Coatings appropriate for medical devices

Details Coatings appropriate for medical devices Coatings appropriate for medical devices

2001-06-12

2004-01-06

Jackson, Monique R. (Department: 1773)

Stock material or miscellaneous articles

Composite

Including interfacial reaction product of adjacent layers

C428S500000, C428S507000, C428S508000, C428S510000, C428S520000, C428S522000, C427S002100, C427S002300, C427S002310, C427S385500, C427S388400, C427S407100

Reexamination Certificate

active

06673453

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the field of hydrophilic coatings which are applied to medical devices, especially devices intended to be implanted, temporarily or permanently, in the body.
Among the many advances in medical practice in recent years is the development of medical devices that supplement the surgeon's skills. Examples of these are a variety of vascular catheters and guide wires that can be used to treat remote areas of the circulatory system otherwise available only by major surgery. Another is the stent, a device that retards restenosis after angioplasty. Another is the intra-ocular lens that restores youthful eyesight to the elderly afflicted with cataracts. Heart valves, artificial pacemakers, and orthopedic implants are among a lengthening list.
Nearly all of the above-described devices are constructed of plastics and metals that were never intended to invade, and sometimes reside for prolonged periods, in the human body. They present surfaces that bear little or no semblance to those of the human organs, which are generally hydrophilic, slippery, and obviously biocompatible. The penalty imposed on invasive devices that are not biocompatible is that they tend to be treated as foreign objects by the body's immune system. Inflammation and thrombosis often result.
Equally important for devices that must be inserted and moved through body tissues is their lubricity. Most metals and plastics have poor lubricity against body tissues, which results in mechanical abrasion and discomfort when the device is passed over the tissue.
The surface of devices already designed and manufactured from such materials can be made biocompatible, as well as hydrophilic and slippery, by properly designed coatings. Thus, the way has been opened to construct medical devices from conventional plastics and metals having the particular physical properties required, and then to apply suitable coatings to impart the desired properties to their surfaces.
Polysaccharides have been shown to be useful in making hydrophilic, lubricious coatings on substrates. Such coatings are described in U.S. Pat. Nos. 4,801,475, 5,023,114, and 5,037,677, the disclosures of which are hereby incorporated by reference. In general, these patents disclose bilaminar coatings comprising a primary coat that adheres tightly to a plastic substrate, and a top-coat which comprises a polysaccharide which is hydrophilic, lubricious and durable. The primary coat is sometimes called a “tie-coat” because it ties the top-coat to the substrate; it is also known as a base coat. Both of the terms “tie-coat” and “base coat” are considered equivalent in this specification.
In the coatings described in the above-cited patents, the primary coat and the top-coat are grafted together with covalent bonds, and retain their individual identities even after grafting. These bilaminar coatings can be used on catheters, guide wires, prosthetic devices, intra-ocular lenses, or other devices which are permanently or temporarily inserted into the body.
It is a common feature of the coatings described above that organic solvents are needed at one stage or another of the process for applying the coating to the device. Many problems are associated with the use of those solvents. Virtually any organic solvent is toxic to a degree, and with many such solvents, the level of toxicity is high. The manifestations of this toxicity may include carcinogenic or teratogenic character, sensitization, and, at best, disagreeable odor. These characteristics can make the processing dangerous and unpleasant to the point of being intolerable. A survey of the patent literature discloses the use, in the manufacture of medical devices, of acetonitrile (toxic lachrymator), dimethylformamide (carcinogen), and N-methylpyrrolidinone (strong irritant, possible teratogen), for example. Another problem with the use of organic solvents is their flammability, which imposes the need for extra precautions to be taken during the manufacturing process.
Not only are these solvents a hazard in the workplace, but they also cause a problem due to the need to remove them completely after the manufacturing is completed. Polymers are well known to be highly retentive of traces of solvents even after exhaustive attempts to remove them. The possible threat to health, caused by exposing the patient's blood stream even to traces of toxic solvents, is a factor to give concern to the conscientious manufacturer.
It is clearly important to eliminate the need for all solvents except pure water, in applying the desired surface characteristics to medical devices. Film-forming aqueous emulsions might satisfy the requirements. As a class, such materials have been known for more than fifty years as vehicles for leather coatings, interior and exterior paints, etc. Commercial products of this kind are generally described by their suppliers as “acrylic latex” or “water-based vehicles”, or even “latex selected from the group consisting of isoprene and styrene”, but the actual chemical compositions and the detailed formulations are proprietary information that is not disclosed. For the formulator of coatings for medical devices to be inserted into the human blood stream, this lack of assurance about the presence or absence of biologically harmful components in the products should be cause for concern.
Obviously, these industrial aqueous products were not developed specifically for use on medical devices only, or else such suitability would have been disclosed. Nevertheless, coatings for guide wires and catheters formulated with such products as major components have been patented, and perhaps used with human patients. See, for example, U.S. Pat. Nos. 5,756,144, 5,272,012, and 5,776,611, the disclosures of which are incorporated by reference herein. Furthermore, apart from the safety factor, these polymer compositions were designed and selected to comply with performance specifications for other commercial uses and were not known to be most appropriate in design for application as coatings on medical devices.
It has been found that a suitable coating can be prepared by selecting particular acrylic monomers, out of the large number available, for use in preparing emulsion polymers. Also, the choice of the proportions in which these monomers are used turns out to be a surprisingly critical factor in meeting the special requirements of medical devices.
SUMMARY OF THE INVENTION
The present invention comprises a substrate, typically a device intended to be implanted temporarily or permanently in the human body, having a bilaminar coating. The bilaminar coating includes a base coat which is firmly adhered to the substrate, and a top-coat which is chemically grafted to the base coat.
In the present invention, the base coat comprises an aqueous acrylic emulsion polymer. The polymer comprises a combination of one or more monomers having alkyl groups. The Equivalent Alkyl Number (EAN) of the polymer is defined by
EAN
=
n
1
⁢
N
1
+
n
2
⁢
N
2
+
…
+
n
m
⁢
N
m
N
1
+
N
2
+
…
+
N
m
where n
i
is a number of carbon atoms in an alkyl group of monomer i, and N
i
is a number of moles of monomer i in the polymer, and where m is a positive integer. The EAN of the polymer used in the base coat of the present invention is in a range of about 3.5 to about 4.5. For the special case in which the polymer contains only one such monomer (i.e. m=1), the EAN must be 4, and the polymer comprises a butyl group.
The base coat and/or top-coat also contain functional groups which enable the two coats to be chemically grafted to each other. Preferably, the emulsion polymer of the base coat has a minimum film-forming temperature that is less than the temperature at which the coats are dried and cured.
An important feature of the present invention is that no organic solvents are used during the preparation of the coated substrate. Therefore, there can be no organic solvent in the final product. There is thus no need for a solvent-extraction step, after the coated substra

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