Communications 894 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,2000 0935-9648/00/1206-0894 $ 17.50+.50/0 Adv. Mater. 2000, 12, No. 12 [7] A. Cassell, N. Franklin, T. Tombler, E. Chan, J. Han, H. Dai, J. Am. Chem. Soc. 1999, 121, 7975. [8] P. Yang, T. Deng, D. Zhao, P. Feng, D. Pine, B. F. Chmelka, G. M. Whitesides, G. D. Stucky, Science 1998, 282, 2244. [9] L. Wang, L. Tao, M. Xie, G. Xu, J. Huang, Y. Xu, Catal. Lett. 1993, 21, 35. [10] S. Liu, L. Wang, R. Ohnishi, M. Ichikawa, J. Catal. 1999, 181, 175. [11] J. Kong, A. M. Cassell, H. Dai, Chem. Phys. Lett. 1998, 292, 567. [12] A. Cassell, J. Raymakers, J. Kong, H. Dai, J. Phys. Chem. 1999, 103, 6484. Novel Bioactive Functionally Graded Coatings on Ti6Al4V** By Jose M. Gomez-Vega, Eduardo Saiz, Antoni P. Tomsia,* Takeo Oku, Katsuaki Suganuma, Grayson W. Marshall, and Sally J. Marshall The use of ceramics in medicine has increased tremen- dously during the past decades, and it is anticipated that the use of bioceramics will increase dramatically during the next millennium. An estimated 11 million people in the United States have at least one medical-device implant. [1] Today, more than 500000 arthroplastic procedures and to- tal joint replacements are performed annually in the U.S. alone. [2] Moreover, as the population receiving such im- plants becomes younger, this number is likely to increase. This has motivated significant effort towards improving im- plant performance through the design of artificial bone-like materials. A variety of bioceramics have been developed and produced, but (despite their excellent biological per- formance) only a few of them have been used in clinical applications (mainly in non- or low-load-bearing implants), on account of their poor mechanical strength. As a conse- quence, metals such as Ti and alloys Co±Cr are being used for implants where high strength is required. At best, they are bioinert. For better osteointegration, metals need to be coated with a bioactive material, and the most common technique is to use plasma-sprayed coatings of hydroxyapa- tite (HA). [3,4] Typically, the plasma-sprayed coatings consist of a mixture of amorphous and crystalline phases. [5] The fast dissolution of the amorphous phase and some of the crystalline products like tricalcium phosphate (TCP) de- grade the stability of the coating. [5] Further heat treatment to improve crystallinity often results in cracking and loss of adhesion. [6] Some of the major causes of failure of HA- coated metal orthopedic and dental implants appear to re- side in the coating. [7] Clearly, new coatings for implant ma- terials are needed. Indeed, Kasemo and Gold [8] suggested that future progress in biomaterials must include the devel- opment of coatings with programmed dissolution of multi- layer surfaces. Such surfaces would provide new opportu- nities to optimize the biomaterial coating surface for different periods of the healing-in phase. This programmed dissolution can then be used to expose different micro-ar- chitectures, chemical patterns, and porosities at various times. Fabrication of coatings for medical applications in- volves a compromise between adhesion, mechanical stabil- ity, and bioactivity, but coatings that satisfy all these re- quirements are extremely difficult to develop. Graded bioactive glass coatings can provide an answer to that challenge. Graded or layered materials have been used for years in specific applications to improve bonds between dissimilar materials, coating adherence, and compatibility between fibers and matrices in composites. [9±11] Recently, such materials have been formally classified as functionally graded materials (FGMs), leading to expanded work on improving their processing and comprehending their prop- erties. By controlling the gradient in the glass composition (i.e., in the glass structure) along the coating, good adhe- sion to the metal can be combined with rapid biofixation and long-term stability. As a first step towards the development of graded coat- ings, a new family of glasses in the SiO 2 ±CaO±MgO± Na 2 O±K 2 O±P 2 O 5 system has been developed. The compo- sitions of the glasses are related to that of Bioglass (BG), [12] but with MgO and K 2 O substituted for CaO and Na 2 O. These additions alter the thermal expansion and softening points such that enameling can be carried out be- low the a®b transformation of Ti in the alloy Ti6Al4V (955±1010 C), without generating large thermal stresses. The silica content of the glasses ranges between 45 and 68 wt.-%. The thermal expansion coefficients (CTE) are in the range of 8.8 to 15.1 ´ 10 ±6 C ±1 , which includes the CTE of Ti6Al4V (~9.6 ´ 10 ±6 C ±1 ). The nominal compositions of the glasses and their main thermal properties are listed in Table 1. The coatings were fabricated using a conventional enam- eling technique. Because of the high reactivity of Ti alloys, a fast-firing procedure has been developed to control reac- tivity and provide excellent adhesion (see experimental section). The reaction sequence at the glass/metal inter- faces during firing was evaluated using a combination of X-ray diffraction (XRD), scanning electron microscopy (SEM) of cross sections and fracture surfaces, and high-res- olution transmission electron microscopy (HRTEM) com- bined with energy-dispersive spectroscopy (EDS). During ± [*] Dr. A. P. Tomsia,Dr. J. M. Gomez-Vega, Dr. E. Saiz Lawrence Berkeley National Laboratory Materials Sciences Division Berkeley, CA 94720 (USA) Dr. T. Oku, Prof. K. Suganuma Institute of Scientific and Industrial Research Osaka University Osaka 567-0047 (Japan) Prof. G. W. Marshall,Prof. S. J. Marshall University of California Department of Restorative Dentistry San Francisco, CA 94143-0758 (USA) [**] This work was supported by the NIH/NIDCR grant 1R01DE11289. Jose M. Gomez-Vega thanks the Spanish Ministry of Education (MEC) for financial support. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 at Lawrence Berkeley National Laboratory.