WO1992009310A1

WO1992009310A1 - Process for sterilizing heat-labile material

Info

Publication number: WO1992009310A1
Application number: PCT/NL1991/000238
Authority: WO
Inventors: Frederik Reinder Rozema; Jacobus Arnoldus Antonius Maria Van Asten
Original assignee: Dsm N.V.
Priority date: 1990-11-26
Filing date: 1991-11-25
Publication date: 1992-06-11
Also published as: NL9002564A

Abstract

A process for sterilizing heat-labile material using steam in an autoclave. In the first phase of the process, air in the autoclave is replaced by steam. This step also preheats the autoclave. In the second phase of the process, the material is sterilized in the presence of moisture. In the third phase of the process, steam is removed from the autoclave and the material is dried. The steam is supplied in such an overheated condition in the first phase that no condensation occurs in this phase. Using a process according to the present invention allows heat-labile materials to be sterilized without loss of mechanical properties.

Description

PROCESS FOR STERILIZING HEAT-LABILE MATERIAL

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The invention relates to a process for sterilizing heat-labile material with the aid of steam in an autoclave.

2. Description of the Prior Art; A process disclosed in GB-A-2.160.208 teaches using steam to sterilize a 3-hydroxybutyrate polymer powder for use as a surgical glove dusting powder.

Sterilization is commonly used to make objects such as implants, sutures, surgical clamps, clothing and instruments suitable for surgical applications.

Sterilization involves the killing of microorganisms, including bacterial spores, present on or in the object to be sterilized.

In current sterilization processes employing steam, the material to be sterilized is brought into an autoclave, into which steam and/or water is introduced for the purpose heating it. To make the sterilization process effective and reproducible, saturated steam is often used that is introduced with the aid of pressure-controlled steam generators. The temperature is increased by increasing the pressure. Because the material to be sterilized has a lower temperature than the steam, the steam condenses on the material to be sterilized and thereby heats the material. The autoclave is maintained at this temperature and pressure for a time generally known as the sterilization time. This is described in the 'Richtlijnen Steriliseren en Steriliteit' , Samson Publishers BV, Alphen a/d Rijn, 1989. To achieve effective sterilization, all surfaces of the material must be in contact with saturated steam for at least the sterilization time. As a result of this process, the material is exposed to condensed water for a fairly long time.

A process for steam sterilisation is also described in AT-B-382.314, but this patent publication does not describe sterilizing heat-labile material, and the process can not be used for sterilising such materials. Such a method further has a serious flaw when used with heat-labile material that is susceptible to moisture. If the material is heat-labile, prolonged exposure to moisture, which is at an elevated temperature, leads to deterioration of the mechanical properties of the material. Although no serious problems arise if the heat-labile material is to be used as a surgical glove dusting powder, if the material is to be used in fields where greater demands are imposed on its mechanical properties, any deterioration thereof is undesired.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for sterilizing heat-labile materials, wherein mechanical properties of the material clearly are subject to less deterioration.

This is achieved by breaking the inventive process of the present invention into a series of phases. In a first phase of the process, air in the autoclave is replaced by steam during the preheating process. During a second phase of the process the material is sterilized in the presence of moisture. In a third phase, steam is removed from the autoclave and the material is dried. The steam is supplied to the autoclave in such an overheated condition in the first phase that no condensation occurs during preheating. Steam is understood to be gaseous water. The supply of steam and the heating are controlled so that the steam is not saturated until the second phase, i.e. the sterilization phase. Only then, condensation occurs at the surface of the material to be sterilized. The material is sterilized by the combination of moisture and temperature during the sterilization time.

A process for operating an autoclave is described in EP-A-138.688, which also describes that steam can be applied that is overheated. But EP-A-148.688 does not describe sterilising heat-labile material and does not describe a process according to the present invention in which the steam is overheated in a first phase and oversaturated in a following phase.

Heat- and moisture-labile materials can also be sterilized by means of radioactive radiation or through exposure to ethylene oxide gas. However, radiation presents rather serious drawbacks, such as the possibility that the chemical composition of the material may be affected and the possibility that the source of radiation might be a hazardous radiation source. Exposure to ethylene oxide also presents drawbacks such as the fact that traces of ethylene oxide may remain in the material, thereby posing detrimental health effects.

Sterilization may comprise, for example, two different germ-destroying effects. The first such effect is cell destruction through oxidation of organic matter in the germ as a result of high temperatures. A fairly long time and a fairly high temperature are required to make this destruction effective, for example 45 minutes heating at 220°C. The total heat load of the material is fairly high. The second destroying effect is the coagulation of protein in the germ through contact of the germ with water a an elevated temperature. This destruction may already be effective on treatment for a relatively short time at a certain relatively low temperature. The total heat load of the material is fairly low in this case. A relatively large amount of activation energy is required for this coagulation. This may be supplied by, for example, the condensation heat of the water. The process conditions can be determined with the aid of the IMO concept as described in Van Asten, J. and Dorpema, J.W., "A new approach to sterilization conditions. The IMO concept". Pharmaceutical Journal, Scientific Edition, 4, 49-56 (1982). The IMO concept is a mathematical model that can be used to calculate the efficiency of a sterilization. This concept is based on an estimated initial degree of contamination by germs, the sterilization time, the temperature, the estimated difference in temperature required to reduce the number of germs by a factor of 10, the estimated resistance of a microorganisms and the desired maximum final degree of contamination. The missing parameters can be calculated by filling in the estimated and known parameters in a mathematical formula.

Steam and air are poorly miscible with one another under the conditions of this process. It is possible to dissolve a maximum of about 0.5 volume% air in steam. For effective sterilization, the steam must reach all parts of the material to be sterilized. This is not the case if there is still residual, undissolved air in, for example, any packing of the material. That is why the air must be removed to the greatest extent possible before the sterilization phase occurs.

In the first phase of the process according to the present invention, the contents of the autoclave are therefore usually evacuated to a certain level prior to the injection of steam. This process may be carried out once or several times in successive cycles. Said level of evacuation can e.g. be chosen between 0,01 bar and 0,3 bar.

If the autoclave is evacuated in two cycles to a pressure lower than 1/10 of the initial pressure, the autoclave will thereafter contain only 1% of the original amount of gas. If the sterilization takes place at a pressure of two or more times the initial pressure, the amount of original gas remaining in the autoclave will be less than 0.5%. This amount is soluble in the steam and is therefore no longer a problem. The evacuating cycles may take place within a short time period and occur with great rapidity. In the case of carefully produced materials having a small degree of initial contamination, it is possible to complete the sterilization process within a short time period. An object subject to normal use is estimated to contain approximately 10⁶ bacteria per unit of area. A material produced under so-called Good Manufacturing

Practice (GMP) conditions is estimated to contain less than 10⁴ bacteria per unit of area. A material produced under GMP^* conditions can therefore be sterilized in a relatively short sterilization time. The sterilization procedures that are now commonly used in hospitals and the like are carried out at temperatures of 121, 126 or 134°C for 15, 10 or 3 minutes, respectively, based on the aforementioned initial degree of contamination of 10⁴. A sterilisation at 134°C and a pressure of 2,2 bar is described in NL-A-7905838, but this patent publication is not directed to sterilising heat-labile materials and does not describe a process according to the present invention in which the steam is overheated in a first phase and oversaturated in a following phase.

The sterilization according to the invention is preferably carried out at a temperature of between 126 and 140°C, for a time period of as much as 10 minutes and as low as 20 seconds, more preferably at a temperature in the range of 130 to 135°C for between 5 minutes and 60 seconds to keep the load of the material to be sterilized as low as possible.

In order to keep the heat load of the material as low as possible, the sterilization time is usually kept as short as possible.

It is advantageous to use an automated system to control the process conditions. Such an automated steam sterilization system can be composed from commercially available components. DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENT

The process according to the present invention can be carried out as follows, although variations in this process are possible. An autoclave with a heating jacket is equipped with a vacuum pump and a steam generator. The temperature in the autoclave can be constantly measured.

Steam can be fed from the steam generator into the autoclave via a pipe, which may be closed off by means of a cock or other such mechanism. Preferably, the cock is an accurately controllable valve, for example a so-called proportional integral differential control valve. This is a programmable diaphragm. The walls of the autoclave are heated to a temperature slightly higher than the desired sterilization temperature, for example 0,5 to 5°C higher and preferably 1 to 1.5°C higher. This can be done, for example, by using steam that is passed through the jacket. Steam of a desired temperature and pressure is generated with the aid of the steam generator. This steam is not oversaturated, i.e. no condensation has occurred. The pressure in the steam generator is preferably 0.01 to 0.5 bar above the desired sterilization pressure. More preferably, the pressure in the steam generator is 0.1 to 0.3 bar above the desired sterilization pressure. This can be steam from the same source as the steam that is lead into the autoclave or different. In a closed system the pressure P and temperature T are directly related to one another. This means that the temperature of the steam in the steam generator is also slightly above the desired sterilization temperature.

The material to be sterilized is usually placed in the autoclave packed in a laminated bag. A laminated bag is a bag consisting of plastic and paper. Via the paper the steam can penetrate into the bag and exercise its sterilizing effect. After drying, the paper is no longer penetrable by germs and the material inside the bag remains sterile. This laminated bag is subsequently usually packed in a moisture-proof protective outer bag. By chosing the right packaging it is possible to obtain products that remain sterile for any requested period of time.

All kinds of packaging materials are described in EP-A-402.922, hereby incorporated by reference. EP-A-402.922 is directed to a process of sterilising with use of high pressure, and therefor not relevant for the process of the present invention. By choosing the right packaging, the material can remain sterile for a time period sufficiently long for practical use.

In the first phase the autoclave is evacuated to e.g. 1/10 of its initial pressure. Then, an amount of steam is injected from the steam generator until the pressure has almost returned to its pre-evacuation level. As the steam expands, it cools slightly but condensation does not occur. The autoclave is then once again evacuated so that the pressure drops to 1/10 of the initial pressure. Approximately 1% of the original amount of air still remains in the autoclave. If the sterilization is carried out at more than about two times the initial pressure, the amount of air remaining in the autoclave is less than 0.5%, which is soluble in the steam. The fact that air must be removed before steam sterilisation is confirmed by US-A-4,372.916, which is directed to establishing and ascertaining desired air removal in steam sterilisation. US-A-4.372.916 does not describe sterilising heat-labile materials and does not describe a process according to the present invention in which the steam is overheated in a first phase and oversaturated in a following phase.

The autoclave is now filled by injecting steam again. This is done carefully, with constant measurement of the temperature in the autoclave. The steam is hotter than the autoclave, thus enabling the autoclave to be slowly heated. The pressure in the autoclave also slowly increases due to the direct relationship between pressure and temperature. Steam is constantly allowed to enter the autoclave in a controlled manner via the valve such that the steam in the autoclave does not become (over)saturated. This can be checked by comparing the measured temperature with a normal pressure/temperature curve. The temperature is measured and the pressure is adjusted. Preferably, an automated system is used to adjust the pressure relative to the temperature. The material to be sterilized is heated by the steam present in the autoclave. As the steam is not saturated, but merely overheated, no condensation occurs at the surface of the material in this phase. The autoclave is heated at such a rate that the difference in temperature with respect to the material does not become too great and cause condensation. No condensation occurs on the walls of the autoclave either, as they are heated to a higher temperature.

As the temperature and pressure approach the sterilization temperature and pressure the rate of flow of the steam into the autoclave is reduced. This is facilitated by the fact that the pressure difference between the generator and the autoclave is smaller at this point than when the heating began. The second phase starts when the steam in the autoclave becomes saturated. This happens because the rate of flow of the steam from the generator to the autoclave becomes increasingly more gradual, and as a result, the steam expands less and cools less. As the temperature of the material to be sterilized has increased somewhat more slowly than the temperature of the steam in the autoclave, the material is cooler than any remaining contents of the autoclave. As a result, the steam condenses onto the surface of the material to be sterilized. It is preferable to have the steam in the autoclave only slightly saturated in the second phase so that only a thin film of water condenses and no drops are formed on the surface of the material. Drops could cause problems during the drying phase. Due to the surface tension of such a thin layer no water penetrates the material.

The condensation energy of the water is sufficient to effect the aforementioned coagulation. As a result of the condensation of a relatively small amount of water, the temperature of the material soon rises to that of the steam (the heat capacity of the material is small relative to the condensation heat of the water) , after which the condensation stops.

If a relatively large amount of material is to be sterilized, the first phase is carried out relatively slowly' so that the large amount of material has the opportunity to heat up. At the beginning of the second phase the temperature of the material is preferably only a few degrees celsius lower than that of the steam in the autoclave, otherwise too much water would condense onto the surface of the material before the material has heated up. This would produce an undesired thick layer of water formed on the material.

A particular aspect of the process is the use of a heated jacket, which contains water and water vapor. The temperature of the jacket is set 0,1 to 5°C and preferably about 0.5°C above the sterilization temperature. The jacket then contains saturated steam with a temperature e.g. 0.5°C higher than the sterilization temperature. The autoclave is heated using steam from a steam generator, the steam being produced at a certain temperature and pressure until phase two is reached. Use is then made of the aforementioned accurately conditioned steam of the jacket. The advantage of this method is that, in principle, any steam generator producing steam at any temperature and pressure can be used, although the present description suggests exemplary conditions for a steam generator. To achieve the desired degree of sterilization, the autoclave is maintained at this temperature and pressure for a certain time. This time is determined based on the IMO concept as described above.

After completion of the second phase, the overpressure of the autoclave can be relieved by opening a cock or by switching on a vacuum pump. While not necessary, he steam may be cooled before being sucked into the vacuum pump. Evacuating the autoclave accelerates the drying process. As a result, the temperature in the autoclave decreases. Because the sterilized material still has a high temperature, the material provides the heat required to evaporate the water film.

Once the drying process is complete, the autoclave can be opened and the material removed.

Use of the process according to the present invention enables one to sterilize heat-labile materials without losses in their mechanical properties. Examples of materials are the polymers and copolymers of cyclic esters such as lactones or cyclic carbonates. Examples of lactones are lactide, zoals L-lactide, D-lactide, DL-lactide, glycolide, ε-caprolactone, dioxanone, 1,4-dioxanone- 2,3-dione, beta-propiolactone, tetramethyl glycolide, beta-butyrolactone, gamma-butyrolactone, delta-valerolacton, and pivalolactone. Examples of cyclic carbonates are trimethylene carbonate and 2,2-dimethyl-trimethylene carbonate. Mixtures of polymers and/or copolymers may also be used. Such materials are described in for example EP-B-0,108,635. Aliphatic polyesters, depsipeptides, polyethylene oxide, methacrylate N-vinylpyrrolidone- copolymers, polyesteramides, polyesters of oxalic acid, polydihydropyrans, poly(alkyl-2-cyanoacrylates), poly- urethanes, poly(vinylalcohol), polypeptides, poly-β-malic acid, poly-β-alkanoic acid and poly(ortho) esters are also examples of materials that can be sterilized with the aid of the process according to the present invention.

EP-A-402.922 further lists all kinds of polymers for which the normal sterilisation processes are not fitted and is therefor also incorporated by reference with that respect.

The invention will be illustrated with respect to the following examples. The present invention is not limited to the disclosed examples, but is meant to include variations thereof. The intrinsic viscosity was measured with the aid of an Ubbelohde Viscosimeter, type Oa, according to ASTM D-445, at 25°C. The viscosity average molecular weight was determined from the intrinsic viscosity using the formula 1^*1 = 5.45 x 10-4 x M_v°.73, according to Leenslag, J.W. et al., "Resorbable materials of poly(L-lactide) . VI Plates and screws for internal fracture fixation", Biomaterials 8, 70-73 (1987).

The stress-strain curve was determined using an Instron 4301 Tensile Tester (Limited High Wycombe), with a 5000 N load cell and a crosshead speed of 10 mm-min-1. The tensile strength at break (σ^b) and the elongation at break ( ε^b ) were determined from this stress-strain curve.

Example I Materials; Polylactide (PLLA) was synthesized according to the method described by Leenslag, J.W. et al., "Resorbable materials of poly(L-lactide) . VI Plates and screws for internal fracture fixation", Biomaterials, 8, 70-73, (1987). Test bars measuring 2.2 x 6.0 x 30.0 mm were sawn from a block of as-polymerized PLLA. The molecular weight of the PLLA was M_v ■ 7.2 x 10⁵.

A test bar was placed in an autoclave with a heating jacket. A vacuum pipe and a steam generator, closed off by means of a programmable diaphragm, were connected to the autoclave. The steam generator supplied steam with a pressure of 340 kPa and at a temperature of 137°C. The heating jacket was set to a temperature of 130.5°C.

The method of the IMO concept was used to calculate that a sterilization cycle of 60 s at 129°C is sufficient to make the material sterile for surgical applications. First phase: In the first phase, the contents of the autoclave were evacuated with the aid of a vacuum pump to a pressure of approx. 10 kPa. Steam was then injected to raise the pressure to approximately 50 kPa. Then, the pressure was returned to approximately a pressure of 10 kPa. Next, overheated steam was again injected. The pressure in the autoclave raised to 264 kPa in about 100 s. The temperature in the autoclave rose to 129°C as the pressure was raised.

Second phase; At the moment that the autoclave reached the temperature of 129°C the diaphragm was virtually closed. Then, a little more steam was slowly injected. A film of water condensed onto the test bar. The steam pipe was completely closed, at which point the temperature of the test bar had increased to 129°C.

Third phase: After being maintained in the second phase for 60 s, the vacuum pipe was opened. The pressure was reduced to about 10 kPa. The temperature decreased from

129°C to about 50°C. The test bar was left in the autoclave to dry at reduced pressure for another 120 s. Then, the pressure was increased to atmospheric level so that the autoclave could be opened and the test bar removed for analysis.

Analyses: The molecular weight, σ^b and ε^b of the test bar were determined. The results are shown in Table I.

Comparative Experiment A

The molecular weight, σ^b and ε^b of an unsterilized test bar according to example I were also determined. The results are also given in Table I.

Comparative Experiment B

A test bar was sterilized according to the method of example I but rather than use overheated steam in the first phase, saturated steam was supplied. This led to condensation occurring during the heating phase. The molecular weight, σ^b and ε^b were determined of this bar too. The results are also shown in Table I. The method according to comparative experiment B corresponds to the sterilization method according to the prior art commonly used in hospitals, as described in Richtlijnen Steriliseren en Steriliteit, Samson Publishers B.V. Alphen a/d Rijn, 1989, except that the initial degree of contamination was estimated to be 10⁴ , as in example I.

Table I Results of the analyses

17 20

8

Table I illustrates that the material of example I has virtually the same mechanical properties as the unsterilized material of comparative experiment A, whereas the material sterilized by conventional processes as depicted in comparative experiment B exhibits much poorer mechanical properties than does material sterilized via the present invention. Comparative example B illustrates that with a sterilisation process according to the state of the art the molecular weight is lowered, while the molecular weight of the product of example I has hardly changed.

The invention has been described in relation to what is considered to be the presently preferred embodiment. However, various modifications and arrangements are permitted that fall within the spirit and scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A process for sterilizing heat-labile material with the aid of steam in an autoclave, comprising the steps of: replacing air in the autoclave by overheated steam during a first phase, said replacing step including preheating the autoclave in a manner such that no condensation occurs; sterilizing a heat-labile material within the autoclave in a second phase of said process, the material being sterilized in the presence of moisture; and removing the steam from the autoclave during a third phase and drying the material.

2. A process according to claim 1, wherein said sterilizing step includes supplying an amount of steam such that water condenses onto the material.

3. A process according to claim 1 or claim 2, wherein said replacing step includes evacuating the air in the autoclave at least partially and replacing it by said steam in at least one cycle until the residual amount of air is soluble in the steam.

4. A process according to any one of claims 1-3 wherein the sterilizing step occurs at a temperature of from 126° to 140°C.

5. A process according to any one of claims 1-4 wherein the sterilizing steps occurs over a time span between 10 minutes and 20 seconds.

6. A process according to any one of claims 1-5 wherein said replacing step includes generating the steam in a steam generator connected to the autoclave by means of a valve.

7. A process according to any one of claims 1-6, wherein walls of the autoclave are heated to a temperature that is 1° to 1.5°C higher than a temperature of said steam at which the sterilization is performed.

8. A process according to claim 6 or claim 7, wherein pressure in the steam generator is 0.1 to 0.5 bar higher than a pressure desired for said sterilizing step.

9. A process according to claim 8, wherein the pressure in the steam generator is 0.1 to 0.3 bar higher than the pressure desired for said sterilizing step.

10. A process according to any one of claims 6-9, further comprising the step of constantly measuring temperature in the autoclave.

11. A process according to any one of claims 1-10, wherein said sterilizing steps includes introducing steam with a temperature and pressure greater than a temperature and pressure of the autoclave at such a low rate that the steam in the autoclave becomes saturated.

12. A process according to any one of claims 1-11, further comprising heating the contents of the autoclave at such a rate so that a temperature difference between the contents and the heat-labile material is maintained below a predetermined value.

13. A process according to any one of claims 1-12, further comprising the step of positioning a heatable jacket containing water and steam around the autoclave in a manner such that the jacket is communicable with the autoclave.