synovial fluid from patients with the tissue reactions requiring extrusion of the implant.2 inflammatory joint disease rheumatoid arthritis were Ϫ The reactive oxygen species peroxynitrite (OONO ) found to contain 3-nitrotyrosine markers, indicating has been shown to play a role in inflammation. Per- peroxynitrite formation while body fluids from nor- ⅐Ϫ mal patients contained no detectable 3-nitrotyrosine. oxynitrite is formed from superoxide (O2 ) and (NO⅐),3 and it is a potent oxidant capable of a Similarly, no 3-nitrotyrosine markers were detected in wide range of reactions.4–6 Peroxynitrite directly in- body fluids from patients with osteoarthritis, a largely non-inflammatory joint disease.10 It is important to Correspondence to: J.A. Frangos @ Bioengineering Institute; note that it has been reported that 3-nitrotyrosine e-mail: [email protected] markers for peroxynitrite also have been observed at Contract grant sponsor: NIH; contract grant number: the interface membrane of hip implants suffering from EB00823 aseptic loosening, which is characterized by local in- Contract grant sponsor: NIH; contract grant number: 11,12 AR47032 flammation. Previously, it was shown that titanium dioxide is © 2003 Wiley Periodicals, Inc. capable of inhibiting the reactivity of peroxynitrite.13 OXIDE COATINGS ON SILICONE 397

Titanium dioxide was shown to enhance the break- Nitration of phenol (4-HPA) by peroxynitrite donor down of peroxynitrite and inhibit the nitration reac- tions of peroxynitrite at physiologic pH levels. Tita- Peroxynitrite has a half- of 1.9 s at physiologic pH.16 The nium surfaces retained the ability to inhibit short half-life makes experiments difficult in this pH range. peroxynitrite even in the presence of 10% fetal bovine This problem was circumvented through the use of 3-morpho- serum, fibrinogen, and bicarbonate. linosydnonimine-N-ethylcarbamide (SIN-1) (Alexis Chemicals, Others have shown that a surface with the ability to San Diego, CA). SIN-1 slowly decomposes to release NO and superoxide at physiologic pH, which then react to form per- breakdown reactive oxygen species can improve the 4,15 biocompatibility of . Polyethylene implants oxynitrite. Peroxynitrite is highly reactive and can nitrate phenolic coated with superoxide dismutase mimics showed a 15 residues, such as tyrosine. It nitrates 4-hydroxyphenylac- notable decrease in capsule thickness compared to 14 etic acid (4-HPA) to form 4-hydroxy-3-nitrophenyl acetic uncoated controls. These results indicate that super- acid (NHPA). NHPA absorbs at 432 nm, and its concentra- oxide, a precursor of peroxynitrite, plays a role in the tion was calculated by measuring the change of absorbance inflammatory response to biomaterial surfaces. at pH 6.0–6.5 and pH 10.0–10.5.17 The objective of this study was to determine if the Solutions of PBS buffer (Irvine Scientific, Irvine, CA) with ability to inhibit reactive inflammatory species also 0.5 mM of 4-HPA (Aldrich, Milwaukee, WI) were placed could be imparted to a by coating the surface over coated silicone substrates in airtight containers. After with a thin film of titanium dioxide. Such coatings autoclaving, SIN-1 was added to yield a final concentration may to improvement in biocompatibility and mit- of5mM. A second oxide-coated silicone substrate was igation of the inflammatory response of implants. placed over the first sample, trapping the solution between the two samples. The containers were sealed and placed in the dark in 37°C incubators for 14 days.

MATERIALS AND METHODS Surface interaction with superoxide from activated macrophages Sample preparation Mouse macrophages from the cell line J774A.1 (ATCC, Oxide-coated silicone elastomer samples were fabricated Manassas, VA) were plated on oxide-coated silicone sub- using radio frequency (RF) plasma magnetron sputtering. strates lining the bottoms of petri dishes. The cells grew to The silicone substrates were cut from sheets of non-rein- form a monolayer that adhered to the coated surfaces. Oxide forced, translucent silicone sheeting (SF Medical, Hudson, coatings at the thickness deposited were transparent, allow- MA) and were 1.5 mm in thickness. ing for spectrometry assays. Deposition rates of the oxide layer were calibrated using The macrophages were grown in DMEM on the oxide- film deposition on quartz substrates under conditions sim- coated silicone substrates contained in sterile petri dishes ilar to those on silicone substrates. The thickness of the oxide (Irvine Scientific, Irvine, CA) with 5% fetal calf serum (Hy- layer was measured with a Dektak IIa (Digital Instruments, clone) and 1 mM of pyruvate (Aldrich). The cells Santa Barbara, CA). The typical range of thickness for oxide were stimulated to produce superoxide by addition of 15 coatings was 100–200 nm. ␮g/mL of phorbol 12-myristate 13-acetate (PMA). Superox- The continuity of the coatings was probed using energy- ide production was measured by the chemiluminescent re- dispersive X-ray (EDS) in a mapping function action with 2-methyl-6-[p-methoxyphenyl]-3,7-dihydroimi- (Oxford Instruments X-ray spectrometer, Concord, MA). El- dazo[1,2-␣]pyrazin-3-one (MCLA).18,19 MCLA is two orders emental information was obtained from points on the sur- of magnitude more specific for superoxide detection than face of the sample corresponding to the scanning electron lucigenin or luminol.20 microscope (SEM) image (Cambridge 360 SEM LEO Electron Macrophages were incubated in a solution of PBS with Microscopy, Thornwood, NY). No preferential location of MCLA (1.5 ␮M). The cells were stimulated with PMA (15 elements was seen, indicating there was no disruption of ␮g/mL) and placed in a receptacle with an attached photo- continuity of the coating within the resolution of the instru- multiplier tube and photon counter, allowing measurement ment. of the resulting chemiluminescence. The receptacle was lo- cated in a darkroom and was equipped with a temperature control system that maintained the cells at 37°C.

Peroxynitrite degradation rates Cytokine production from activated macrophages Peroxynitrite was synthesized using a quenched-flow re- actor system.15 The breakdown of peroxynitrite over sub- Mouse J774A.1 macrophages were plated on silicone sub- strate samples was monitored by the decrease in absorbance strates with titanium dioxide coatings. Aluminum oxide- at 302 nm using a DU 640 Beckman spectrophotometer. coated silicone served as controls. The macrophages were 398 SUZUKI ET AL.

coated silicone and a 55% decrease compared to un- coated silicone. Superoxide is a precursor to peroxynitrite and pro- duced by stimulated macrophages.21 Figure 3 com- pares the chemiluminescence from stimulated macro- phages plated on various substrates with different oxide coatings. The signal values are 5000 s after stim- ulation and normalized to the initial baseline signal. Macrophages were plated on uncoated silicone and titanium dioxide-coated silicone [Fig. 3(A)]. Cells plated on titanium dioxide-coated substrates exhibited a decrease in their chemiluminescent signals after stimulation. There was a 55% decrease in signals from cells on titanium dioxide-coated silicone compared to uncoated silicone. Macrophages also were plated on uncoated polystyrene and titanium dioxide-coated polystyrene [Fig. 3(B)]. Macrophages on titanium di- oxide-coated polystyrene exhibited a 165% decrease in their chemiluminescent signals after stimulation com- pared to uncoated polystyrene. Macrophages were plated on uncoated and oxide- coated silicone and stimulated to produce interleu- kin-6 (Fig. 4). There was a 77% decrease in the pro- duction of IL-6 from stimulated macrophages cultured

Figure 1. Decomposition of peroxynitrite after 6 h. Per- oxynitrite measured by absorbance at 302 nm. Solution pH ϭ 13.4 [*p Ͻ 0.01 compared to plain and aluminum coated silicone (Student–Newman–Keuls test)]. Error bars given as Standard Error of the Mean.

stimulated with PMA (15 ␮g/mL), and IL-6 levels were measured by ELISA (R&D Systems; Minneapolis, MN).


Figure 1 shows the decomposition of peroxynitrite in solution (pH ϭ 13.4) after6hofexposure to silicone substrates with different oxide coatings. There was a 79% increase in the decomposition of peroxynitrite in solutions exposed to titanium dioxide-coated silicone compared to uncoated silicone and a 77% increase compared to aluminum oxide-coated silicone. Experiments at physiologic pH (7.4) were con- ducted using solutions of 4-HPA with SIN-1 as a per- oxynitrite donor. Solutions were exposed to silicone Figure 2. Nitration of 4-hydroxyphenylacetic acid (4-HPA) substrates with different oxide coatings during the by 3-morpholinosydnonimine (SIN-1) breakdown in the breakdown of SIN-1 to peroxynitrite. Titanium diox- presence of silicone with different oxide coatings. Concen- ide-coated silicone exhibited a decrease in nitration of tration of nitrated 4-HPA was measured by absorbance at 4-HPA to NHPA compared to aluminum oxide-coated 432 nm. Samples measured after 14 days of SIN-1 break- down. [*p Ͻ 0.05 vs. aluminum oxide coating (Student– silicone and uncoated silicone (Fig. 2). There was a Newman–Keuls test) and p Ͻ 0.05 vs. silicone control (Stu- 61% decrease in the nitration of 4-HPA with titanium dent–Newman–Keuls test)]. Error bars given as Standard dioxide-coated silicone compared to aluminum- Error of the Mean. OXIDE COATINGS ON SILICONE 399

Figure 3. Chemiluminescent signal produced by J774.1A murine macrophages after stimulation by PMA. Macrophages cultured on A) silicone polymer substrates with and without coatings [*p Ͻ 0.05 vs. Silicone (Student’s t test)] and B) tissue cultured treated dishes with and without amorphous titanium oxide coatings [*p Ͻ 0.1 vs. culture dish (Student’s t test)]. Values are normalized ratios of luminescent signal after 5000 s to baseline signal. Error bars given as Standard Error of the Mean. on titanium dioxide-coated silicone compared to un- flammation, in contrast to plugs comprised of the coated silicone controls. Cytokine production was less polymers Teflon and Delrin, which induced chronic than half that of aluminum oxide-coated silicone. inflammation and never fully integrated with the tis- sue.25 Titanium is widely used as an implant material with DISCUSSION excellent clinical results. The exact mechanisms of its superior performance in the biologic environment cur- rently are unknown. Upon exposure to air, titanium These results show that coatings of titanium dioxide readily forms a stable surface layer of oxide that con- applied to silicone substrates can result in the decrease sists predominantly of titanium dioxide, TiO .26 After in the reactivity and production of inflammatory me- 2 insertion of the implant, recruited inflammatory cells diators such as peroxynitrite and superoxide. encounter this oxide layer of the titanium, and it has A few studies have compared the biocompatibility been proposed that the oxide layer plays a fundamen- of titanium and polymer implants. Titanium particu- 27 lates showed less initial reaction when injected into tal role in tissue response. rats than particulates comprised of polymethyl- Titanium dioxide is known to catalytically break methacrylate.22 Titanium implants showed less in- down , the product of the superox- 28 flammatory response compared to polyethylene im- ide dismutase-catalyzed reaction of superoxide. It plants when placed in normal and arthritic joints of also is known that TiO2 can act as a catalyst in reac- 29,30 rats.23 Leukocytes associated with the surface of tita- tions involving reactive oxygen species. - nium implants in rats were less responsive to stimu- line TiO2 powder has been examined as a photocata- lation than were leukocytes from polytetrafluoroeth- lyst for the purification of .31 Hydroxyl radicals ylene implants.24 Plugs of titanium implanted in the that initiate oxidation of hydrocarbons to di- abdominal walls of rats were seen to become inte- oxide, water, and water-soluble organics are involved grated with the surrounding soft tissues without in- in these reactions.32–34 These findings indicate that 400 SUZUKI ET AL.

catalyst™O ϩ reductant 3 catalyst ϩ reductant™O (1A)

catalyst ϩ oxidant™O 3 catalyst™O ϩ oxidant (1B)

This results in the transfer of oxygen from the cat- alyst to the reductant and then from the oxidant back to the catalyst. Surface lattice oxygen in metal oxide catalysts participates in these reactions. A similar cat- alytic reaction scheme has been suggested for the ob- served breakdown of by titanium oxide.39 The possibility exists that other reactive oxy- gen species could undergo these reactions as well.

A neighboring pair of titanium dioxide [Ti(IV)O2] sites could serve as the oxygen-donating catalyst in Equation (1A). Superoxide could serve as the reduc- tant, with two free radicals donating their extra elec- trons to reduce the two titanium dioxide sites to a

single titanium sesquioxide [Ti(III)2O3] site, with the resulting generation of oxygen and water. Peroxyni- trite is known to undergo two electron oxidation re- actions,5 and this species could serve as the oxidant in Figure 4. Interleukin-6 production by stimulated J77A.1 Equation (1B). Substituting these species into Equa- macrophages cultured on silicone substrates with different tions (2A) and (2B) gives the following reactions: oxide coatings. [*p Ͻ 0.005 vs. aluminum oxide (Student’s t ϩ ⅐Ϫϩ ϩ 3 test)]. Error bars given as Standard Error of the Mean. Ti(IV)O2 2O2 2H Ti(III)2O3 ϩ O ϩ H O (2A) titanium oxide can act as a catalyst in reactions involv- 2 2 ing free-radical species. ϩ Ϫ 3 ϩ Ϫ Ti(III)2O3 OONO 2 Ti(IV)O2 NO2 (2B) The observation has been made that the tissue around titanium implants, although not inflamed, Furthermore, in Equation (2B) hydrogen peroxide takes on a bluish discoloring.35 This discoloring results also could be substituted for peroxynitrite, resulting in from nonabrasive leaching of the metal into the sur- the breakdown of hydrogen peroxide and the forma- 39 rounding tissue. The bluish color of titanium com- tion of 2Ti(IV)O2 sites. plexes caused by leaching indicates that the titanium These reactions provide a pathway through which is complexed in the Ti(III) (3ϩ valence) state.35,36 The titanium oxide can directly remove superoxide, which exact mechanism for the leaching is unknown, but in agrees with the results shown in Figure 3. Addition- spite of this behavior, the discoloring is regarded as ally, this same mechanism provides the means for harmless, and it does not affect the biocompatibility of titanium oxide to remove the reactive superoxide titanium. product, peroxynitrite, as shown in Figures 1 and 2. It has been suggested that this leaching phenome- The mechanism requires the formation of Ti(III) in non may be due to an interaction with reactive oxygen the oxide, and this may account for the bluish discol- species produced during the inflammatory response.36 oring around titanium implants caused by Ti(III), as Such reactive species would interact with the surface observed in vivo. Although the mechanism is catalytic, of the titanium implant that is covered with a titanium with Ti(III) returning to the Ti(IV) valence state, it is oxide layer. conceivable that excess Ti(III) may be formed by su- Catalytic materials have high surface areas and peroxide produced during the inflammatory response, possess surface-active sites. The chemical reactivity resulting in Ti(III) leaching out into the surrounding of metal oxide surfaces has been shown to be related tissue. directly to the coordination environment of the surface Other mechanisms through which titanium oxide cations37 and to be most affected by oxygen de- could catalytically break down reactive species also ficiency.38 The oxygen defect concentration is con- are possible. Peroxynitrite can undergo similar reac- trolled by aliovalent doping or annealing in a reducing tions as superoxide.17,40–42 Given the fact that per- atmosphere. Catalytic oxidation reactions can be de- oxynitrite can undergo reactions in place of the super- scribed by a general mechanism: oxide, it is natural to hypothesize that peroxynitrite OXIDE COATINGS ON SILICONE 401

may interact with the titanium oxide surface layer in a References similar manner. Figures 3 and 4 show that superoxide production 1. Anderson JM. Host reactions to biomaterials and their evalu- and cytokine production of stimulated macrophages, ation: Inflammation, wound healing, and the foreign body response. In: Ratner BD, Hoffman AS, Schoen FJ, Lemons JE, respectively, were reduced in the presence of titanium editors. Biomaterial science: An introduction of materials in dioxide surfaces. This is consistent with reports dem- . New York: Academic Press; 1996. p 165–173. onstrating antioxidant activity affecting the cellular 2. Thomsen P, Ericson LE. Inflammatory cell response to bone redox state and, in turn, mediating proinflammatory implant surfaces. In: Davies JE, editor. The bone–biomaterial cytokine gene expression.43 Therefore, we hypothesize interface. Toronto: University of Toronto Press; 1990. p 153– 164. that degradation of reactive oxygen species by tita- 3. Huie RE, Padmaja S. The reaction of NO with superoxide. Free nium dioxide surfaces leads to reduced oxidant stress Rad Res Comm 1993;18:195–199. in macrophages, and then to decreased production of 4. Crow JP, Beckman JS. Reactions between nitric oxide, super- cytokines such as interleukin-6. oxide, and peroxynitrite: Footprints of peroxynitrite in vivo. Polyethylene implants coated with superoxide dis- Adv Pharm 1995;34:17–43. 5. Koppenol WH, Moreno JJ, Pryor WA, Ischiropoulos H, Beck- mutase mimics showed a notable decrease in capsule man JS. Peroxynitrite A cloaked oxidant formed by nitric oxide 14 thickness compared to uncoated controls. This is and superoxide. Chem Res Toxicol 1992;5:834–842. consistent with the reported direct link between su- 6. Pryor WA, Squadrito GL. The chemistry of peroxynitrite: A peroxide and the production of proinflammatory me- product from the reaction of nitric oxide with superoxide. Am J diators, including cytokines, through two transcrip- Physiol 1995;12:L699–L722. ␬ 43 7. Rachmilewitz D, Stamler JS, Karmeli F, Mullins ME, Singel DJ, tional activators, NF- B and AP-1. These results Loscalzo J, Xavier RJ, Podolsky DK. Peroxynitrite-induced rat agree with the results shown in Figure 4, which indi- colitis—A new model of colonic inflammation. Gastroenterol cate that species such as superoxide, a precursor of 1993;105:1681–1688. peroxynitrite, play a role in the inflammatory response 8. Miller MJ, Thompson JH, Zhang X, Sadowska-Krowicka H, to biomaterial surfaces. Interestingly, this group con- Kakkis JL, Munshi UK, Sandoval M, Rossi JL, Eloby-Childress S, Beckman JS, Ye YZ, Rodi CP, Manning PT, Currie MG, Clark cluded that peroxynitrite was a likely mediator of the DA. Role of inducible nitric oxide synthase expression and proinflammatory effects of superoxide in the implant peroxynitrite formation in guinea pig ileitis. Gastroenterol environment. 1995;109:1475–1483. 9. Salvemini D, Wang Z, Bourdon D, Stern M, Currie M, Manning P. Evidence of peroxynitrite involvement in the carrageenan- CONCLUSIONS induced rat paw edema. Eur J Pharm 1996;303:217–220. 10. Kaur H, Halliwell B. Evidence for nitric oxide-mediated oxi- dative damage in chronic inflammation: Nitrotyrosine in se- The results of these presented experiments suggest rum and synovial fluid from rheumatoid patients. FEBS Lett that a crucial difference between titanium and poly- 1994;350:9–12. 11. Hukkanen M, Corbett S, Batten J, Konttinen Y, McCarthy I, mer implants may be the ability of titanium to inhibit Maclouf J, Santavirta S, Hughes S, Polak J. Aseptic loosening of reactive oxygen species, which results in the mitiga- total hip replacement. Macrophage expression of inducible tion of the inflammatory response. This may explain nitric oxide synthase and cyclo-oxygenase-2, together with the differences in tissue responses at an implant. peroxynitrite formation, as a possible mechanism for early Polyethylene surfaces coated with superoxide dis- prosthesis failure. J Bone Joint Surg Br 1997;79:467–474. 12. Hukkanen M, Corbett SA, Platts LA, Konttinen YT, Santavirta mutase mimics enhance the breakdown of superoxide S, Hughes SP, Polak JM. Nitric oxide in the local host reaction 14 at the surface of the implant, but such mimics can to total hip replacement. Clin Orthop Rel Res 1998;352:53–65. convert superoxide only to hydrogen peroxide. Hy- 13. Suzuki R, Frangos JA. Inhibition of inflammatory species by drogen peroxide is a reactive species and the enzyme titanium surfaces. Clin Orthop Rel Res 2000;372:280–289. catalase is required to further break hydrogen perox- 14. Udipi K, Ornberg RL, Thurmond KB, Settle SL, Forster D, Riley DP. Modification of inflammatory response to implanted bio- ide down to water and oxygen to prevent damage to medical materials in vivo by surface bound superoxide dis- surrounding tissue. Titanium dioxide coatings have mutase mimics. J Biomed Mater Res 2000;51:549–560. the advantage of catalytically breaking down hydro- 15. Beckman JS, Chen J, Ischiropoulos H, Crow JP. Oxidative gen peroxide28 as well as promoting the breakdown of chemistry of peroxynitrite. Met Enzymol 1994;233:229–241. superoxide and peroxynitrite. 16. Beckman JS, Beckman TW, Chen J, Marshall PA, Freeman BA. Apparent production by peroxynitrite: Impli- These experimental findings offer the potential of cations for endothelial injury from nitric oxide and superoxide. combining the range of physical characteristics and Proc Natl Acad Sci USA 1990;87:1620–1624. cost effectiveness of silicone and other polymers with 17. Beckman JS, Ischiropoulos H, Zhu L, Van der Woerd M, Smith the biocompatibility exhibited by titanium, resulting C, Chen J, Harrison J, Martin JC, Tsai M. Kinetics of superoxide in a new and novel class of biomaterials for use in dismutase and -catalyzed nitration of phenolics by per- oxynitrite. Arch Biochem Biophys 1992;298:438–445. implant applications. 18. Nishida A, Kimura H, Nakano M, Goto T. A sensitive and specific chemiluminescence method for estimating the ability Ϫ The authors thank Dr. Lars M. Bjursten of the University of human granulocytes and monocytes to generate O2 . Clin of Lund, Sweden, for his valuable input and assistance. Chim Acta 1989;179:177–182. 402 SUZUKI ET AL.

19. Uehara K, Maruyama N, Huang CK, Nakano M. The first 30. Kraeutler B, Bard A. Heterogeneous photocatalytic decompo-

application of a chemiluminescence probe, 2-methyl-6-[p-me- sition of saturated carboxylic acids on TiO2 powder. Decar- thoxyphenyl]-3,7-dihydroimidazo[1,2-␣]pyrazin-3-one (MCLA), boxylative route to alkanes. J Am Chem Soc 1978;100:5985– Ϫ for detecting O2 production, in vitro, from Kupffer cells 5992. stimulated by phorbol myristate acetate. FEBS Lett 1993;335:167– 31. Matthews R. Photooxidation of organic impurities in water 170. using thin films of titanium dioxide. J Phys Chem 1987;91: 20. Oosthuizen M, Engelbrecht M, Lambrechts H, Greyling D, 3328–3333. Levy R. The effect of pH on chemiluminescence of different 32. Matthews RW, McEvoy SR. Photocatalytic degradation of phe- probes exposed to superoxide and single oxygen generators. nol in the presence of near-UV illuminated titanium dioxide. J J Biolum Chemilum 1997;12:277–284. Photochem Photobiol A 1992;64:231–246. 21. Anderson J, Miller K. Biomaterial biocompatibility and the 33. Nair M, Luo ZH, Heller A. Rates of photocatalytic oxidation of macrophage. Biomaterials 1984;5:5–10. crude oil on salt water on buoyant, cenosphere-attached tita- 22. Cuckler JM, Mitchell J, Baker DG, Ducheyne P, Imonitie V, nium dioxide. Ind Eng Chem Res 1993;32:2318–2323. Schumacher HR. A comparison of the biocompatibility of poly- 34. Trillas M, Pujol M, Domenech X. Phenol methyl methacrylate debris with and without titanium debris: over titanium dioxide. J Chem Technol Biotechnol 1992;55:85– A comparison of two in vivo models. In: St. John, KR, editor. 90. Particular debris from medical implants: Mechanisms of for- 35. Meachim G, Williams D. Changes in nonosseous tissue adja- mation and biological consequences. ASTM STP 1144. Phila- cent to titanium implants. 1973;7:555–572. delphia: American Society for Testing and Materials; 1992. p 36. Tengvall P, Lundstrom I. Physico-chemical considerations of 118–126. titanium as a biomaterial. Clin Mater 1992;9:115–134. 23. Overgaard L, Danielsen N, Bjursten LM. Anti-inflammatory 37. Barteau MS. Site requirements of reactions on oxide surfaces. J properties of titanium in the joint environment? An experi- Vac Sci Technol A 1993;11:2162. mental study in rats. J Bone Joint Surg Br 1998;80-B:888–893. 38. Ying JY, Tscha¨pe A. Synthesis and characteristics of non-stoi- 24. Eriksson A, Thomsen P. Ex vivo analysis of leukocyte hydro- chiometric nanocrystalline oxide-based catalysts. Chem gen peroxide production using a bi-plate model in mice. J Cell Eng J 1996;64:225–237. Physiol 1996;166:138–143. 25. Thomsen P, Bjursten L, Ericson L. Implants in the abdominal 39. Tengvall P, Lundstrom I, Sjoqvist L, Elwing H, Bjursten LM. wall of the rat. Scand J Plast Reconstr Surg 1986;20:173–182. Titanium–hydrogen peroxide interaction: Model studies of the 26. Sibum H, Guther V, Roidl O, Wolf H. Titanium and titanium influence of the inflammatory response on titanium implants. alloys, In: Elvers B, Hawkings S, editors. Ullman’s encyclope- Biomaterials 1989;10:166–175. dia of industrial chemistry. New York: VCH Publishers; 1985. 40. Ischiropoulos H, Zhu L, Chen J, Tsai M, Martin JC, Smith CD, p 95–121. Beckman JS. Peroxynitrite-mediated tyrosine nitration cata- 27. Albrektsson T, Branemark PI, Hansson HA, Kasemo B, Larsson lyzed by superoxide dismutase. Arch Biochem Biophys 1992; K, Lundstrom I, McQueen DH, Skalak R. The interface zone of 298:431–437. inorganic implants in vivo: Titanium implants in bone. Ann 41. Beckman JS, Koppenol WH. Nitric oxide, superoxide, and per- Biomed Eng 1983;11:1–27. oxynitrite: The good, the bad and the ugly. Am J Physiol 28. Tengvall P, Elwing H, Sjoqvist L, Lundstrom I, Bjursten L. 1996;271:C1424–C1437. Interaction between hydrogen peroxide and titanium: A pos- 42. Sampson JB, Rosen H, Beckman JS. Peroxynitrite-dependent sible role in the biocompatibility of titanium. Biomaterials tyrosine nitration catalyzed by superoxide dismutase, myelo- 1989;10:118–120. peroxidase, and horseradish peroxidase. Meth Enzymol 1996; 29. Carey JH, Lawrence J, Tosine HM. Photodechlorination of 269:210–219. PCB’s in the presence of titanium dioxide in aqueous suspen- 43. Sen CK, Packer L. Antioxidant and redox regulation of gene sions. Bull Environ Contam Toxicol 1976;16:697. transcription. FASEB J 1996;10:709–720.