Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-02-23
2002-03-05
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
Reexamination Certificate
active
06353038
ABSTRACT:
The invention relates to a composite which contains a plastic-based bioactive component and is intended for medical, in particular surgical or therapeutic, use.
BACKGROUND OF THE INVENTION AND STATE OF THE ART
To elucidate the background of the invention and the state of the art, the publications used, to which reference is made below, are to be viewed as being incorporated into the description of the invention below.
Previously known composites made up of a plastic component and a bioactive component include combinations of hydroxyapatite and methylmethacrylate (literature references (1)-(5)).
However, a thermoplastic, plasticizable at a relatively low temperature, has not been used as the plastic component in the known composites mentioned above.
OBJECT OF THE INVENTION
The object of the invention is to provide a novel composite which is intended for medical use, in particular surgical or therapeutic use, and which contains a thermoplastic component and can be machined and molded into a piece-like, coherent, shock-resistant and load-resistant form.
It is a particular object to provide a composite which can be plasticized at a relatively low temperature.
It is also an object of the invention to provide a composite which is moldable for a certain period even after its temperature has been lowered to a temperature below the setting temperature of the plastic component.
It is a further object of the invention in particular to provide a composite in which the biodegradability, plasticization temperature and setting rate of the plastic component can be controlled separately.
SUMMARY OF THE INVENTION
The characteristics of the invention are given in claim 1. A composite according to the invention is characterized in that it comprises
a thermoplastic component, plasticizable within a temperature range of −10° C. . . . +100° C., which is substantially made up of hydroxy acids or structural units derived from hydroxy acid derivatives, which has a molar mass within a range of 10,000-1,000,000 g/mol, and which degrades in the body typically within a period ranging from a few days to several years, and which in its solid state is a mechanically strong plastic or rubbery material and
a bioactive component, which is a bioactive glass, a bioactive xerogel, a bioactive ceramic material, or a bioactive glass ceramic material.
Preferred Embodiments and a Detailed Description of the Invention
The term “medical” is to be understood in the wide meaning of the word and thus also covers dental and veterinary applications.
Bioactive Component
By the bioactive component used in the composite according to the present invention is meant a material which reacts in the physiological conditions of the body. The bioactive component may have one or more of the following properties: bondable to tissues, bioresorbable, bondable/resorbable, releasing active agents, mineralizing, biocompatible, and antimicrobial. The bioactive component is a bioactive glass, a bioactive ceramic material, a bioactive glass ceramic material, coral or a coral-based product, or a bioactive xerogel. The concept of bioactivity is discussed in, for example, Heikkilä's doctoral dissertation (Ref. 1).
In a composite according to the invention, the bioactive component is present as particles separate one from another. The word “particle” here covers particles of different sizes and shapes, such as fibers, solid or porous pieces, rods, microparticles and glass beads.
The bioactive glasses described in references (1)-(5) can be mentioned as examples of suitable bioactive glasses.
Suitable bioactive ceramic materials include Ca phosphates, such as hydroxyapatite.
An especially suitable bioactive component is xerogel. By xerogel is meant a dried gel, which is described in the literature (9, 11). Silica xerogels are partly hydrolyzed oxides of silicon. Hydrolyzed oxide gels can be produced by the sol-gel process, which has been used for the production of ceramic and glass materials for many years.
The sol-gel process is based on the hydrolyzation of metal alkoxides and a subsequent polymerization of the metal hydroxides. As the polymerization reaction progresses, additional chains, rings and three-dimensional networks are formed, and a gel, made up of water, the alcohol of the alkoxy group and the gel itself, is formed. The sol may also contain other additives, such as acids or bases, which are used for the catalysis of the reaction. If the alcohol and the water are removed thereafter from the gel by washing and evaporation, a xerogel is obtained.
The polymerization of the remaining OH groups continues during the drying. The polymerization continues for a long time even after the gelation. This is called aging. The further the polymerization proceeds, the more stable the gel or xerogel becomes. At room temperature, however, the polymerization will in fact stop after an ageing of a few weeks, and the xerogel will not become completely inert. If the temperature is raised, the polymerization reaction can be accelerated, the gel becomes more stable and shrinkage occurs, and internal stresses appear in the xerogel to an increasing degree.
If the temperature is raised to a sufficiently high level (approx. 1000° C. for monolithic silica gels), the gel or xerogel becomes a pure oxide and no OH groups are left in the material. However, in the case of pure oxides, the dissolution reaction rate is very slow. If the oxides are added together with other ions, such as Na, K, Mg or Ca, the reaction rate can be greatly increased. By these methods, bioactive glasses have been developed, which can form a silica gel layer on their surface through an ion exchange reaction. The dissolution rate of these glasses is controlled by the composition and surface area of the glass. The glasses are melted at a temperature above 1000° C. Therefore it is not possible to add any organic compounds to the structure of the glass.
Sol-Gel glasses have been used, for example, as implant materials, in particular in bone implants (11). These materials do not dissolve completely. The material is formed at a high temperature, and organic compounds cannot be incorporated into its structure. Klein et al. described that when used as implants, silica gels sintered at a lower temperature caused a strong cell reaction in macrophages and in lymphocytes.
Plastic Component
The thermoplastic component used in the present invention, plasticizable within a temperature range of −10° C. . . . +100° C., may be to a varying degree biodegradable or even bioactive. The term “biodegradable” covers all plastics which are not inert. This group thus includes all bioresorbable plastics (degrading under the action of cells) and biodegradable plastics (degrading under the. effect of mere moisture). The use of the composite will determine whether it is expedient to select a plastic which. is biodegradable at a slower or a more rapid rate.
Biodegradable types of plastic are suitable for most of the uses of the composite according to the invention. A biodegradable plastic component disappears at the desired rate or is biologically nearly stable, and thus promotes the tissue contact and the desired tissue reaction of the bioactive component. The plastic of the composite thus keeps the particles of the bioactive component in place but does not necessarily prevent the bioactive material from coming into contact with tissue fluid. Since the plastic component gradually decomposes, the water of the tissue fluid comes through diffusion into contact with the bioactive component. Likewise, ions and any active additives released from the bioactive material can become diffused through the plastic and affect their surroundings. The surrounding and/or contact surface tissue grows, filling the void formed by the degradation of the plastic. Ultimately the plastic component decomposes completely and releases any possibly remaining bioactive component.
Alternatively, the plastic component may be nearly inert. A composite made up of a bioactive material and an inert plastic may, if the composite possi
Aho Allan
Heikkilä Jouni
Kangasniemi Ilkka
Seppälä Jukka
Yli-Urpo Antti
Bioxid Oy
Lydon James C.
Michl Paul R.
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