US20120282412A1 - Method for polymerizing a monomer solution within a cavity to generate a smooth polymer surface - Google Patents

Abstract

In preferred embodiments, the present invention relates to methods for polymerizing a monomer solution within a cavity covered by a porous membrane to generate a smooth polymer surface. More specifically, the method can be used to provide a medical device or sensor with a smooth polymer surface.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of U.S. patent application Ser. No. 12/026,396, filed Feb. 5, 2008 and entitled “Method for Polymerizing a Monomer Solution within a Cavity to Generate a Smooth Polymer Surface,” which claims the benefit of U.S. Provisional Patent Application No. 60/888,475, filed Feb. 6, 2007. All of the above referenced applications are incorporated by reference herein in their entirety.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• In preferred embodiments, the present invention relates to methods for polymerizing a monomer solution within a cavity such that the outer surface of the cavity has a smooth surface. More specifically, methods are disclosed for making a sensor comprising functional chemistry immobilized within a polymeric matrix disposed within a cavity along the sensor, wherein the sensor has a smooth outer surface.
• 2. Description of the Related Art
• Polymers are widely used for coating surfaces in a wide variety of applications. For example, polymers are used to coat metals, fabrics, paper and glass to provide corrosion resistance, water resistance and insulation. With respect to biomedical applications, polymers can be used to increase the biocompatibility of a surface or to provide other desirable properties, such as immobilizing functional chemistries for intravascular deployment.
• A variety of surface coating methods exist. One method involves dipping the surface to be treated in a solution or emulsion of a polymer and then either letting it dry or transferring the surface into a coagulation bath which is capable of extracting the solvent from the polymer solution. If the coat needs to be made thicker, the process can be repeated to add another layer of polymer to the coated surface.
• In another method, the polymer is formed into a powder that is electrostatically sprayed onto a neutrally or oppositely charged surface. The charged polymer powder particles electrostatically adhere onto the surface. Heat treatment of the powdered surface cures and finishes the coated surface.
• In another method, the surface is heated and immersed in a fluidized bed of powdered polymer particles. The fluidized bed of polymer particles is formed by aerating a bed of polymer particles with a gas. The powder adheres to the heated surface, which is then removed from the fluidized bed and further heated to cure and finish the coated surface.
• There remains an unmet need for methods of making an analyte sensor, by immobilizing function chemistries in a polymeric matrix within a cavity in the sensor, such that the chemistries retain their functionality and wherein the outer surface of the sensor is smooth and non-thrombogenic.
SUMMARY OF THE INVENTION
• A method is disclosed for making an analyte sensor having a smooth outer surface. The method comprises the steps of: providing an optical fiber comprising a cavity covered by a membrane having pores; loading the cavity and the membrane pores with a solution comprising polymerizable monomers, an analyte indicator system and a polymerization initiator; and initiating polymerization of the monomers.
• In a preferred variation, prior to initiating polymerization, the loaded cavity and membrane pores are coated with wax and the solution is deoxygenated. In a further variation, the wax coating is removed after polymerization is completed to leave a smooth outer surface, wherein the analyte indicator system is immobilized within the cavity. Removing the wax may comprise contacting the wax with an organic solvent. Preferably the organic solvent is hexane. In another variation, the step of removing the wax may further comprise application of ultrasonic energy.
• In one embodiment, the indicator system comprises a fluorophore and an analyte binding moiety.
• In one embodiment, the loading step comprises vacuum filling.
• In one embodiment, initiating polymerization comprises application of a second initiator selected to undergo a redox reaction with the first initiator.
• In other embodiments, initiating polymerization comprises application of thermal or radiation (e.g., UV) energy.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a cut-away view of a sensor where a portion of the porous membrane sheath is cut away to expose the optical fiber and hydrogel beneath the membrane.
• FIG. 2 is a cross-sectional view along a longitudinal axis of a sensor with a hydrogel disposed distal the optical fiber.
• FIG. 3A is a cross-sectional view of a portion of the porous membrane sheath, optical fiber and cavity before polymerization.
• FIG. 3B is a cross-sectional view of a portion of the porous membrane sheath, optical fiber and cavity after polymerization.
• FIG. 4 is a cut-away view of the sensor shown inFIG. 1 disposed in a second solution.
• FIG. 5 is a cut-away view of the sensor shown inFIG. 1 covered with a coat of wax.
• FIG. 6A is a side view of the optical fiber showing the arrangement of the cavities on the optical fiber.
• FIG. 6B is a side view of the optical fiber showing the arrangement of alternatively shaped cavities on the optical fiber.
• FIG. 6C is a side view of the optical fiber showing the arrangement of yet another alternatively shaped cavities on the optical fiber.
DETAILED DESCRIPTION
• FIG. 1 shows asensor2 comprising anoptical fiber10 with adistal end12 disposed in aporous membrane sheath14. Theoptical fiber10 hascavities6, such as holes, in the fiber optic wall that can be formed by, for example, mechanical means such as drilling or cutting. Thecavities6 in theoptical fiber10 can be filled with a suitable compound, such as a polymer. In some embodiments, the polymer is ahydrogel8. In other embodiments of thesensor2 as shown inFIG. 2, theoptical fiber10 does not havecavities6, and instead, thehydrogel8 is disposed in a space distal to thedistal end12 of theoptical fiber10 and proximal to themirror23. In some embodiments, thesensor2 is a glucose sensor. In some embodiments, the glucose sensor is an intravascular glucose sensor.
• In some embodiments, theporous membrane sheath14 can be made from a polymeric material such as polyethylene, polycarbonate, polysulfone or polypropylene. Other materials can also be used to make theporous membrane sheath14 such as zeolites, ceramics, metals, or combinations of these materials. In some embodiments, theporous membrane sheath14 is microporous and has a mean pore size that is less than approximately two nanometers. In other embodiments, theporous membrane sheath14 is mesoporous and has a mean pore size that is between approximately two nanometers to approximately fifty nanometers. In still other embodiments, theporous membrane sheath14 is macroporous and has a mean pore size that is greater than approximately fifty nanometers.
• In some embodiments as shown inFIG. 2, theporous membrane sheath14 is attached to theoptical fiber10 by aconnector16. For example, theconnector16 can be an elastic collar that holds theporous membrane sheath14 in place by exerting a compressive force on theoptical fiber10. In other embodiments, theconnector16 is an adhesive or a thermal weld.
• In some embodiments as shown inFIG. 1, amirror23 andthermistor25 can be placed within theporous membrane sheath14 distal thedistal end12 of theoptical fiber10. Thermistor leads27 can be made to run in a space between theoptical fiber10 andporous membrane sheath14. Although athermistor25 is shown, other devices such as a thermocouple, pressure transducer, an oxygen sensor, a carbon dioxide sensor or a pH sensor for example can be used instead.
• In some embodiments as shown inFIG. 2, thedistal end18 of theporous membrane sheath14 is open and can be sealed with, for example, an adhesive20. In some embodiments, the adhesive20 can comprise a polymerizable material that can fill thedistal end18 and then be polymerized into a plug. Alternatively, in other embodiments thedistal end18 can be thermally welded by melting a portion of the polymeric material on thedistal end18, closing the opening and allowing the melted polymeric material to resolidify. In other embodiments as shown inFIG. 1, apolymeric plug21 can be inserted into thedistal end18 and thermally heated to weld the plug to theporous membrane sheath14. Themoplastic polymeric materials such as polyethylene, polypropylene, polycarbonate and polysulfone are particularly suited for thermal welding. In other embodiments, thedistal end18 of theporous membrane sheath14 can be sealed against theoptical fiber10.
• After theporous membrane sheath14 is attached to theoptical fiber10 and thedistal end18 of theporous membrane sheath14 is sealed, thesensor2 can be vacuum filled with afirst solution15 comprising a monomer, a crosslinker and a first initiator. Vacuum filling of a polymerizable solution through a porous membrane and into a cavity in a sensor is described in detail in U.S. Pat. No. 5,618,587 to Markle et al.; incorporated herein in its entirety by reference thereto. Thefirst solution15 is allowed to fill thecavity6 within theoptical fiber10. In addition, as shown inFIG. 3A, thefirst solution15 can also fill the void volume within theporous membrane sheath14 which comprises pores17 andchannels19 that are capable of being filled with thefirst solution15.
• In some embodiments, thefirst solution15 is aqueous and the monomer, the crosslinker and the first initiator are soluble in water. For example, in some embodiments, the monomer is acrylamide, the crosslinker is bisacrylamide and the first initiator is ammonium persulfate. In other embodiments, the monomer is dimethylacrylamide or N-hydroxymethylacrylamide. By increasing the concentrations of the monomer and/or crosslinker, the porosity of the resulting gel can be decreased. Conversely, by decreasing the concentrations of the monomer and/or crosslinker, the porosity of the resulting gel can be increased. Other types of monomers and crosslinkers are also contemplated. In other embodiments, thefirst solution15 further comprises an analyte indicator system comprising a fluorophore and an analyte binding moiety that functions to quench the fluorescent emission of the fluorophore by an amount related to the concentration of the analyte. In some embodiments, the fluorophore and analyte binding moiety are immobilized during polymerization, such that the fluorophore and analyte binding moiety are operably coupled. In other embodiments, the fluorophore and analyte binding moiety are covalently linked. The indicator system chemistry may also be covalently linked to the polymeric matrix. Some preferred fluorophores include HPTS-triLys-MA and HPTS-triCys-MA, and some preferred analyte binding quencher moieties include 3,3′-oBBV and derivatives thereof; these and other fluorophores and quenchers are described in detail in U.S. Provisional Application No. 60/833,081 and U.S. patent application Ser. No. 11/671,880, entitled OPTICAL DETERMINATION OF pH AND GLUCOSE, filed on the same day as the present application; these disclosures are incorporated herein by reference in their entirety.
• In some embodiments as shown inFIG. 4, after thesensor2 is filled with thefirst solution15, theoptical fiber10 and thefirst solution15 filledporous membrane sheath14 andcavity6 are transferred to and immersed into asecond solution24 comprising a second initiator. In some embodiments, thesecond solution24 is aqueous and the second initiator is tetramethylethylenediamine (TEMED). In some embodiments, thesecond solution24 further comprises the same fluorescent dye and/or quencher found in thefirst solution15 and in substantially the same concentrations. By having the fluorescent dye and quencher in both thefirst solution15 and thesecond solution24, diffusion of fluorescent dye and quencher out of thefirst solution15 and into thesecond solution24 can be reduced. In some embodiments where asecond solution24 is used, thesecond solution24 further comprises monomer in substantially the same concentration as in thefirst solution15. This reduces diffusion of monomer out of thefirst solution15 by reducing the monomer gradient between thefirst solution14 and thesecond solution24.
• In some embodiments as shown inFIG. 3A, at or approximately at theinterface26 between the first andsecond solutions15 and24, the first initiator and the second initiator can react together to generate a radical. In some embodiments, the first initiator and the second initiator react together in a redox reaction. In other embodiments, the radical can be generated by thermal decomposition, photolytic initiation or initiation by ionizing radiation. In these other embodiments, the radical may be generated anywhere in the first solution. Once the radical is generated, the radical can then initiate polymerization of the monomer and crosslinker in thefirst solution15.
• As shown inFIG. 3B, when the radical is generated via a redox reaction as described herein, the polymerization proceeds generally from theinterface26 to the interior of theporous membrane sheath14 and towards thecavity6 in theoptical fiber10. Rapid initiation of polymerization at theinterface26 can help reduce the amount of first initiator that can diffuse from thefirst solution15 and into thesecond solution24. Reducing the amount of first initiator that diffuses out of thefirst solution15 helps reduce polymerization of monomer outside theporous membrane sheath14 which helps in forming a smooth external surface. Polymerization of the monomer and crosslinker results in ahydrogel8 that in some embodiments substantially immobilizes the indicator system, forming thesensor2.
• In some embodiments, thefirst solution15 is aqueous while thesecond solution24 is organic. In some of these embodiments, the monomer is substantially soluble in the aqueousfirst solution15 but is not substantially soluble in the organicsecond solution24. Because the monomer is not soluble in thesecond solution24, it will not diffuse into thesecond solution24 and polymerization occurs within thefirst solution15. Interfacial tension at theinterface26 between thefirst solution15, thesecond solution24 and the surfaceporous membrane sheath14 can affect the amount of penetration of the organicsecond solution24 into the pores17 of theporous membrane sheath14. Penetration of thesecond solution24 into theporous membrane sheath14 reduces the smoothness of theporous membrane sheath14 after polymerization because the resultinghydrogel8 is not flush with the opening of the pores17.
• In some embodiments, a surfactant that interacts with the surface of theporous membrane sheath14 and the aqueousfirst solution15 is added to the hydrophobicsecond solution24. By changing the amount and type of surfactant added, the interfacial tension can be changed so that theinterface26 between the aqueousfirst solution15 and the organicsecond solution24 forms at the opening of the pores17 in theporous membrane sheath14. If theinterface26 forms at the opening of the pores17, the resultinghydrogel8 will be flush with the opening of the pores17, resulting in a smooth surface.
• In some embodiments, theporous membrane sheath14 can be pretreated to change its hydrophilicity. Changing the hydrophilicity of theporous membrane sheath14 changes the surface tension between theporous membrane sheath14 and thefirst solution15 and thesecond solution24. For example, theporous membrane sheath14 can be plasma etched to increase its hydrophilicity, which in some embodiments helps in maintaining the interface between thefirst solution15 and the second solution at the opening of the pores17.
• In some embodiments having an organic second solution, the first initiator in thefirst solution15 is a thermal initiator that generates radicals upon thermal decomposition. Use of a thermal initiator generally removes the need for a second initiator in thesecond solution24. In some embodiments, the thermal initiator decomposes and generates radicals below a temperature of 55 degrees Celsius. Use of a thermal initiator is particularly suitable for thermally stable dyes, quenchers, monomers and crosslinkers.
• In some embodiments as shown inFIG. 5, in order to reduce water loss and to facilitate deoxygenation, after theporous membrane sheath14 andcavity6 are loaded with thefirst solution15 comprising a thermal initiator, theporous membrane sheath14 is coated with awax28. This can be accomplished by dipping thesensor2 intoliquid wax28, which is allowed to harden around theporous membrane sheath14.
• In some embodiments, thewax28 has a melting point above the thermal initiation temperature of the thermal initiator. Therefore, in order to reduce the likelihood of initiation during the wax coating process, thesensor2 can be dipped and withdrawn from the liquid wax rapidly, thereby reducing the exposure of the initiator to the elevated temperature of theliquid wax28. If desired, thesensor2 comprising thefirst solution15 and thermal initiator can be cooled or chilled so that the exposure to thehot wax28 does not result in initiation and so the wax solidifies rapidly on the sensor surface. In addition, chilling thefirst solution15 can be done during the loading of thefirst solution15 into thesensor2, which can be a relatively long process in some embodiments, thereby reducing the premature decomposition of the initiator which can result in early initiation of polymerization. Also, the thickness of the wax coating can be controlled in part by the temperature of thesensor2 and/or the temperature of theliquid wax28 when thesensor2 is dipped into theliquid wax28. The colder thesensor2, the thicker the wax coating, and the hotter theliquid wax28, the thinner the wax coating. After thecoated sensor2 is withdrawn from the liquid wax bath, thewax28 is allowed to harden. In some embodiments, hardening of the wax coating can be facilitated by dipping thecoated sensor2 in a cold water bath. Additional coats ofwax28 can be put on thesensor2 by simply dipping the wax coatedsensor2 into the liquid wax bath for an additional coat of wax.
• In preferred embodiments, the coating ofwax28 is substantially impermeable to water and water vapor but permeable to oxygen. This reduces the loss of water from thefirst solution15 that can occur though net diffusion of water or water vapor out of thefirst solution15 while allowing the deoxygenation of thefirst solution15.
• In some embodiments, deoxygenation is performed by placing thewax28coated sensor2 into an aqueous bath while bubbling a gas, such as nitrogen, in the bath. The oxygen in thefirst solution15 diffuses into the bath and is carried away by the nitrogen gas. The aqueous bath can be made to have the same osmolarity and water vapor pressure as the first solution, thereby reducing the loss of water from thefirst solution15 within thesensor2. Because of the multiple measures used to reduce water loss out of thefirst solution15, thesensor2 can remain in the deoxygenation bath for extended periods of time, such as for example 1, 2, 4, 8, 12, 16 or 24 hours or more. In other embodiments, deoxygenation is substantially complete in less than 24 hours and therefore the next step in the polymerization process can be initiated in less than 24 hours. Deoxygenation reduces the formation of peroxides during the polymerization process which interfere with the polymerization of the monomer and the performance of the dye and quencher. In addition, because oxygen can function as an inhibitor of the polymerization reaction, the presence of oxygen in thefirst solution15 can reduce the efficiency of the polymerization reaction by reducing the conversion of monomer into polymer and by inhibiting the initiation of the polymerization reaction.
• In some embodiments, after the deoxygenation step, polymerization can be initiated by heating thesensor2 andfirst solution15 comprising the thermal initiator above the thermal initiation temperature but below the melting point of the wax. For example, in some embodiments, thesensor2 is heated to 37 degrees Celsius for 24 hours. After polymerization of the hydrogel is complete, the wax can be removed from thesensor2, e.g., by immersing in hexane and optionally including application of ultrasound energy. In other embodiments, a different solvent can be used instead of hexane. For example, other alkane hydrocarbons such as pentane and heptane and mixtures of these alkane hydrocarbons would also be suitable to remove the wax. In addition, it should be understood that a hexane solvent can comprise a mixture of hexane isomers, or more generally, an alkane solvent can comprise a mixture of the alkane isomers. The hexane wash and ultrasound may be repeated as necessary. The sensor thereby stripped of its wax coating is optionally transferred into an alcohol bath, and preferably finally to a water bath.
• In another embodiment, thefirst solution15 further comprises a second monomer that has at least one functional group. After polymerization into a hydrogel, different chemical moieties can be attached to the functional groups. Examples of chemical moieties with useful traits are molecules with anti-thrombogenic properties or anti-immunogenic properties.
• In another embodiment, after thefirst solution15 is loaded into theporous membrane sheath14 andcavity6, theoptical fiber10 andfirst solution15 filledporous membrane sheath14 are removed from thefirst solution15 and polymerization is initiated in air without the use of asecond solution24. Photolytic initiation or ultraviolet light (UV) initiation can be used in these embodiments by selecting a photosensitizer or a UV initiator, as appropriate. Alternatively, in some embodiments, the monomer or crosslinker itself may generate radicals upon absorption of visible light or UV radiation. In some embodiments, it is desirable to have the air and first solution interface at the opening of the pores. In some embodiments, a surfactant is added to thefirst solution15 so that the interfacial tension between thefirst solution15 and the surface of theporous membrane sheath14 results in the interface forming at the opening of the pores. Additionally, UV initiation may be combined with the wax coating procedure described above by selecting a wax that transmits UV. Additionally, UV initiation may be done with the use of asecond solution24. For example thesecond solution24 may be chosen so as to minimize water loss during the UV polymerization process.
• In some embodiments, the selection of monomer and crosslinker results in a polymer with acidic properties. For example, a monomer with an acidic functional group such as a phenol group can result in a polymer with acidic properties. In other embodiments, the selection of monomer and crosslinker results in a polymer with basic properties. For example, a monomer with a basic functional group such as an amine group can result in a polymer with basic properties. In additional embodiments, the selection of monomer and crosslinker results in a polymer with substantially neutral properties.
• In some embodiments, polymerization is carried out at temperatures below 50° C. For dyes and quenchers that are unstable at high temperatures, polymerization at low temperatures is desirable. In other embodiments, polymerization is carried out at temperatures above 50° C. Polymerization at elevated temperatures is suitable for thermally stable dyes, quenchers, and reaction components.
• In some embodiments, thefirst solution15 is organic and the first initiator, the monomer and the crosslinker are soluble in organic solvents. In these embodiments, thesecond solution24, if present, is aqueous in some embodiments and organic in other embodiments. By having an aqueoussecond solution24, diffusion of organic soluble first initiator, monomer and crosslinker from thefirst solution15 and into thesecond solution24 is reduced. When thefirst solution15 is organic, the method of polymerization is generally more flexible. For example, in some embodiments, polymerization proceeds as an addition reaction. In other embodiments, polymerization proceeds as a ring opening reaction or a condensation reaction. In some embodiments, radical polymerization is used, and in other embodiments anionic or cationic polymerization is used.
• In some embodiments, the monomer is not polymerized in a solvent, but instead, undergoes bulk polymerization. In these embodiments, the monomer is generally a liquid and the initiator, crosslinker, dye and/or quencher are generally soluble in the monomer, allowing the polymerization to be carried out without a solvent.
• In some embodiments, the monomers and/or crosslinkers polymerize to form a hydrophilic polymer. In other embodiments, the monomers and/or crosslinkers polymerize to form a hydrophobic polymer. In some embodiments, the polymer is gas permeable.
• In some embodiments, the polymerization can be accomplished at a basic pH by using an appropriate initiator combination. For example in some embodiments, the first initiator is TEMED, which is basic, and the second initiator is ammonium persulfate. This is advantageous when the dye and quencher are more soluble at a basic pH.
• In other embodiments, the polymerization can be accomplished at an acidic pH by using an appropriate initiator combination. For example in some embodiments, the first initiator is ascorbic acid, which is acidic, and the second initiator is ammonium persulfate. Another example would be to use ascorbic acid with Fenton's reagent, where Fenton's reagent comprises hydrogen peroxide and a ferrous salt. Another example of an initiator pair is t-butyl hydroperoxide and sodium formaldehyde sulfoxilate.
• In some embodiments as shown inFIG. 6A, thecavities6 are cylindrically shaped holes that are spirally arranged on theoptical fiber10. The spiral arrangement of thecavities6 can be accomplished by, for example, drilling afirst cavity6 in theoptical fiber10 along a transverse axis of thefiber10, rotating theoptical fiber10 by a set amount between0 and 360 degrees, and drilling thesecond cavity6 distal thefirst cavity6. To add anadditional cavity6, thefiber10 is again rotated by the set amount and theadditional cavity6 is drilled distal theprevious cavity6. By offsetting thecavities6, a more complete coverage of the cross-sectional area of theoptical fiber10 can be obtained, thus increasing the likelihood that excitation light passing through theoptical fiber10 will irradiate a sufficient and preferably an optimal amount of the indicator system immobilized within the hydrogel in the cavities.
• In some embodiments, thecavity6 is drilled completely through theoptical fiber10. In other embodiments, thecavity6 is drilled partially through theoptical fiber10. In embodiments where thecavity6 is drilled partially through theoptical fiber10, thecavity6 can penetrate approximately halfway through thefiber10, less than halfway through thefiber10, and more than halfway through thefiber10.
• In some embodiments, thecavities6 are not cylindrically shaped, but instead are rectangular-shaped as shown inFIG. 6B or wedge-shaped as shown inFIG. 6C.Other cavity6 shapes are contemplated, such as hemispherical or a spiral or helical cut that winds longitudinally down thefiber10, and the embodiments disclosed herein are not meant to be exhaustive. Indeed, any cavity geometries may be used in accordance with aspects of the invention.
• While a number of preferred embodiments of the invention and variations thereof have been described in detail, other modifications and methods of using and medical applications for the same will be apparent to those of skill in the art. Accordingly, it should be understood that various applications, modifications, and substitutions may be made of equivalents without departing from the spirit of the invention or the scope of the claims.

Claims (11)

1. A method for making an analyte sensor having a smooth outer surface, comprising:

providing an optical fiber comprising a cavity covered by a membrane having pores;

loading the cavity and the membrane pores with a solution comprising polymerizable monomers, an analyte indicator system and a polymerization initiator; and

initiating polymerization of said monomers.

2. The method ofclaim 1, wherein prior to initiating polymerization, the loaded cavity and membrane pores are coated with wax and the solution is deoxygenated.

3. The method ofclaim 2, wherein the wax coating is removed after polymerization is completed to leave a smooth outer surface, wherein the analyte indicator system is immobilized within the cavity.

4. The method ofclaim 3, wherein removing the wax comprises contacting the wax with an organic solvent.

5. The method ofclaim 4, wherein the organic solvent is an alkane.

6. The method ofclaim 5, wherein the alkane is hexane.

7. The methods ofclaim 3, wherein removing the wax further comprises application of ultrasonic energy.

8. The method ofclaim 1, wherein the indicator system comprises a fluorophore and an analyte binding moiety.

9. The method ofclaim 1, wherein the loading step comprises vacuum filling.

10. The method ofclaim 1, wherein initiating polymerization comprises application of a second initiator selected to undergo a redox reaction with the first initiator.

11. The method ofclaim 1, wherein initiating polymerization comprises application of thermal or radiation energy.

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