Electric heating – Microwave heating – With diverse device
Reexamination Certificate
2002-02-08
2003-11-11
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
With diverse device
C219S682000, C219S686000, C219S687000, C219S746000, C219S759000, C422S021000, C422S298000, C422S307000
Reexamination Certificate
active
06646241
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to sterilization of instruments and, more particularly, to the combined application of microwave and thermal energy in the presence of steam and liquid water to the sterilization of dental and surgical instruments and other objects.
BACKGROUND OF THE INVENTION
Several instrument sterilization procedures are presently in use. Autoclaving is most commonly employed, but slowly dulls sharp metal instruments. In U.S. Pat. No. 4,865,814 for “Automatic Sterilizer” which was issued to Bobby B. Childress on Sep. 12, 1989 a microprocessor-controlled heater which generates steam inside of a sealed chamber is described. The pressure level rather than the temperature is used to control the heater. Air is caused to be displaced from the sterilization chamber by the generation of the steam; however, this process does not remove all of the air. The presence of air interferes with production and maintenance of steam at the optimally desired temperature and pressure in the chamber and causes corrosion. Instances of failed sterilization using steam sterilizers are common. Such instances may be triggered by admixture of steam by trace air.
In “A Report Of An Outbreak Of Postoperative Endophthalmitis” by W. Swaddiwudhipong et al., J. Med. Assoc. Thailand 83, 902 (2000) defects in surgical sterilization including possible inadequacy in the autoclave sterilization of surgical instruments is reported. In “The Use Of Autoclaves In The Dental Surgery” by N. W. Savage and L. J. Walsh, Australian Dental Journal 40, 197 (1995), the authors state that although autoclaving is the absolute method of achieving instrument sterilization in any health-care setting, its effectiveness relies on an effective pre-sterilization routine for instrument handling and the subsequent correct loading and operating of the autoclaves. Similar findings are reported in “Autoclave Performance And Practitioner Knowledge Of Autoclave Use: A Survey Of Selected UK Practices” by F. J. T. Burke et al., Quintessence International 29, 231 (1998). In “Disinfection And Sterilization Practices In Mexico” by M. Zaidi et al., J. Hospital Infection 31, 25 (1995), the authors report the use of too short an exposure time in steam sterilizers or dry heat sterilizers as contributing to ineffective sterilization of surgical instruments.
Heat sterilization at approximately 160° C. is also used. However, this method requires heat generators capable of rapid heating which are not commonly available, and rubber and plastic parts may be damaged. Chemical sterilization techniques have the disadvantage that hazardous materials such as ethylene oxide or alkaline glutaraldehyde must be handled and disposed of in a hospital or dental clinic environment. Moreover, sterilization times are lengthy.
Sterilization of medical and dental instruments by directly and indirectly using microwaves is known. In both U.S. Pat. No. 5,019,359 for “Method And Apparatus For Rapid Sterilization Of Material” which was issued to Barry S. Kutner et al. on May 28, 1991 and U.S. Pat. No. 5,039,495 for “Apparatus For Sterilizing Articles Such As Dental Handpieces” which was issued to Barry S. Kutner et al. on Aug. 13, 1991, a liquid sterilant solution and the material to be sterilized are placed in a sealable, vapor-impermeable collapsible pouch. Microwave energy vaporizes the sterilant solution and the instruments are exposed either to the vaporized sterilant alone or to both microwave radiation and the vaporized sterilant. The vaporized sterilant prevents arcing and assists in sterilizing the instruments when used in conjunction with the microwaves. In U.S. Pat. No. 5,417,941 for “Microwave Powered Steam Pressure Generator” which issued to Bernard A. McNulty on May 23, 1995, an apparatus which produces high temperature and pressure steam derived from microwave energy is described. Microwave energy is coupled into a guiding structure such that essentially all of the energy is transferred to a reaction fluid contained in a holder located at the end of the guiding structure. The reaction fluid is rapidly vaporized and the resulting vapors expand into a high-pressure chamber through a metal screen that also prevents transmission of microwave energy. No mention is made of whether the resulting temperature and pressure permit steam sterilization to occur, whether the sterilization chamber is free of air during the sterilization cycle, or whether arcing of the metal parts is avoided.
In “Nonthermal Killing Effect Of Microwave Irradiation” by Seigo Sato et al., Biotech. Techniques 10, 145 (1996), the elucidation of the lethal effects of microwave radiation at constant temperatures is described. It was found that the death rates for
E. coli
exposed to microwave irradiation were higher than those obtained in conventional heat sterilization at the same temperatures. In “Heat Transfer Analysis Of
Staphylococcus aureus
On Stainless Steel With Microwave Radiation” by C. B. A. Yeo et al., J. Appl. Microbiol. 87, 396 (1999), the authors show that the microwave killing pattern of Staph. aureus is principally due to heat transfer from the stainless steel substrate which absorbs microwave energy in the surface regions, and that little direct energy is absorbed by the microbes from the incident microwave radiation. Complete bacterial inactivation was achieved at 61.4° C. with an irradiation time of 110 s.
Metallic instruments are problematic in microwave-assisted sterilization processes because such instruments reflect microwave energy and, when placed in microwave field, will arc. In U.S. Pat. No. 5,599,499 for “Method Of Microwave Sterilizing A Metallic Surgical Instrument While Preventing Arcing” which was issued to Jeffery S. Held and Robert F. Schiffmann on Feb. 4, 1997, and in U.S. Pat. No. 5,607,612 for “Container For Microwave Treatment Of Surgical Instrument With Arcing Prevention” which was issued to Jeffery S. Held and Robert F. Schiffmann on Mar. 4, 1997, a container for preventing arcing of a metal object placed therein and subjected to microwave radiation is described. To reduce arcing between metal surgical instruments, the container includes a tray upon which the instruments are located a suitable distance apart. Moreover, the container has at least one surface for absorbing microwave energy which impinges on the exterior surfaces of the container for converting the absorbed microwave radiation into heat that sterilizes the instruments. Iron oxide (that is, Fe
2
O
3
) materials are employed for this purpose, and prevent substantially all of the microwave radiation impinging on the exterior surface of the container from entering the volume of space therein.
In U.S. Pat. No. 4,861,956 for “Microwave/Steam Sterilizer” which issued to Calice G. Courneya on Aug. 29, 1989 a microwave/steam sterilizer is disclosed. The authors state that the sterilizer hydrates potential pathogens, including spores, and subjects them to relatively uniform electromagnetic energy without arcing and without self-destruction of the microwave source from reflected microwave energy. According to Courneya et al. microwave energy is used to vaporize water forming steam which is rapidly absorbed by dry spores making them vulnerable to killing by direct microwave energy. The water vapor also keeps electrical charges sufficiently low that arcing and sparking are overcome. The sterilizer provides an adequate availability of water, as steam, to allow the dry spores to hydrate without flooding with excess water which acts as a coolant and prevents the formation of super-heated steam internally within the spores. Expected sterilizer temperatures are in the region of 98.9° C. Excess steam is preferentially attracted to the coolest area in the chamber, namely, the chamber walls. Thus, liquid water is not present on the instruments being sterilized. Additionally, 98.9° C. and steam at near atmospheric pressure are inadequate for sterilization of instruments based on experience with autoclaves.
In “A Microwave Based Device For Sterilisation/Disinfection Of Su
Varma Ravi
Vaughan Worth F.
Ecofriend Technologies, Inc.
Freund Samuel M.
Leung Philip H.
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