Coating processes – Medical or dental purpose product; parts; subcombinations;... – Implantable permanent prosthesis
Reexamination Certificate
2000-05-02
2002-07-30
Langel, Wayne A. (Department: 1754)
Coating processes
Medical or dental purpose product; parts; subcombinations;...
Implantable permanent prosthesis
C423S305000, C423S308000, C423S311000, C433S212100, C623S023560
Reexamination Certificate
active
06426114
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to novel sol-gel calcium phosphate, in particular, hydroxyapatite, ceramic coatings and processes of making same at low temperature. Such coatings are useful, inter alia, for dental implants and other bone-metal contact appliances.
BACKGROUND OF THE INVENTION
Osseointegration, or development of mechanical strength between an implant and bone, decides the success or failure of the implantation procedure. The implant will fail if good osseointegration is not achieved. Considerable research has been conducted to address this issue, primarily for dental and orthopaedic implants. The explored approaches generally involve modification of the surface and/or shape of the implant to facilitate the process of osseointegration.
Although there is no universally accepted solution to osseointegration, the concept of new, in-growing bone interlocking with a macroporous surface of the implant has attracted increasing interest among researchers and practitioners. The Endopore Dental Implant System, a Canadian invention marketed by Innova Corp. of Toronto, is one example of such surface-modified implant, wherein bone grows into the void space between sintered spheres of titanium alloy. For early strength development in such systems it is critical that bone in-growth into the pores is relatively rapid.
One promising approach is to use ceramics and specifically a sol-gel process to produce the ceramic. A sol-gel (SG) process provides superior chemical and physical homogeneity of the final ceramic product compared to other routes, such as solid-state synthesis, wet precipitation, or hydrothermal formation. The SG process allows the desired ceramic phase to synthesize at temperatures much lower than some of the alternate processes referred to above. In the SG coating process, substrate metal degradation due to thermally-induced phase transformations, microstructure modification or oxidation, is avoided. SG widens green-shaping capability, for example, and it is a very convenient method for deposition of ceramic coatings. The SG ceramic coating process has demonstrated a better structural integrity, purity and phase composition than the conventional methods, such as thermal spraying. The SG process also offers a cheaper and easier-to-form alternative for bioactive coating uses.
Hydroxyapatite (HA, Ca
10
(PO
4
)
6
(OH)
2
) ceramics belong to a class of calcium phosphate (CaP)-based materials, which have long been widely used as bone substitutes [1-3]. Recently, HA has been used for a variety of biomedical applications, including matrices for drug release control [4-6]. Due to the chemical similarity between HA and mineralized bone, synthetic HA exhibits strong affinity to host hard tissues. However, poor mechanical properties, e.g. low strength and toughness, restrict monolithic HA applications to those that require little or no load-bearing parts. HA coatings on metallic substrates (titanium alloys), offer great improvement in orthopaedic and dental applications [7-10]. Other members of the family of CaP-based materials, such as dicalcium phosphate (CaHPO
4
.2H
2
O) or tricalcium phosphate (Ca
3
(PO
4
)
2
), have also been used for the same purpose.
Sol-gel (SG) processing of CaP allows molecular-level mixing of the calcium and phosphor precursors [11-23], which improves chemical homogeneity of the resulting calcium phosphate, e.g. in particular HA, in comparison with conventional methods such as solid state reactions [24], wet precipitation [25,26], and hydrothermal synthesis [27]. The versatility of the SG method opens an opportunity to form thin film coatings in a rather simple process, and provides an alternative to thermal spraying which is currently widely used for biomedical applications [21, 28]. Numerous reports [29-31] in the prior art have indicated decomposition of the plasma sprayed HA to form other phases, such as tetracalcium phosphate, calcium oxide, and amorphous calcium phosphate. Some of these compounds are undesirable due to fast dissolution in vivo. Additionally, severe cracking of the plasma-sprayed layer (an nearest feature of plasma sprayed ceramics) frequently leads to accelerated implant failure.
The SG process provides significantly milder conditions of the synthesis of calcium phosphate films. This results in a much better structural integrity whereas the defects originated from plasma spraying can be largely avoided [28]. Furthermore, the lower temperature synthesis particularly benefits the metal substrates. However, thermal treatment of calcium phosphate, such as HA, sol-gel films under vacuum is frequently required to avoid metal oxidation. This leads to structural instability of the HA coating (i.e., evolution of structural water under vacuum environment) during thermal treatment. At temperatures below ~400° C., further oxidation of the underlying Ti or Ti alloy is negligible due to the surface presence of a natural protective oxide layer [32, 33].
Existing sol-gel hydroxyapatite (HA) synthesis methods require calcination temperatures higher than 500° C. to develop a well-crystallized HA phase. A high degree of HA crystallinity is required for bioactive applications, because partially crystalline, or amorphous calcium phosphate, such as HA, coatings are rapidly resorbed by living tissue. Metal alkoxides such as calcium diethoxide and phosphorus esters (for example, trialkyl phosphites and trialkyl phosphates), albeit expensive, are used as Ca and P precursors, respectively, in SG synthesis of CaP. However, these precursors are hygroscopic. This makes it necessary that the sol preparation to be conducted at a controlled (i.e. water-free) atmosphere. Furthermore, the inherent low hydrolysis activity of the trialkyl phosphates requires a prolonged time period for HA formation. For a highly-active phosphite, an aging time period over at least 24 hours is necessary to form the apatitic structure. The loss of the alkoxy phosphorus compounds due to their volatility also leads to the formation of non-stoichiometric products which are non-desirable for bioactive applications. Although the use of other phosphorus compounds, such as phenyldichlorophosphine, offers some improvement in this respect, a much higher temperature, for example, >800° C., is required to form a pure, crystallized HA. If the HA coating, or other CaP coating, is being processed, this temperature can easily damage the underlying metallic substrate, in particular Ti, which is one of the most commonly used metals for implants. Another shortcoming is that synthesized HA cannot be in-situ combined with bioactive polymers, or other organic materials.
The following is a brief review of pertinent literature in this field. Takahashi et al. [22] developed a gel route using calcium nitrate and phosphonoacetic acid (HOOCCH
2
PO(OH)
2
) in an aqueous solution and obtained a pure HA powder at 700° C. The crystallinity of HA increased with temperature up to 1100° C. Chai et al. [28] compared two calcium precursors, namely calcium diethoxide and calcium propionate, reacted with triethyl phosphite to form HA coating. They found that HA phase appeared at 500° C. for calcium propionate solution, but no HA formed when calcium ethoxide was used. However, they did not explain the influence of chemical nature of the precursors on phase formation.
Qiu et al. [35] used calcium nitrate and ammonium dihydrogen phosphate (NH
4
H
2
PO
4
) to synthesize HA in highly basic solution. They obtained HA at calcination temperatures of 400° C.-1100° C. and indicated that the crystallinity of the HA improved with increasing temperature.
Haddow et al. [23] used calcium acetate with a number of phosphorus precursors, i.e. phosphoric acid (H
3
PO
4
), phosphorus pentoxide (P
2
O
5
), and triethyl phosphite for HA coating applications. They found the films prepared from triethyl phosphite and calcium acetate showed the best wetting characteristic and
Liu Dean-Mo
Troczynski Tomasz
Langel Wayne A.
Oyen Wiggs Green & Mutala
The University of British Columbia
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