Method to develop an organized microstructure within an implantable medical device

Abstract

A method to facilitate the ability to engineer an organized microstructure within a monolithically homogeneous implantable biomaterial, particularly metallic alloys, is provided. By starting with an ultra-fine grained, equiaxed microstructure having an averaged linear grain size of less than 5 microns, a functionally graded microstructure is developed by establishing a temperature gradient across the cross sectional dimension. In the case of a tubular product form, this dimension would be framed by the outer and inner surfaces. By ensuring one surface is held at a substantially lower temperature than the other surface a temperature gradient is developed. The present invention also provides an implantable medical device which includes an organized microstructure. In accordance with the present invention, the organized microstructure includes a first surface region having said ultra-fine grained, equiaxed microstructure and a second surface region comprising a recrystallized, coarser grained microstructure having an average linear grain size of greater than 25 microns, wherein the first and second surface regions are separated by a transition region.

1 . A method comprising: 
 providing a monolithically homogeneous implantable biocompatible material comprising at least two exposed surfaces, said biocompatible material comprising an ultra-fine grained, equiaxed microstructure having an average linear grain size of less than 5 microns; and    heating said biocompatible material under conditions in which one of said exposed surfaces is held at a first temperature T 1  and another of said exposed surfaces is held at a second temperature T 2 , wherein T 1  is less than T 2  and T 1  is below a recrystallization temperature of the biocompatible material to provide an organized microstructure comprising a first surface region having said ultra-fine grained, equiaxed microstructure and a second surface region comprising a recrystallized, coarser grained microstructure having an average linear grain size of greater than 25 microns, wherein said first and second surface regions are separated by a transition region.    
   
   
       2 . The method of  claim 1  wherein said biocompatible material comprises a metal, a metal alloy, a ceramic material or a polymer.  
   
   
       3 . The method of  claim 2  wherein said biocompatible material is a metal alloy or a compositionally pure monolithic metal that is selected from the group consisting of iron-based alloys, cobalt-based alloys, refractory metal-based alloys, precious metal-based alloys, precious metals and titanium-based alloys.  
   
   
       4 . The method of  claim 1  wherein said biocompatible material is process to be in tubular form.  
   
   
       5 . The method of  claim 1  wherein the average linear grain size of said first surface region having said ultra-fine grained, equiaxed microstructure is from about 1 micron to about 100 nanometers.  
   
   
       6 . The method of  claim 1  wherein said average linear grain size of said recrystallized, coarser grained microstructure is from about 40 microns to about 50 microns.  
   
   
       7 . The method of  claim 1  wherein T 1  is at nominal room temperature or a temperature that is below nominal room temperature.  
   
   
       8 . The method of  claim 7  wherein T 1  is at the temperature of liquid nitrogen at standard atmospheric conditions.  
   
   
       9 . The method of  claim 1  wherein T 2  is at a temperature that is greater than about ½ the melting point of said biocompatible material.  
   
   
       10 . The method of  claim 9  wherein T 2  is from about 0.75 to about 0.90 homologous temperature or greater, where the homologous temperature is the industrially accepted liquidus temperature.  
   
   
       11 . An implantable medical device comprising: 
 a biocompatible material comprising a first surface region having an ultra-fine grained, equiaxed microstructure having an average linear grain size less than 5 microns and a second surface region comprising a recrystallized, coarser grained microstructure having an average linear grain size of greater than 25 microns, wherein said first region and second surface regions are separated by a transition region.    
   
   
       12 . The implantable medical device of  claim 11  wherein said biocompatible material comprises a metal, a metal alloy, a ceramic material or a polymer.  
   
   
       13 . The implantable medical device of  claim 12  wherein said biocompatible material is a metal alloy or a compositionally pure monolithic metal that is selected from the group consisting of iron-based alloys, cobalt-based alloys, refractory metal-based alloys, precious metal-based alloys, precious metals and titanium-based alloys.  
   
   
       14 . The implantable medical device of  claim 11  wherein said biocompatible material is process to be in tubular form.  
   
   
       15 . The implantable medical device of  claim 11  wherein the average linear grain size of said first surface region having said ultra-fine grained, equiaxed microstructure is from about 1 micron to about 100 nanometers.  
   
   
       16 . The implantable medical device of  claim 11  wherein said average linear grain size of said recrystallized, coarser grained microstructure is from greater than about 40 microns to about 50 microns.  
   
   
       17 . The implantable medical device of  claim 11  wherein said implantable medical device is one of a stent, a blood filter, an artificial heart valve, vascular occluders, aneurismal coils, prosthetic grafts including stent-grafts, a cochlear implant, a visual prosthesis, a neurostimulator, a muscular stimulator or a deep brain stimulator.  
   
   
       18 . The implantable medical device of  claim 11  wherein said first surface region has a ductility from about 10% (engineering strain to failure) or less.  
   
   
       19 . The implantable medical device of  claim 11  wherein said second surface region has a ductility from about 40% (engineering strain to failure) or greater.  
   
   
       20 . The implantable medical device of  claim 11  wherein said transition region is an area in which there is a change from said ultra-fine microstructure at a surface nearest to the first surface region to said coarser grained microstructure at a surface nearest to the second surface region.