The assignee for this patent application is
Reporters obtained the following quote from the background information supplied by the inventors: "Strict pharmacological and good mechanical integrity of a drug eluting medical device are required to assure a controlled drug release. Significant technical challenges exist when developing an effective and versatile coating for a drug eluting medical device, such as a stent.
"The art recognizes a variety of factors that might play a significant role in a stent coating's ability to retain its integrity when subjected to external loadings, such as crimping and balloon expansion. According to the art, many of these factors are not fully understood, nor their significance fully appreciated. To the inventor's knowledge, the art has only been able to characterize the mechanisms of action as pertains to coating integrity at a theoretical level. A discovery of a relationship between the material, process used to coat, deformation of the stent in relation to that of the coating, etc. that might reduce the inherent uncertainty in this field, however, has yet to be articulated. As will be appreciated, the fracture mechanics for a coating is a highly non-linear problem that cannot be reduced down to a few variables.
"Nevertheless, the art has tried to develop a methodology capable of reducing the fracture problem down to a few unknowns--e.g., choosing a temperature range, a crimping protocol, and a polymer having the desired elastic properties as a function of the stent and clinical/therapeutic objectives. For example, attempts have been made to predict inter-laminar shear, crack propagation and related structural-mechanical behavior of a coating when subjected to an external load using a finite element-based analytic approach. To the inventors' knowledge, however, none of these efforts have proven successful, at least to the extent that their results might enable those in the medical device field to predict with any useful degree of certainty whether a particular drug-eluting stent will suffer from cracks, peeling, etc. in its coating absent device-specific testing.
"In light of the foregoing, there is a need for a method that improves the integrity or resilience of a drug-polymer coating carried by a polymer-based medical device, such as a polymer stent, when the medical device is subjected to external loads. It is important to avoid cracking and/or peeling of a drug-polymer coating. Potentially, such coating damage during stent radial expansion/contraction can cause severe thrombogenic response after stent implantation and compromise control of the drug dosage and/or rate of release of the drug-eluting stent."
In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventors' summary information for this patent application: "The invention improves upon the art by providing a method for making a drug eluting polymeric stent including the steps of forming a stent body from a polymer tube having a diameter about equal to the maximum deployed stent diameter, reducing the stent diameter to an intermediate diameter, applying a drug-polymer coating to the stent, and then crimping the stent down to the delivery or minimum stent diameter. A significant reduction in the instances of cracking/peeling has been observed using this method.
"Methods according to the invention may be practiced without substantially affecting, compromising or altering other prior established/confirmed functionality of the drug-eluting stent, such as the integrity of the polymer scaffolding which receives the coating, the drug release rate, the rate of biodegradation, etc. According to this aspect, the invention helps to simplify the development process. For example, suppose it becomes necessary, after all stent functions have been verified through clinical trials, to increase the deployed diameter while maintaining the same stowed diameter, or decrease the stowed diameter. If cracking/peeling then appears when the stent is expanded, i.e., as related to an increased range of deformation required of the stent scaffolding and coating; there is an important, yet heretofore unfulfilled need in the art to address this type of problem without changing a polymer, a stent pattern, the coating thickness, ratio of drug to polymer, etc. While in some cases it may become inevitable that a drug-eluting stent needs to be modified substantially to address a cracking problem in the coating, the invention provides methods that can, in some cases, obviate this need, thereby avoiding the need for additional clinical trials.
"Unless otherwise indicated, the terms 'stowed diameter' and 'deployed diameter' carry the same meaning as crimped diameter and deployed diameter in
"According to one aspect of invention, a method for crimping a drug-eluting polymer stent to avoid crack propagation in the drug-polymer coating includes the step of applying a coating by a spraying technique after reducing the stent diameter to an intermediate diameter, followed by baking the polymer coating and scaffolding to increase fracture resistance in the coating and scaffolding. The method may further include a final crimping step including four stages. After each reduction in diameter a relaxation period is used to relieve stress/strain in the coating and scaffolding.
"According to another aspect of invention, a baking step is used to anneal material following crimping and coating. After a final coating weight is reached, the stent is placed in an oven for between 15 minutes and one hour at about 50 degrees Celsius. The baking period may be longer, e.g., 2 hours, when a suitable baking temperature is selected for this length of time. This baking step increases the fracture resistance of the drug-eluting polymer stent. It is believed that during the baking period the internal stress concentration is reduced, thereby increasing the fracture resistance. In addition, it is proposed that the shear resistance of the binding, transition, or intermediate layer between the semi-crystalline scaffolding polymer and amorphous (or more amorphous) coating polymer could be increased by potentially allowing the two materials to bind together, which improves fracture resistance.
"Another aspect of the invention is a method for increasing fracture resistance including a diameter reduction step of about 50-80%, the diameter reduction step performed on a polymer scaffolding, e.g., one having a W-shaped closed cell pattern and strut displacement between crimped and deployed diameters of above 130 degrees about a hinge element, a coating process including depositing a drug-polymer mixed with a solvent in an amount of between about 90-98% by weight solvent, an annealing step of about 15-60 min at a relaxation temperature of about 5-20 degrees below a glass transition temperature for an amorphous and semi crystalline polymer, and a crimping process including a three part diameter reduction at a relaxation temperature of between 5-20 degrees below a glass transition temperature for an amorphous polymer (coating) and semi crystalline polymer (scaffolding).
"A method according to the invention may also include building a drug-eluting polymer stent with a higher-than-deployed initial diameter (i.e., greater than the design deployed diameter for the vessel type), followed by a diameter reduction, then a coating step to minimize crack propagation in the drug-polymer coating. Wherein the stent is capable of being reduced in size to about 40% of its starting size without crack propagation in the coating. The coating thickness may be about 2-10 microns and the ratio of drug to polymer coating is about 1:1 to 1:3.
"According to another aspect of invention a method for crimping a drug-eluting polymer includes reducing the stent diameter to a crimped diameter and loading the stent having the crimped diameter onto a delivery catheter, wherein the stent is a drug-eluting polymer stent having a drug to polymer loading ratio of 1:1 to 1:3 and the ratio of deployed to crimped diameters is between 2 and 5.
"According to another aspect of invention, there is a method of making a drug-eluting stent having a PDLA to drug ratio of 1:1 for the coating including the steps of making a stent body having a first diameter, applying the drug-polymer coating, then reducing the stent size to a second diameter, wherein the ratio of first diameter to second diameter is about 3:1."
For more information, see this patent application: Castro, Dan; Wang, Yunbing. Methods to Increase Fracture Resistance of a Drug-Eluting Medical Device. Filed
Keywords for this news article include: Cardio Device, Medical Devices, Clinical Trials and Studies,
Our reports deliver fact-based news of research and discoveries from around the world. Copyright 2014, NewsRx LLC
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