Electrocrystallization of Strongly Adherent Brushite Coatings on Prosthetic Alloys

s references are European Patent applications 50 lected from the group consisting of alkali metal phos0,264,353 and 0,264,354, both made by Shimamune and phates and ammonium phosphate; the solution having a Hosonuma. These disclosures teach a composite mateconcentration of phosphoric anions of 0.05 mole/L or rial and a process for the production thereof which more and a pH of from 4 to 7. Thus, the noteworthy comprises a metallic substrate having thereon an oxide factors in the Shindow process are the electrolytic forlayer, the oxide layer consisting essentially of the oxide 55 mation of the coating on an anode at a pH between 4 of one or more metals denoted in the previously menand 7. In contrast to the procedure I have developed, it tioned group, and thereafter a calcium phosphate overis particularly noteworthy that the process of Shindow lay on the oxide layer. The composite is made by oxidizet al. is an oxidation which occurs in an electrolyte ing a metallic substrate, either thermally or electrolytisolution that does not contain the cation to be deposited. cally, to form a layer of the oxide or the metallic sub60 The contention that an electrophoretic method of phosstrate component alone or a layer of mixed oxide of the phate deposition on a metal serving as the cathode remetallic substrate component and a metal component in flects the current state-of-the-art, finds substance in thr the electrolyte. Alternatively, heating of the metallic issuance of a patent in 1989 to Hemminger et al. (U.S. substrate is accomplished to stabilize the surface Pat. No. 4,806,218). Therein, electrophoresis is termed thereof; and, then a coating of calcium phosphate com65 cataphoretic because the final coating is placed on the pound is formed on the surface. Essentially, the Shimatungsten element or wires while they act as a cathode. nune et a1 methodology comprises two distinct proHemminger et al. avoid a certain degree of erosion of cases. The first, the perhaps preferred embodiment, is a the electrode by first applying polarity and treating it as 5,3 10,464 3 4 an anode for the purposes of incipient coating; thereaf: apparatus. The process may be set up in any simple ter, the polarity is reversed and the element to be coated laboratory by anyone having rudimentary knowledge spends the duration of the electrophoretic processing of the electrolysis technique and the capability of preperiod as a cathode. As in the previous teachings, it is paring the requisite electrolyte solutions containing the purpose of these patentees to place an oxide coating 5 calcium ions and dihydrogen phosphate ions. on the electrodes and, therefore, the electrolyte is generally devoid of phosphate ions. Hemminger et al. differ OF THE from Shindow et a1. in that the electrode of interest is a Of the drawings: cathode which is used electrophoretically, while ShinFIGS. 1A and 1B are scanning electron micrographs dew uses the electrode of interest as an anode in an 10 of electrolytically deposited calcium phosphate coatelectrolytic cell (as was the case in the procedure of ings on 316L stainless steel; Shimamune et aI.). FIG. 2 is an X-ray powder pattern of brushite scraped Electrophoretic deposition of phosphate materials from the surface of an electrode coated according to has recently received a good deal of professional atteninvention; and tion. It is seen that, in most conventional and state-of15 FIGS. 3A and 3B are scanning electron micrographs the-art coating processes, particles are suspended in a of electrolytically deposited brushite coatings on titsliquid and, in the presence of a large electric field, are ilium mesh. driven onto an electrode. Analysis of these coatings, after sintering, indicates the presence of hydroxyapatite DETAILED DESCRIPTION O F THE and tricalcium phosphate. These techniques are particu20 PREFERRED EMBODIMENT . larly attractive because irregularly shaped substrates, such as are employed in the various prostheses, can be coated conveniently and relatively inexpensively. Generally speaking, electrolytic deposition methods should grow to greater popularity since most of the heretofore conventional methods of preparing implants with phosphate coatings have employed flame spraying and plasma spraying. With these latter procedures, heating of the substrate surface can be extensive and thermal decomposition of the material being sprayed is often observed. Other high temperature techniques, such as dip coating and sputter coating, can also suffer from thermal degradation, inconveniences and disabilities which make electrolytic deposition a superior coating procedure. Given the number of arthroplastes performed yearly, the need for a physically stable, biocompatible material which can be easily deposited on implant surfaces is considerable. It has been recently reported, in a prominent medical journal, that an estimated 120,000 total hip implants are required yearly in the United States. As early as 1986, it was estimated that, by 1990, the annual joint prostheses requirements would reach 500,000. Thus, a process for economically coating a metallic prostheses with phosphate ceramic, notably the various calcium phosphate coatings, becomes compelling when viewed in the light of the ability of these coatings to accelerate bone fixation during the early recuperative stages after implantation. Before detailing the various Drocesses which enabled SUMMARY OF THE INVENTION me to obtain the desired crystalline phosphate coatings, I have devised an electrolytic method for preparing I will clarify the difference between the form of present phosphate coatings on cathodes which is superior in the state-of-the-art electrodeposition and the instant invenway of obtaining that goal but avoids the disabilities and 40 tion. Electrophoresis is the impelling of charged ~ a r t i disadvantages of the earlier and conventional art. This cles (suspended in a liquid medium) towards an anode new electrolytic process is especially attractive because and/or cathode (electrodes) between which there is highly irregular objects can be coated relatively quickly established an electric field. At rates depending upon at low temperatures, Additionally, a high degree of the magnitude of the electric field, the mass of the particontrol of deposit crystallinity can be obtained using my 45 cles in suspension, and other physical Parameters, posiprocedure and, because the coating is formed on an tively charged particles will be impelled onto the cathalloy used as a cathode, a departure from the preceedode, while negatively charged particles will impact the ing art, corrosion of the metal surface is minimized anode. during the deposition process. Electrolysis, on the other hand, is associated with a All deposits disclosed herein were formed at room 50 change in oxidation state of species at both the cathode temperature from aqueous solutions which contained and the anode of an electrochemical cell. In many cases, calcium ions and dihydrogen phosphate ions. Deposits electrolysis is a deleterious process to be avoided if it were typically formed while controlling the current occurs during an electrophoretic deposition. It is on this throughout the course of the deposition, although dedistinction that I base my new electrodepositional posits can also be obtained by controlled potential elec55 method, which I define as an electro-stimulated crystaltrolysis. Oxygen was not excluded. The process is disline phosphate growth on metallic conductive subtinct and is one of electrolysis, not electrophoresis. By strates. The electro-stimulation is associated, as previapplying a negative potential to the object to be coated, ously mentioned, with the passage of an electrical curhydrogen is evolved and the pH at the surface of the rent through the working electrode. The crystalline cathode is abruptly increased. The pH increase converts 60 phosphate growth is produced by a redox process not HzP04-ions to HP042-ions and results in the crystailiby an impacted aggregation of particles. I deviate from zation of the less soluble calcium phosphates from the the general case in that my electrode of deposition (the electrolyte onto the cathode. By increasing the current, prosthetic or alloy element) is used as the cathode the pH and the pH gradient near the cathode increase, which controls the pH in the vicinity of the substrate thus producing more nucleation sites and faster crystal 65 throughout the depositional scheme. This procedure growth. Thus, I teach a form of controlled crystal permits one to exercise a high degree of control over growth on a cathodic prosthetic alloy or other suitable deposit crystallinity and minimizes (or obviates) corroelectrode which is placed into an electrolysis process sion of the metal surface during the depositional activ5,3 10,464 5 6 ity. Additionally, by controlling the current density, it is 40% at 1 mA/cm2, but only 10% at 0.1 mA/cm2, bepossible to control the deposit morphology. cause the pH perturbation near the electrode surface is Having set the actual stage for the following experinot large enough to produce extensive localized crystalment, it is only necessary to describe the physical plant lization. The current efficiencies were determined that is used to realize the instant invention. A conven5 gravimetrically using the assumption that 100% current tional electrolysis apparatus is set up in which the deefficiency corresponds to 1 mole of CaHP04.2H20 vice to receive phosphate deposition is established as being deposited per mole of e-passed. All current effithe cathode. The electrolyte comprises an aqueous soluciencies were calculated for electrodes on which 5 tion containing Ca2+and dihydrogen phosphate ions. C/cm2 coatings had been deposited. No deliberate attempt is made to exclude oxygen and I 10 The general applicability of electrolytically deposited feel that there is no degradation in the results because of phosphate coatings on metal substrates is further exemthis P U ~ ~ O S C ~ U ~ n ~ n e ~ ~ l ~ s i ~ n . Crystallization Occurs, plified in FIG. 3, wherein scanning electron microonce the electrical current is passed, as a result of an graphs of the same coating (at two different magnificaelectrolytically induced pH gradient near the cathode tions) are as shown. The substrate in this case is titanium surface. Upon reduction of the electrolyte solution, a 15 mesh on the stem of a commercially available Ti-6AIvariety of half reactions can occur. The dominant reac4v hip implant. m i s mesh provides an anchor for bony tion is a reduction of water to hydrogen gas and hyingrowth into the implant. Strands of the mesh are the droxyl ions (OH)-. This reaction results in a localized large cylindrical features in FIG. 3, which are 250 pm PH increase in the the cathode. The pH in diameter. Crystallization was carried out at 1.0 increase near the surface of the electrode converts 20 mA/cm2 until 8 Clcm2 of charge had passed. The area H2P04-to HPO4-and results in the crystallization of used to determine the current density was based on the less phosphates. increasing the curoverall dimensions of the electrode, not the actual surrent* both the pH and the pH gradient near the face area of the mesh, which was not measured. All of trode increase, thus producing more nucleation sites and faster crystal growth. The following experimental 25 the surfaces visible in FIG. 3 are coated and there are no discernable locations where preferential crystallization regimine indicates results obtained in the laboratory that are totally consistent with the aforementioned raappears to have occurred. The morphology and physitionale. cal integrity of calcium phosphate coatings produced by electrolytic deposition of the instant invention also EXPERIMENTAL , may be varied using controlled potential electrolysis in J" A cathode is prepared by using a device (prosthesis) Order produce the crysta11izaiion. comprised of 316L stainless steel. To form the deposit The results reported in this disclosure are considered depicted in FIG. 1 ~ , the electrolysis is run over a perto be nominal and continued use of the invention proiod of 2 hours 13 minutes at 1 mA/cm2 for a total of 8 cess will yield successful results as herein indicated. clcm2. ln FIG. 1 ~ , a calcium phosphate deposit at 10 35 Variation in concentrations of electrolytes, electric m ~ / c m 2 resulted from a total of 5 ,--/cm2 being passed current densities used and/or the totals of coulombic over 8 minutes 20 seconds. Larger crystals are procharges passed may be had without departing from the duced, as expected, at the lower current density. ~ d d i intent and the spirit of this teaching. Such variations tional to nucleation arguments, the vigorous rate of should be considered and readily employed, consistent hydrogen evolution observed at 10 rnA/cm2 is partially 4 with the hereinafter appended claims. responsible for the decreased crystal size in this case. What is claimed is: Deposits shown in FIGS. 1A and 1B are crystalline 1. The method of covering a metal surface portion of CaHP04.2H20 (brushite). An X-ray powder pattern of a prosthetic appliance with a coating of uniform thickmaterial produced at 10 mA/cm2 is shown in FIG. 2. ness of pure brushite of predetermined crystal size and Reflections that are marked by asterisks (*) in FIG. 2 45 morphology which comprises the steps of: denote those reflections which are expected based upon a) contacting the metal surface portion as a cathode d spacings known for brushite. No reflections for brushwith aqueous electrolyte containing calcium and ite expected to be greater than the background level are dihydrogen phosphate ions, missing from this pattern. Additionally, there are no b) passing an electric current through the electrolyte, reflections present which might be associated with 50 and other materials. Determination of the amount of calc) controlling the number of crystal nucleation sites cium in this material by atomic absorption spectroscopy and the rate of crystal growth by adjusting the indicates 22% calcium by weight, which is in good electrical current in the electrolyte. agreement with the value of 23.3% predicted for brush2. The method of claim 1 in which the electrolyte is ite. 55 maintained at room temperature throughout the course Current efficiencies for production of the brushite of deposition. coatings on 316L stainless steel are 50% at 10 rnA/cmZ, * * * * *