Methods and devices for controlled release

Abstract: A device for sustained release of an active ingredient from a solution has a membrane of polyethylene, polypropy­ lene or copolymers thereof and a thickness of less than 6 mil.

Description: This invention relates to the controlled release of various substances through membranes. In particular, the invention relates to methods for controlling the rate of release at substantially constant rates over long periods of time and over considerable temperature ranges. The invention relates specifically to membrane release systems for pheromones and other attractants. Controlled release of materials through membranes has been studied extensively. US-A-2,572,669 and US-A-2,661,991 disclose the release of a volatile solid, p-dichlorobenzene, through polyethylene polymer films of various thicknesses. US-A-3,578,545 discloses release of odours from an absorbing material laminated to a porous, flexible plastic film; DE-C-2,740,497, published 16 March 1978, discloses the use of laminates to release pheromones, and US-A-4445641 shows the use of a microporous reservoir to retain an active ingredient such as a pheromone behind a membrane. US-A-4,017,030 discloses release of substances, including pheromones, using plastic capillary hollow fiber tubes, employing a stagnant gas layer at an open end as the "membrane". A similar approach is disclosed in EP-A-9,374, published 2 April 1980. US-A-4,145,001 discloses package form release dispensers for deodorizers. JP-A-82/72904, published 7 May 1982, discloses release of "neat" pheromones containing small amounts of marker material to facilitate detection, from polyethylene capillaries. Other related disclosures include EP-A-31,719, and EP-A-103,953, Additional release systems using cartridges inside permeable or impermeable containers, or, for concentrated liquids, envelopes of two superposed plastic layers, are disclosed in US-A-3,770,199 and US-A 3,795,556, respectively. US-A-3,951,622 discloses a system for controlled release of the unchanged odor of a fragrance through polyethylene or polypropylene membranes. Fragrance is broadly defined, but must be a mixture of mutually soluble components, and the components must include an alcohol and an essential oil. The essential oil is a plant-derived volatile material. The release systems are not designed to maintain constant release over variations in temperature. Control with respect to temperature is significant in the design and success of releasing systems for certain applications, such as the release of substances in connection with insect control. In addition, release systems designed for commercial applications which require large numbers of devices must be economically constructed, and the release persistent over long periods of time. It would therefore be desirable to obtain release of the desired material without the presence of an extraneous costly additive such as an essential oil. None of the methods or devices of the existent art are satisfactory for the purpose of low cost, high volume, pest control systems. Disclosure of the Invention It has now been found that it is possible to design methods and devices which assure not only persistence of a constant rate for release of a desired material into the surroundings over days, weeks or months, but also to design methods and devices that control and stabilize the rate of release over a workable temperature range. The devices can be manufactured at low cost and with simplicity, and are designed to provide long time lines of controlled release of such important substances as insect pheromones for control of pests. It has been found that the presence of an essential oil is not necessary, nor is the mutual solubility of the components. Therefore, in one aspect, the invention relates to a device which releases a constant amount of an active ingredient over a long time period, in the absence of an essential oil. Further, the mixture from which the active ingredient is released is not required to be a homogeneous solution. The device is basically a container which has, as at least a portion of its surface a membrane composed of polyethylene, polypropylene, or their copolymers, having the correct thickness of less than 6 mil (0.015 cm) so that the membrane will be permeable to the desired active ingredient under the conditions of its storage in the device. The active ingredient is typically an attractant, such as a pheromone, and is a component of a mixture that contains, in addition to the active ingredient, a solvent, typically an alcohol of 5 carbons or less and, optionally, one or more release modulators. Solvents are preferred which are biodegradable and non-toxic. In one embodiment, to maintain a constant rate of release of the active ingredient, the solution of the ingredient is in emulsion in the container with a liquid immiscible with the solution In another aspect, the invention relates to a device which releases a constant amount of active ingredient at variable temperatures over a range of between 5° C and 45° C. In one embodiment, this device is similar to the foregoing one, but only a portion of the surface of the container comprises the membrane. The remainder of the container surface is impermeable to the solution of active ingredient stored in the container. The solution must occupy that entire portion of the surface that constitutes the membrane when the device is at the lowest temperature in the range, but can occupy only so much of the remainder of the container as will permit expansion of the solution at the highest temperature in the range. The resulting expansion increases the volume, and lowers the concentration of the active ingredient, thus compensating for an increased emission rate at high temperatures. Another embodiment useful for the same purpose is a container having the permeable membrane surface wherein the container houses a heterogeneous (at low temperature) mixture in which one component is a solution containing the active ingredient. The solution is in contact with an additional negative release modulator, such as a polymer with an affinity for active ingredient. At the lowest temperature of the range, the release modulator and the solution are only minimally mutually soluble, but are relatively mutually more soluble at the highest temperature. At high temperature release is slowed by the influence of release modulator now in more effective contact with the active ingredient, thus, again, compensating for the effect of high temperature. Additional aspects of the invention involve methods to control the rate of release of an active ingredient by the addition of release modulators and methods of releasing desired substances, particularly pheromones, in accordance with the above-mentioned devices. Modes of Carrying Out the Invention A. Pheromones Release Systems One application in which the devices and methods of the present invention are extremely useful is that of pest control in agriculture. The current methods of pest control are both expensive and fraught with dangers to the environment at large. The most efficient method of control is aerial spraying. This requires the spread of toxic materials and constitutes a hazard to beneficial insects, livestock, and people in surrounding areas. However, because the threat of economic, and, in the long run, sociological, damage of uncontrolled pest infestation is so potent and frightening, it has been necessary to carry out such control measures for prevention as well as for cure. For example, the potential for damage by certain agricultural pests is so great that an ongoing program of monitoring is required, and even in the absence of detectable pests, spraying at the cost of approximately $30 per acre at least annually is mandated. For a typical 6,000-acre operation, this amounts to $180,000 per year expended for merely preventive pest control, The devices and methods of the invention would obviate the need for these expenditures in providing an ongoing control mechanism at little cost. Briefly, the ability of the devices to maintain a constant release rate of an active ingredient over a long period of time and over a considerable temperature range can be used advantageously to obtain constant release of attractants for a particular insect over a wide area by strategic placing of the devices. In addition, these devices can be used to release constant amounts of insecticides which will result in the demise of the insects once they are attracted to the location. This provides a matrix of ongoing pest management not possible with the techniques of the prior art. It is especially important in this application that a constant release rate be maintained, since a lower limit of pheromone is needed to be effective, but higher concentrations result in confusion of, rather than attraction of, the insects. As used herein, "active ingredient" refers to a material whose controlled release into the environment is desired. This "active ingredient" is, in a preferred embodiment, a pesticide or an attractant for pests. Chemical structures for a number of attractants are shown and their method of use disclosed in the Kirk-Othmer Encyclopedia of Chemical Technology. Third Edition (19 ). Interscience. NY, 13 :479-483. Pheromones are among preferred embodiments of attractants. They are materials secreted in small amounts by insects (and, there is some evidence, by higher organisms including mammals and humans) which have sufficient vapor pressure to be detected and sensed by other members of the same species at low ambient concentrations at some distance from the source. When detected, the pheromones cause physiological responses in the recipient related to social functions among species members. Typical pheromones may have a variety of chemical structures, and generally have low molecular weights of less than 1000. Many are mixtures of several components, often closely related structurally. The effect of these pheromones is to serve as, for example, sex attractants, warning signals, or signals to indicate a desire for cooperative activity. Examples of such pheromones whose chemical structures have been studied include grandlure, codling moth pheromone, fruit fly pheromone, gypsy moth hormone, and the like. Synthetic materials which may or may not be identical with naturally occurring pheromones, and capable of performing the same functions as those served by pheromones are, of course, also useful as attractants in the device of the invention. One example of such a material is trimedlure, an attractant for the Medfly. Other active ingredients include repellents (see. e.g., Su. H.C.F., et al, Agric Food Chem (1981) 29:115; Su, H.C.F., et a, J Agric Food Chem (1982) 30 :290), or pesticides such as pyrethrin. Active ingredients generally may include any material; it is not necessary that such ingredient be an essential oil. The solution containing the active ingredient has as a solvent any convenient liquid capable of dissolving the active ingredient and any other optional additives, and may be, for example, an alcohol of 5 carbons or less, preferably ethanol. The solution may or may not contain, depending on the release rate desired, a "release modulator" which accelerates or slows the release of the active ingredient. Release modulators which regulate the permeation of the membrane by the mixture include such materials as limonene, which is itself strongly penetrant of the membrane, and various polymeric compositions such as the cellulose polymer. Klucel H-7805, polyvinyl pyrrolidone (PVP), and the silicone oil DC-200, which interact with the components of the mixture so as to increase or decrease their availability for penetration. Several additional methods are available for slowing or increasing the rate of release of the active ingredient. Of course, increasing the surface area of the membrane increases the rate of release, although not linearly. Similarly, as expected, increasing the thickness of the membrane slows the rate of release. Also, providing the mixture in the form of an emulsion with a material which is relatively incapable of penetrating the membrane and which does not itself modulate release decelerates the release of the solution containing the active ingredient, presumably by decreasing the volume of penetrating material which is in contact with the membrane. Also, clearly, the concentration of the active ingredient in the mixture contained will influence the amount of active ingredient released. B. Temperature Compensation The temperature-compensating aspects of the invention are applicable to the release of the desired active ingredient, which is dissolved in a compatible solvent, typically an alcohol of 5 carbons or less. Generally speaking, an increase in temperature would accelerate the release of both solvent and active ingredient due to increased kinetic energy of the molecules. The devices of the invention compensate for this effect as felt by the active ingredient in two different ways. In one approach, the increased release rate for the active ingredient is offset by a decrease in its effective concentration. By maintaining a standard surface area which occupies only part of the surface of the release container, the constant volume of solution facing the releasing surface contains an effectively lower concentration of the active ingredient due to the expansion of the total volume of solution in the container. Thus, although without this provision the active ingredient would be released more quickly at high temperature, the release is maintained at substantially the same level by lowering the effective concentration at the membrane surface. This effect is workable in a temperature range of at least 5° C to 45° C. In the second approach, the compensation for increased kinetic energy takes the form of regulation of release by a negative release modulator. Such materials inhibit the release of active ingredients by associating with them and preventing their penetration of the membrane. A negative release modulator is used which is relatively immiscible with the solution at low temperatures; but achieves a higher degree of mutual solubility at elevated temperatures. Thus, at low temperature. e.g., 5° C, the negative release modulator and the solution containing active ingredient are separated into two layers; at higher temperatures, e.g., 40° C, they merge into a single homogeneous layer. Thus, at higher temperatures, more of the active ingredient is in contact with the modulator, and release is inhibited. At low temperatures, the effects of the modulator are physically separated from the active ingredient, and it is allowed to penetrate the membrane relatively unimpeded, despite its lower kinetic energy. The two effects offset each other, and a constant rate of release for the active ingredient is thus maintained. Suitable temperature ranges for both the foregoing embodiments are in the range of 5° -45° C, which comprise the general temperature range needed for control of agricultural pests. Examples In all of the following examples 1-4, 1 ml of solution containing active ingredient was sealed into an envelope, which envelope is constructed having one surface of relatively impermeable polyester and the other of low density polyethylene. The total surface area of the permeable polyethylene membrane is 20 cm². The total volume of sample released from the envelopes per day was measured by periodic weighing at ambient temperature. The duration of release, until the envelope was devoid of sample, was also noted. Example 1 Effect of Membrane Thickness The effect of membrane thickness was determined using a 10% solution of trimedlure in ethanol. The results using two membrane thicknesses are shown below (1 mil = .00254 cm): thickness g/day days 2 mil 0.009 90 0.8 mil 0.023 33 As expected, an increase in membrane thickness dramatically lowers the rate of release. Example 2 Concentrations and Components The effect of the nature and concentration of the active ingredient pheromone or other attractant on the release rate was also determined. The results are clear that within the approximately 10% concentration range, the nature of the pheromone has relatively little influence on the rate of emission. Also, large deviations in the concentration of the attractant (over a factor of 10) influence the rate of emission of the total solution only slightly. As a first order approximation, the rate of emission of the total solution is relatively independent of solution concentration, and increasing the concentration of attractant in the solution effectively results in a corresponding increase in the concentration of the attractant in the immediate environment: sample (% wt/vol) thickness (mil) g/day days 1.0% trimedlure 0.8 0.029 27 10.0% trimedlure 0.8 0.022 37 8.0% codling moth pheromone 0.8 0.022 37 10.0% trimedlure 2.0 0.009 90 11.5% oriental fruit fly pheromone 2.0 0.0086 93 Example 3 Effect of Release Modulator Additives The rate of release into the environment can, however, be regulated by additives to the solution of pheromone. As shown below, limonene (an essential oil component) greatly enhances the total release rate. On the other hand, polymeric materials exert an unpredictable effect on total rate. sample (%) additive thickness g/day days 1.0 fragrance -- 0.8 0.023 33 1.0 fragrance 0.1g PVP 0.8 0.022 37 10.0 trimedlure -- 0.8 0.022 37 10.0 trimedlure 0.1g limonene 0.8 0.038 21 8.0 codling moth -- 0.8 0.022 36 8.0 codling moth 0.1g limonene 0.8 0.028 28 11.5 fruit fly -- 2.0 0.0086 93 11.5 fruit fly 0.1g Klucel H7805 2.0 0.010 77 As shown in the above table, limonene consistently increases the rate of emission. This is expected, as limonene migrates easily through the membrane. Limonene is an essential oil component. The polymer PVP impedes its release, and the polymer Klucel is an aid. Example 4 It is not necessary that the mixture for release be supplied as a mutually soluble mixture. It may also be supplied as an emulsion. The release rate is relatively slow, but can be aided by a release modulator such as Aerosil, a silica gelling agent. Samples contain 10% trimedlure emulsion in 50-50 ethanol/water. Without the addition of Aerosil, the emission was less than .004 g/day, and the total volume had not been released through a 4 mil membrane even after 180 days. However, with the addition of 0.25 g Aerosil, the emission per day more than doubled, to 0.008 g/day, and the sample packet was empty after 48 days. Example 5 Construction of a Temperature-Constant Release Device-I A cylindrical "straw" having a diameter of 0.4 cm, and made of low density polyethylene, is sealed at both ends when in use, and covered for all but approximately 1 cm of its length with an impermeable polyester film. A 1 ml sample is supplied to the cylinder, and the end sealed so as to leave sufficient air space for expansion into the straw. As the temperature changes, due to the expansion of solvent, the total volume of solution in the straw changes as well, thus diluting the average concentration of the solution. Since the active ingredient emitted is proportional to its concentration, this factor will generate a lower concentration of released pheromone at increased temperatures. On the other hand, since temperature increases the rate of emission, there is a competing effect due to this factor. The net result is that the total amount of pheromone released remains substantially constant. The effect of temperature on the concentration of a 10% solution of pheromone in the sample subject to release is shown in the table below: T °C volume of a 1 ml (20°) sample effective conc. (mg/ml) 10 0.89 110 20 1.00 100 30 1.11 90 40 1.23 80 Thus, a solution which is made up to be 10% solution of pheromone in ethanol at 20° C (i.e., 100 mg/ml) will appear at 10° C to be an approximately 11% solution and at higher temperatures to have lower percentage amounts of the pheromone ingredient. Example 6 Construction of a Temperature-Control Release Device-II In an alternative manner of regulating the release rate with temperature, grandlure (boll weevil pheromone), ethanol, and DC-200 are used in a three-component system which regulates the amount of grandlure available for release. In this system, grandlure is soluble in ethanol at all temperatures, but only slightly soluble in the release modulator DC-200, a silicone oil. DC-200 is miscible with ethanol at 40° C, but less soluble at lower temperatures. A 10% solution of grandlure in ethanol is used in this illustration, and 0.7 g of ethanol solution containing 10% grandlure was added to 2.9 g DC-100 at 20° C. to give a two-layer system. At higher temperatures, as shown in the table below, the entire mixture becomes homogeneous, thus placing the grandlure in effective contact with the (negative) release modulator. However, at lower temperatures the components undergo increasing degrees of separation, thus diminishing the amount of grandlure subject to the modulator. Again two countervailing influences are balanced--the increase in release rate due to increase in temperatures is offset by the increased influence of the negative release modulator. The impact of temperature on the physical separation of the components is shown in the table below (relative layer thicknesses are shown in mm). T °C appearance 58 clear 50 cloudy 45 cloudy 32 two layers: 4 mm/20 mm 26 two layers: 6 mm/20 mm 14 two layers: 8 mm/21 mm

Claims: A device for the constant release of an active ingredient into the surroundings through a membrane over long time periods which device comprises a container containing the active ingredient and which has, as at least a portion of its surface, a membrane consisting essentially of polyethylene, polypropylene, or copolymers thereof, of a thickness less than 6 mil (about 0.015 cm), and characterized in that, to maintain a constant rate of release of the active ingredient despite temperature change, a solution of the active ingredient is in emulsion in the container with a liquid immiscible with the solution. A device which releases an active ingredient into the surroundings through a membrane over long time periods, which device comprises a container containing the active ingredient and having as a portion of its surface a membrane consisting essentially of polyethylene, polypropylene, or copolymers thereof, having a thickness of less than 6 mil (about 0.015 cm), characterized in that, to maintain a constant rate of release of the active ingredient despite temperature change, a solution of the active ingredient in a solvent is in the container in an amount whereby the solution at least covers the entire permeable membrane surface at 5 °C. and when thermally expanded at 45 °C maintains cover of that membrane but is still contained in the container. A device which releases an active ingredient into the surroundings through a membrane over long time periods, which device comprises a container containing the active ingredient and having as a portion of its surface a membrane consisting essentially of polyethylene, polypropylene, or copolymers thereof, having a thickness of less than 6 mil (about 0.015 cm), characterized in that, to maintain a constant rate of release of the active ingredient despite temperature change, a solution of the active ingredient in a solvent is in the container wherein the solution is in contact with a negative release modulator which is less miscible with the solution at 5° than at 45°C. A device according to Claim 2 or Claim 3 wherein the active ingredient is an insect attractant. A method of controlledly releasing an active ingredient from a device to its surroundings at a given rate over a long time period by permeation of the ingredient through a membrane of the device characterized in that, to maintain the rate constant despite temperature changes, the ingredient is in solution in the device. A method according to Claim 5 wherein the solution of the ingredient is emulsified with an immiscible liquid. A method according to Claim 5 wherein the solution is in contact with a negative release modulator which is less miscible with the solution at 5° C than at 45° C. A method according to Claim 5 wherein the permeable membrane of the device comprises only a portion of its surface, the method including arranging that the whole of the surface of the permeable membrane will be contacted with the solution at 5° C and that upon expanding the solution thermally in the device it is maintained in contact with the permeable membrane with its expansion accommodated elsewhere in the device.

Inventors: WILK IMMANUEL J, WILK, IMMANUEL J.
Applicants: WILK IMMANUEL J, WILK, IMMANUEL J.
Application year: 1986
Main classification: A01N25
IPC codes: A01N25, A61L9