Patent Application
20150266056
The present invention relates to porous coating with controlled structure in the micro and nano-size domain. The coatings have a thickness between 10 nanometers and 10 millimeters and their porosity is created in such a way that the pore size distribution is anisotropic.
It also relates to a process for fabricating porous coatings with controlled structure in the micro and nano-size domain. In particular, but not exclusively, it relates to a process for fabricating coatings with an anisotropic pore size distribution.
The invention finally relates to objects covered with said coatings.
Coatings may be used in a great variety of technical fields, in particular in the medical field.
Failure of a therapy based on systemically administered drugs has many origins, but one classical reason is the inability to achieve the required dose at the site to be treated without generating detrimental side effects. This is especially true for treatments needed following implantations. For example, when an implant is placed into the body, it provokes small injuries that will most of the time induce a reaction of the surrounding tissue that may be detrimental on the medium or long run to its perfect integration. An other example is the risk of infection during the implantation procedure. It is well known that the bacteria carried by the patient himself are an important source of infections during surgeries. A way to tackle these issues is to use a drug (in the two examples one can think of a respectively an antibiotic) that will locally counteract the reaction.
The local delivery of a drug after an implantation may also be used to enhance a reaction of the body just after implantation and improve the chance of success of the procedure. For example a bone implant can be covered with proteins that will favor the new tissue growth and therefore reduce the convalescence period and improve the long term outcomes.
Drug eluting coatings have therefore created strong interest over the recent years. They are used quite extensively today in cardiology on drug eluting stents for angioplasty and other developments are conducted in orthopedics for hip and knee implants, in maxillofacial surgery, etc. . . . . They can be classified into two major groups. In the first group, the drug to be eluted is mixed to the coating and will be released either in parallel to the dissolution of the coating or by diffusion through the coating or a part of the coating. In the second group, the drug is contained into the porosity of the coating that acts as a series of reservoirs. It is released as the body fluid penetrates the porosity and dissolves it.
For any type of coating, thickness is a critical aspect that has a direct impact on its stability. It is well known from the literature that thick coatings are weaker and have a higher tendency to break over time. By reducing the thickness of the coating, lifetime is improved, but as a consequence, the amount of drug that is stored is reduced. By creating small cavities that can be filled with a drug and act as reservoirs, this amount can be maintained despite the reduction in thickness.
Prior art shows different ways of creating a porous coating for drug release applications. For example Reed, Looi and Lye (CA 2 503 625) use a differential attack of a metallic alloy. By removing one component of the alloy, they create a porous layer. Brandau and Fischer (U.S. Pat. No. 6,709,379) create the porosity by an electrolytic oxidation combined with an anodization. Herlein, Kovacs and Wolf (EP 1 319 416) create pores at the surface of a metallic stent through electrochemically induced pitting. These holes are then covered with a ceramic layer. In all these cases, the created porosity has the disadvantage of being homogeneous in size, or at least having a homogeneous size distribution. As a result, the loaded amount of drug will either be low (small pores) or the release will occur over a short period of time (large pores) i.e. over a few hours.
As a matter of fact, in order to store and release over a few days to a few weeks a large amount of drug, the coating must combine two porosities: one of large size acting as a reservoir and where the drug is stored and another of size similar to the released molecules that acts as a diffusion membrane.
In a previous application (EP 1 891 995), we described a coating combining two porosities disposed anisotropically, one being mainly used as drug reservoirs and the other being mainly used as diffusion membrane. We also described a method to synthesize such coatings. This approach offers however some disadvantages when it comes to the fabrication of coatings with a high density of reservoirs. In order to get a high density of reservoirs while maintaining a thickness as thin as possible, the best approach is to use spheres as template particles. In the close-packed hexagonal arrangement, they will generate a reservoir porosity of about 60%. From a mechanical point of view, this arrangement is not ideal for the final ceramic coating. As a matter of fact, the diffusion membrane part of the coating will be maintained over the open cavities of the reservoirs by a series of columns having very thick bases and tops but a shaft with a very narrow neck. This shape is defined by the shape of the space between three spheres in contact.
In this application, we describe an approach that allows to improve drastically the mechanical adhesion of the coating without affecting is storage capabilities.
Obtaining simultaneously a high drug loading and a slow drug release is achieved with anisotropic porous coatings having a pore size distribution in the micro or nano-size domain. These coatings are produced by the following process
In another possible embodiment the coatings are produced by the following process (
In another embodiment the coatings are produced by the following process (
Coatings obtained by the present invention are characterized by the fact that the pores are different in size and disposed in an anisotropic way. Micro-pores are created near the surface of the object and are used to store the drug, while nano-pores are disposed on the outside, towards the free surface of the coating, and act as a release membrane.
They are also characterized by the fact that the pores used to store the drug are very close to each other and form a dense network of cavities but are separated by, sometime very thin, walls.
As a non-limiting example, we will discuss afterwards mostly ceramic coatings on a metallic substrate. But the same description can be applied to any type of coating material and the coating can be made of different types of materials: metals, ceramics, polymers, hydrogels or a combination of any of these materials. It is therefore understood in that the word ceramic in the following can be replaced by polymer, metal or a combination and the same applies to metal. For differences in the process steps, table 1 describes for most of the possible combinations the modifications that need to be applied.
As the porous coating is in contact with a living body, it is preferably made of a biocompatible material. Depending on the applications this can be, but not exclusively, an oxide, a phosphate, a carbonate, a nitride or a carbonitride. Among the oxide the following ones are preferred: tantalum oxide, aluminum oxide, iridium oxide, zirconium oxide or titanium oxide. In some cases the coating can also be made of a biodegradable material that will dissolve over time and may be replaced by the living tissue. Substrates are made of materials such as metals, ceramics, polymers or a combination of any of these. Metals such as stainless steel, Nitinol, titanium, titanium alloys, or aluminum and ceramics such as zirconia, alumina, or calcium phosphate are of particular interest. It can also be a biodegradable material that will dissolve over time and may be replaced by the living tissue.
The coating has a thickness between 10 nanometers and 10 millimeters, preferably between 200 nanometers and 30 micrometers. A thicker coating allows creation of larger reservoir cavities while a thinner coating will be mechanically more resistant. The thickness is therefore chosen as an optimum to load enough drug while maintaining perfect stability.
The porosity is created in such a way that the size distribution is anisotropic. Preferably, the median value of the pore size distribution in the coating varies from the surface of the object to the free surface of the coating, said free surface being the surface of the coating which is away from the support. Preferably, the mean value of the pore size distribution at the free surface of the coating is less than a few μm.
In another variant, the coating is made of distinct sub-layers with distinct porosity size distributions. In this case, one of the sub-layers has a mean pore size distribution of less than a few μm and preferably, the sub-layer with the smallest mean pore size distribution is located near the free surface of the coating. Preferably, both mean pore diameters differ by a factor 5 to 10. In another embodiment, both mean pore diameters differ by a factor of 100 or more.
In a preferred embodiment the smaller porosity is fixed in the nanometer range. Diffusion of a liquid through a membrane is described by the diffusion coefficient. This coefficient varies with the thickness of the membrane, the density of pores, their size as well as their tortuosity. In particular, the size of the pores will influence the diffusion if it is similar to the size of the molecule that will diffuse.
In one possible embodiment micrometer size cavities are created by depositing a template onto the implant. This template is made, for example, of mono disperse polystyrene particles that are deposited by, for example, dip coating or by ink-jet printing onto the substrate. The polystyrene particles are then partially etched using oxygen plasma. The oxygen plasma removes a layer of material from the particle. The longer the treatment, the more plasma is removed. The template layer is then partially covered with the ceramic while the diffusion membrane is produced by adding a second layer of nano-porous ceramic. Finally the template materials are removed by a thermal treatment and cavities are created. This embodiment is schematically shown in cross section in
The
In another embodiment, the cavities are created by first depositing a template layer onto the substrate. This layer is then structures by methods such as, for example, photolithography or electron beam. This structuring will locally modify the solubility behavior of the template layer. The substrate covered with the template layer is then dipped into a solvent that will remove some part of said template layer. The zones whose solubility has been modified will stay in place in a way that is similar to mask structuration on
In a further variant, the pores, or at least their surfaces, are made hydrophobic in order to be filled by a lipophilic solution which can be slowly released in an aqueous medium or wherein the pores, or at least their surfaces, are made hydrophilic in order to be filled by a hydrophilic solution.
General Coating Process
The following is a description of some possible variants of the processes used to obtain such anisotropic coatings. The steps for these variants are shown in
A first embodiment of the coating process comprises the following steps:
In a second possible embodiment of the process, the coating is made of two different coatings, a first dense coating not entirely covering said temporary particles, and by a second nano-porous coating, said first coating being dried before deposition of said second porous coating. Finally particles are eliminated forming pores with an anisotropic size distribution and the coating is consolidated through a fixation step. In this embodiment, the first coating forms a dense structure around the temporary particles.
In a third possible embodiment, the coating process comprises the following steps:
In a fourth possible embodiment of the process, the coating is made of two different coatings, a first dense coating not entirely covering said temporary particles, and by a second nano-porous coating, said first coating being dried before deposition of said second porous coating. Finally the structured template layer is eliminated forming pores with an anisotropic size distribution and the coating is consolidated through a fixation step. In this embodiment, the first coating forms a dense structure around the structured mask layer.
In the present description, the term “eliminating” is used in a broad sense. It covers any commonly used terms related to an important change in the particle morphology, such as for example disintegration, dissolution or removal. For instance, but not exclusively, elimination of the temporary particles may comprise a thermal step, a chemical step, a mechanical step, an electro-mechanical or an irradiation step. In the case of a thermal, a chemical or an irradiation step, the temporary particles are either completely destroyed or only partially, e.g. the particles can be made hollow. In the case of a mechanical step, the temporary particles can be mechanically removed. In the case of an electro-mechanical step (e.g. sonication or ultrasonic vibrations), the particles can be swelling (e.g. by use of polymeric particles, such as PLGA) or disintegrated. More the elimination step and the consolidation step may be one single step.
The term “temporary” has to be understood as “present only for a limited time during the process”. Temporary particles can be viewed as templates that create the tri-dimensional structure and porosity of the coating.
The expression “mono-layer of particles” means that the particles are at the same level relatively to the surface of the support. For each mono-layer, no particle will sit on top of another.
Temporary Particles Deposition or Templates
In the present invention, this step is preferably carried out by dip-coating or by ink-jet printing. Any other deposition method can however be used such as for example spin-coating or solvent evaporation.
In the case of dip-coating, the temporary particles are in a solution (for example water) and the substrate is dipped in said solution. As is known in the art, the density of particles present on the substrate will depend on the concentration of particles in the solution, the rate of withdrawal of the substrate and also the surface treatment of the substrate. All these parameters may be adjusted by the skilled man to attain the desired density of particles on the substrate.
In the case of ink-jet printing, the temporary particles are in a solution which is printed onto the substrate. The solvent of the solution can, as an example, be water mixed with other solvents to inhibit the coffee ring effect, as know in the art. Examples of other solvents are ethylene glycol or formamide. The concentration of particles on the substrate will depend on parameters from the solution as well as printing parameters that may be adjusted by the skilled man to attain the desired value.
Particles
The diameter and the shape of the temporary particles can be chosen arbitrarily. But a preference is given for homogeneous particles in shape and size. The chemical composition of the particles is also free, but it is preferably selected in the group of polymers, starch, silica, metals or biological materials such as cells. A preference is given for polymers materials with a spherical shape and homogeneous diameter: mono-disperse polymer beads. For example, polystyrene bead may be advantageously used. They are readily available in numerous sizes and are very consistent in size. Alternatively, biocompatible polymers (e.g. PLGA or Poly Lactide Glycolide Acid type) can also be used.
When deposited on the support temporary particles can either be in contact with each other or separated by some empty space. When in contact, the contact surface size can be modified by changing the surface chemistry and surface affinity of the particles. It can be increase by using wettable particles or reduced to a point-like contact when using non-wettable particles such as Teflon.
Using hydrophilic and/or hydrophobic temporary particles allows the creation of various structures in the coating. Before the deposition of the temporary particles, the substrate is locally covered with a hydrophilic respectively a hydrophobic layer. In this way specific zones are adapted to fix temporary particles with a similar surface affinity while attachment on the other zones is prevented. In the case of a stent, it may be advantageous to only coat regions which are less subject to deformations; alternatively it may be advantageous to only coat regions which are in contact with the intima of the vessel to target the release of drug to prevent proliferation or inflammation. In the case of bone or dental implants, it may be advantageous to select regions where bone ingrowth should be favored and where it should be hindered.
Particles Etching
After deposition on the substrate, the particles are etched. In a possible embodiment, the particles that are used have a spherical shape and are made of polystyrene. As an example, they can be etched using an Argon-Oxygen plasma. The extent of etching will depend on the time spent in the plasma.
Template Layer Structuring
The template layer structuring approach offers a lot of flexibility in designing the micro-sized porosity. As a matter of fact, the shape in the plan parallel to the substrate can be freely chosen. In a possible embodiment, the micrometer size pores can have a hexagonal shape and the thickness of the wall between the pores can be adapted to modify the generated porosity (
In another possible embodiment, the shape of the micrometer size pores can be adapted to resist to potential deformations. Wall can, for example, be disposed at a very specific angle with the deformation direction, and therefore minimize the effect of such deformation (
Coating Deposition
Different procedures can be considered for the coating deposition. They are chosen according to the coating precursors that are used as well as to the desired properties of the coating. A few examples are given below:
A first procedure to deposit the coating onto the substrate uses a mixture of nanoparticles or a nanopowder in a solvent such as for example water as coating precursor. The substrate is dipped into the precursor mixture and pulled out at a controlled speed. The thickness of the coating varies with the viscosity of the mixture and with the pulling speed.
Another procedure uses a sol obtained through hydroxylation and partial condensation of a metallic alkoxyde as coating precursor. Again, the precursor can be coated onto the substrate using either dip or spin coating.
Another procedure uses a solution obtained by dissolving a precursor into the adapted solvent. Again the mixture can be coated onto the substrate using either dip or spin coating.
In a given embodiment the precursor used can be a hydrophilic material and therefore generate hydrophilic pore surfaces.
In another embodiment the precursor used can be a hydrophobic material and therefore generate hydrophobic pore surfaces.
In all cases the coating can be deposited in several steps or sublayers. Between the depositions of each sub-layer the solvent of the coating precursor can be partially or fully removed by, for example, a thermal treatment. This approach permits the formation of thicker, crack-free coatings. The composition of the coating precursor can also be modified between each step. This allows the creation of coatings with variable chemical composition. For example, the chemical composition of the coating can be very similar to that of the substrate at the coating/substrate interface and can be very compatible at the interface with the body.
Using nanopowders or a sol-gel approach for producing coatings offers the advantage of reducing the necessary temperature for obtaining crystalline coatings. This is particularly favorable for metallic substrates that may go through phase transitions when thermally treated and therefore lose part of their mechanical or shape memory properties.
According to the first embodiment of the process, there is only the deposition of one single first coating entirely covering the temporary particles, said single coating having a nano-porous structure. Such porous structure thus allows the elimination of the particles as indicated here under.
According to the second embodiment of the process, the coating comprises in fact two coatings, a first coating which is treated after deposition (for example dried), said first coating not covering entirely the deposited particles, and then a second coating having a porous structure as in the first embodiment of the process.
In another possible embodiment, the first coating which is treated after deposition and which does not cover entirely the template particles is dense.
Typically, these coatings according to the embodiments indicated above are realized using nano-powders or by a sol-gel route, known per se in the art. Of course, any other methods for creating a porous structure may be envisaged in the present invention.
Particles Removal
The elimination of the temporary particles can be achieved by different methods such as for example, but not exclusively, a thermal, a chemical, a mechanical, an electro-mechanical, a photo-chemical or an irradiation step. It can also take place at different stages of the process, before and/or during and/or after the fixation step, depending on the coating requirements
Fixation
Any appropriate method can be used for the fixation step. Advantageously a drying step is used.
The coating fixation step can take place before the particle elimination step or it can take place simultaneously with the particle elimination step or even after the particle elimination step.
For ceramics this can be sintering where the crystalline phase is formed. For a polymer this can be a photo-chemically (by visible of UV light), a thermally or chemically induced polymerization. For metals or for certain ceramics this can be a thermal treatment under controlled (neutral or reducing) atmosphere.
Pore Filling
In a given embodiment the pore structure can be used for storage and diffusion of an active substance for medical purpose.
In a possible embodiment, the coating can be filled with a drug, an anti-coagulation substance, an anti-proliferative substance, an antibiotic substance, a bacteriostatic substance or a growth factor.
In another embodiment the coating can be filled with cells.
The substance can be introduced in the coating for example by dip coating or ink-jet. Any other method to fill the pores can be envisaged.
The following example describes the realization of a coating having a structure similar to that presented in
In one embodiment, a preferred substrate such as 316L stainless steel is used and will be coated. The substrate preparation can be electrochemically polished as described by Verma et al. (Biomed Mater Eng, 2006, 16, 381-395).
A suspension of TiO2 nanoparticles (Techpowder, Lausanne, Switzerland) is prepared with addition of PVA (Polyvinyl Alcohol), the bonding agent, and ammonia to help stabilize the colloid. The nanoparticle specifications are D10=38 nm, D50=62 nm, D90=82 nm (CPS Disc Centrifuge™)
A dip-coater (PL 3201 from Speedline Technologies™) is prepared and set to the withdrawal speed of 90 [mm/sec]. The substrate is dipped first into a water based suspension of 1 micrometer diameter polystyrene microbeads (Duke Scientific, Berkeley USA). The number of beads deposited per surface area varies with their concentration in the suspension and the withdrawal speed. To manufacture the coating as shown in
The substrate covered with etched microbeads is then dip-coated in the ceramic nanopowder suspension to cover the beads with a layer of TiO2. The ceramic layer can vary in thickness depending on the process parameters and intended use. In this example the beads will be completely covered. The TiO2 suspension will form a compact layer covering the beads. The final layer thickness is approximately 1.5 micrometers.
Finally, after a drying period of 10 minutes in a controlled climate chamber at 10% RH and 37° C., the coated substrate is sintered in an oven in a two stage process. A first burn-off step is performed at 500° C. under air for 1 hour. This step will see the polymer beads burn off and leave a lenticular shaped cavity in the TiO2 layer. A second stage of sintering immediately follows the first at 800° C. for 1.5 hours with a controlled Argon atmosphere. This second stage will serve to consolidate the ceramic layer.
Using Ink-Jet Method to Create and Load the Coating
There are different types of ink-jet printing technologies available today. As an example we describe hereafter the drop-on-demand technology (but this description can easily be extended to continuous ink-jet printing). In the drop-on-demand technology, micro-droplets of a substance are projected at the request of the operator through a nozzle onto a surface. The nozzle and/or the surface can be moved in all spatial directions (for example x, y, z, or r, 0, z, more adapted to cylindrical systems such as stents). This movement allows a precise control on the final localization of the droplet on the surface.
Advantages of the Ink-Jet Approach
The ink-jet method can be applied to every step of the coating deposition as described above as well as for the filling of the coating with an active substance:
For each step it allows a perfect spatial control of the process. Spatial resolution of the inkjet method is of the order of a few tenths of micrometers.
Depositing the Template with an Ink-Jet Method
Defining zones with template particles and zones without template particles allows the creation of zones with reservoirs and zones without such reservoirs (
Another advantage of this approach is the possibility of varying the thickness of the ceramic coating. A critical aspect of ceramic coating is their relatively low resistance to deformation. When the shape of the substrate is modified, the coating has to adapt to the new shape and this may create cracks in the layer. These cracks may generate some delamination and as a consequence particle release. Resistance to deformation can be strongly improved by using thin coatings. A zone without reservoirs will be thinner and therefore more resistant to deformation. In a particular embodiment, template beads can be deposited only in regions where deformation is low. Regions with high deformation can be coated with ceramic only, as shown in
Depositing the Ceramic Coating with an Ink-Jet Method
Here again, the ink-jet method offers a high flexibility. Ceramic with various compositions and porosities can be coated on different parts of the substrate. For example the outside porosity of the coating (diameter and length of the pores, tortuosity) is a key element that will drive the elution profile of a loaded active substance. Having various porosities in different regions of the coating will allow the creation of different release profiles. Ink-jet therefore allows the deposition of different nanoparticle suspensions in different locations on the stent, thus leading to different outside porosities and substance release profiles.
Filling the coating with an ink-jet method
In one embodiment the coating is filled with an active substance according to the following procedure:
By using an ink-jet method to fill the pores, it is possible to specifically load certain regions with a given substance. Different molecule compositions can be loaded on a single coating (
The processes previously discussed allow the manufacturing of coatings and objects carrying such coatings with specific and original features.
A major application for these objects, as can be readily understood from the different embodiments and variants described above, is in the field of medical devices and more specifically, but not limited to, of medical implants. Of particular interest are stents, orthopedic and dental implants. The porosity can be used as a drug reservoir that will release its content in a controlled way over time or it can be used to favor tissue ingrowth and therefore increase the mechanical interlocking between the implant and the living tissue.
For stents the coating can be loaded with one or several drugs. It can be a combination of the following drugs given as non-exclusive examples: an anti-proliferative agent, an anti-coagulation substance, an anti-infectious, a bacteriostatic substance.
The object can also be an orthopedic or dental implant wherein the pores may be adapted in the same manner as for the stent discussed above. In such case, the porosity obtained can either be of interest to store growth factors such as bone growth factors, increase biocompatibility or create regions where bone or cartilaginous tissue can grow and attach in a solid manner to the implant. This can also be achieved by filling the cavities with resorbable bioactive ceramics such as calcium phosphates.
Accordingly the support can be made of metal, of ceramic or polymer. It can also be made of a biodegradable material.
In this application, the coating may comprise non-porous domains. Such domains may have a minimal dimension larger than 10 micrometers and a maximal dimension smaller than 10 millimeters. In a variant, these domains have a minimal dimension larger than 100 micrometers. In another variant, these domains have a maximal dimension smaller than 1 millimeter.
The pore size may also be adapted for diffusing beads, particles or polymers containing an active substance which can be slowly released.
Alternatively the beads or particles can emit an irradiation. Advantageously, in such case, the beads or particles shall remain within the cavities.
Table 1 summarizes different possibilities for manufacturing a porous surface according to the present invention.
The present invention will be more fully understood from the following figures and table:
In
This application is a divisional of U.S. patent application Ser. No. 13/319,815, filed 10 Nov. 2011, which is the U.S. national phase of International Application No. PCT/IB2009/052206, filed 26 May 2009, which designated the U.S., the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13319815 | Nov 2011 | US |
| Child | 14670216 | US |
