US 2635943 A
Description (OCR text may contain errors)
Patented Apr. 21, 1953 STERILIZATION OF STARCH SPONGE Majel M. MacMasters, Peoria, 111., assignor to the United States of America as represented by the Secretary of Agriculture Application March 1'7, 1950,
Serial No. 150,351
4 Claims. (01. 21--56) (Granted under Title 35, U. S. Code (1952),
sec. 266) a The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes throughout the world without the payment to me of any royalty thereon.
This application relates to a novel sterilization procedure whereby starch sponge, a material useful in surgery particularly as an internal surgical dressing material or whereby surgical articles comprising starch sponge may be sterilized with a minimum impairment of the desired properties of the sponge.
Starch sponge, in either block, piece, or powdered form is known; to possess properties which render it valuable as a hemostatic material and as an absorbable surgical dressing. It is naturally important that the sponge-or a dressing comprising said sponge be sterile when used for such purposes. Starch sponge. is a material which does not readily lend itself to sterilization procedure other than that of conventional steam treatment under moderately elevated pressure.
Conventional steam sterilization involves moist heat at temperatures in excess of 100 C. and ordinarily produces unsatisfactory results with starch sponge, for the nature of the sponge is considerably altered and many of its desirable characteristics are lost or seriously impaired. This appears to be true at all effective steam sterilization temperatures. porous material is conveniently carried out by placing containers or packages of the material in the sterilizer and subjecting them to the action of steam under pressure in the usual way. Packagings pervious to the steam, or open containers, are employed in order to permit proper sterilization to take place. Since the steam heats largely through the transfer of its heat of vaporization as it condenses upon the objects to be sterilized, porous materials, such as starch sponge, are impregnated by a substantial amount of condensed moisture. This is a condition readily recognized by those skilled in the art as necessary to achieve satisfactory sterilization.
The final steps of the sterilization, carried out in the usual way, comprise cutting off the steam supply at the end of the sterilization period, permitting the steam pressure in the sterilizer to fall and freeing the sterilizer and its contents of excess moisture. The sterilized contents are then removed. These steps when applied to starch sponge have been found to result in an undesirable physical alteration of the sponge itself. The sponge, instead of possessing the desired resilient and sponge-like properties, is hard,
The sterilization of 1 and the individual fibers making up the sponge are horny in character.
I have determined that the starch, as it exists in starch sponge form, is in a definite physical state, believed to be a coacervate state. I have, moreover, determined that this state which characterizes the starch sponge is relatively unstable under conditions of moist heat and it tends to revert readily toward a gel condition. Starch gel, if dried by heat, results in a horny product. This transformation from the relatively unstable state to the gel state occurs during conventional steam sterilization particularly during the latter stages of cutting off the live steam, and results in the undesirable alteration as previously described. I have discovered that starch sponge after having been subjected to conditions of moist heat may be prevented from reverting to the gel state if it is dried immediately to the state of crispness. The state of crispness represents a physical state of the sponge in which the starch is in a non-reverted state and, at the same time,
' the moisture present is within the proper relationship and proportion to permit a section of the sponge to be crushed between two hard surfaces. It is a state in which the sponge has been prevented from gelling in whole or in part, and upon immersion in liquid, such as aqueous solutions, it is transformed into a soft, pliable and resilient sponge.
It is somewhat surprising that the moisture content of a freshly dried and crisp sponge may vary over wide limits. For instance, a sponge may be crisp when freshly dried and still have associated therewith 20 percent or more moisture. Efiecting the state of crispness during the drying process depends upon several factors, some of which have not been determined. It depends to some degree but not entirely upon the moisture content. It seems also to depend upon the physical relation f the contained moisture to the starch strands or fibers during conditions of elevated temperature. It is only after the state of crispness is obtained that the sponge appears to be stable in the presence of heat and moisture.
The customary periods of vacuum associated with the conventional sterilization procedure will not result in the required state of crispness in sterilized starch sponge. Moreover, air drying the freshly sterilized sponge usually results in reversion. This reversion occurs rapidly, and often the sterilized sponges are seriously damaged during the short period of time involved in 3 reducing the steam pressure of the sterilizer at the usual rate.
I have discovered that visual observation is sufiicient to give ,an accurate end point to my drying process. In practice it is unnecessary to make a more exact determination than merely observing the time when the sponge is crisp. Utilizing my discoveries I dry steam sterilized starch sponge so as to forestall reversion to the gel state. To accomplish this I incorporate immediately an efficientdrying step integral with the steam sterilization. This usually involves a rapid reduction of steam pressure so as to minimize exposure to moisture and high temperature followed by immediate removal of the freshly sterilized sponge to a zone of drying or desiccating capable of rapidly removing moisture. The drying may be accomplished by means of a drying oven swept by heated air or other inert gas. Alternately, the sterilized sponge may be left in the ste rilizer,-;and substantially immediately shutting off the steam followed by rapid reduction of pressure, a considerable vacuum is maintained until the sponge has reached the desired state. This period of vacuum usually takes up to 30 minutes or longer or approximately three times conventionalvacuum drying of sterilized materials v The temperature and pressure employed may he -those customarily employed for sterilizing porous articles such asfabric, dressings, and the like; The temperature may be within the range ofslightly over lQO" C to 125 C. at the corresponding saturatedsteam pressures. For efiicient sterilization I have found that 105-115 C.
and 5 to 20 p. s. i. gage steam pressure give good results.. Y
The following exemplary data illustrate the in-' vention. r a
Starch sponge in block form, prepared by freezing a starch v paste as disclosed by Mac- Masters and Hilbert in US. Patent No. 2,442,928 and MasMasters and Bice in U. S. Patent No. 2,423,475, is wrapped in pervious paper and placed in a conventional steam sterilizer. After sterilization at 110 C. and-15 pounds steam pressure forone hour, the steam is shut off, the pressure permitted to fall at the usualrate, and the hot packages removed and permitted to cool to room temperature. After opening the packages, the sponge was found to be hard and tough. Upon immersion in liquidsitfails to absorb properly and closeexamination of the sponge itself re- Yealed -thttthe fibrous elements had been transformed toj theundesired horny state.
Example 2 The aboveproc'ess is repeated, excepting that immediately after the steam is shut off the pressu'reinthe sterilizer is reduced rapidly and the hot'packagesar'e transferred immediately to a forced draft drying .oven heated to 80 C; and dried for 30 to 45 minutes'to a state of crispness. The dried sponge of this example possessed all been accomplished, cutting off the steam supply,
rapidly reducing the pressure and substantially immediately thereafter drying the sterilized sponge to a state of crispness by exposure to heated air for'at least about 30 minutes, said reduction in pressure and drying being sufficiently rapid to forestall reversion of the sponge to the gel state. V r
2. .The method comprising sterilizing. starch spongeeby subjectingsaid sponge to the actionoi' moist. steam at a temperaturewithin the range of C. to C.'and gagepressure within the. range of 5 to 20 p. s. i. until sterilization has been accomplished, cutting oif the steam supply, rapidly reducing .the pressure an substantially immediately thereafter drying the sterilized sponge to a state ofcrispness, continuing said drying for at least. about30. minutes, said reduction in pressure and drying being .sufficiently rapid to forestall reversion. of the sponge to the gelstate. 1
3; In thesterilization of surgical articles comprising star'ch. sponge .materlalwhereby saidfmaterial is subjected to the action of moist heat and elevated pressureslat temperatures in excess of 100 C. for a period of at least about l hr., the improvement which comprises drying said sponge material, so as toforestall reversion of the starch sponge to a gel state,. by'.rapidly reducing the pressure to atmospheric and substantially immediately sweeping over the freshly sterilized sponge material 'a stream of air heated to approximately 803C; for at least about 30 minutes.
4. In the sterilization of surgical articles comprising starch sponge material whereby said material is subjected to thefaction of .moist heat and elevated pressures attemperatures inexcess of 100C. forlaperiod of at least about 1 hr.,
the improvementwhich' comprises drying said sponge" material, so as toforestall reversion of the starch sponge to. a gel state, by rapidly reducin the pressure to atmospheric and substantially immediately applying a vacuum to'the freshly sterilized sponge material for at least about 30 minutes.
MAJEL M. MACMASTERS.