Abstract:
An actuator that includes an enclosure having a wall impermeable to a first solute and permeable to a solvent and containing, at least temporarily, a catalyst capable of promoting the transformation of at least one second solute into the first solute to vary the osmotic pressure in the enclosure; and a deformable chamber connected to the enclosure, the chamber being capable of increasing in volume under the action of the solvent moving from the enclosure into the chamber by osmosis or the enclosure being designed to be arranged in contact with the solvent, the chamber being capable of increasing in volume under the action of the solvent penetrating into the enclosure by osmosis.

Description:
This application claims the benefit of French Application No. 05/50314, filed Feb. 3, 2005 and Intl. Application No. PCT/FR2006/050092, filed Feb. 2, 2006, the entire disclosures of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to devices that can be used as actuators or as motors which are easy to form, which use low-cost fuel, and which emit little or no waste. 
     Further, the present invention relates to devices which can be used as actuators or as motors capable of operating within a biological medium such as the human body or an animal body. 
     Such actuators and such motors find applications in the medical field, for example, to overcome the impairment of a natural muscle. Muscles that can be replaced or assisted, temporarily or definitively are, for example, the heart muscle, the respiratory muscles, the sphincters, and smooth or striated muscles, in particular skeletal muscles. 
     Such actuators and such motors also find applications in fields other than the medical field. In particular, such a motor may be used in all fields where a low waste generation is an important factor in selecting the motor. It may be, for example, the automobile field where the polluting waste generated by the motor used to drive the vehicle wheels is desired to be decreased as much as possible. 
     2. Discussion of the Related Art 
     US patent application 2004/248269 of the applicant describes an osmotic actuator intended to be dipped in a biological medium and comprising a deformable enclosure having a semi-permeable membrane, the enclosure containing a solute likely to be osmotically active. 
     Patent application EP-A-1481165 of the applicant describes an actuator and an osmotic motor with an operation that can be controlled with more accuracy. For this purpose, patent application EP-A-1481165 provides use of microorganisms which are contained in an enclosure permeable to a solvent and impermeable to a first solute. The microorganisms are capable of transforming a second solute into the first solute. A deformable chamber is connected to the enclosure and can see its volume increase under the action of the solvent penetrating into the enclosure by osmosis as the microorganisms are providing the first solute. 
     A disadvantage of such an actuator and of such an osmotic motor is that keeping microorganisms alive imposes constraining conditions of use. More specifically, it is necessary to dissolve, in the solvent in which the microorganisms are arranged, substances essential to the metabolism of the microorganisms, for example, glucose and oxygen. It is further necessary to provide the discharge of the waste generated by the cellular metabolism, especially the carbon dioxide. Further, it is necessary to maintain many parameters such as temperature or the pH of the solvent in which the microorganisms are arranged within generally very small ranges out of which microorganisms cannot survive. 
     SUMMARY OF THE INVENTION 
     The present invention aims at an actuator and an osmotic motor with a simplified implementation. 
     The present invention also aims at an actuator and an osmotic motor that can operate over a long time period without any constraining maintenance operation. 
     The present invention aims at using, instead of the microorganisms provided in European patent application EP 1481165, one or several catalysts capable of promoting a reaction of transformation of a compound into another compound. Such catalysts may for example correspond to enzymes which are proteins endowed with a very high catalytic power. As compared with patent application EP-A-1481165, the present invention is characterized in a greater room for maneuver as to the conditions of use of the actuator and of the osmotic motor. Indeed, catalysts being non-living compounds, the constraints aiming at ensuring their integrity are less restrictive than those aiming at the survival of microorganisms. 
     More specifically, the present invention provides an actuator comprising an enclosure having a wall impermeable to a first solute and permeable to a solvent and containing, at least temporarily, a catalyst capable of promoting the transformation of at least one second solute into the first solute to vary the osmotic pressure in the enclosure; and a deformable chamber connected to the enclosure, said chambre being capable of increasing in volume under the action of the solvent moving from the enclosure into the chamber by osmosis or said enclosure being designed to be arranged in contact with the solvent, said chamber being capable of increasing in volume under the action of the solvent penetrating into the enclosure by osmosis. 
     According to an embodiment of the present invention, said wall of the enclosure is permeable to the second solute. 
     According to an embodiment of the present invention, said wall of the enclosure is impermeable to the second solute, the catalyst being capable of promoting the transformation of a number of particles of the second solute into a greater or smaller number of particles of the first solute. 
     The present invention also provides a motor comprising an actuator such as previously described, in which the chamber comprises return means which oppose to the volume increase of the chamber and controllable means for lowering the osmotic pressure in the chamber. 
     According to an embodiment of the present invention, the wall of the enclosure is impermeable to the second solute, the catalyst being capable of promoting the transformation of a number of particles of the second solute into a greater number of particles of the first solute. The motor further comprises an additional enclosure having a wall permeable to the solvent and impermeable to the first and second solutes and containing an additional catalyst capable of promoting the transformation of a number of particles of the first solute into a smaller number of particles of the second solute, said additional enclosure being connected to the chamber by a valve. 
     According to an embodiment of the present invention, the wall of the enclosure is impermeable to the second solute, the catalyst being capable of promoting the transformation of a number of particles of the second solute into a greater number of particles of the first solute. The enclosure is arranged in a deformable envelope containing the solvent and the first solute, the enclosure containing an additional catalyst capable of promoting the transformation of a number of particles of the first solute into a smaller number of particles of the second solute, the means for lowering the osmotic pressure in the chamber being a valve capable of connecting up the chamber and the envelope. 
     The present invention also provides a motor comprising an actuator such as previously described, in which the enclosure is at least partly deformable and is connected to the chamber at the level of the wall. The motor comprises first means for supplying the catalyst into the enclosure, and second means for supplying an additional catalyst, capable of promoting the transformation of the first solute into the second solute, into the enclosure. 
     According to an embodiment of the present invention, the wall is permeable to the second solute. The catalyst is capable of promoting the transformation of a number of particles of the second solute into a smaller number of particles of the first solute and the additional catalyst is capable of promoting the transformation of a number of particles of the first solute into a greater number of particles of the second solute. 
     According to an embodiment of the present invention, the second solute is a compound comprising an amine function, the first solute being a complex of the second solute and of an additional solute comprising an aldehyde function, the wall being impermeable to the additional solute. Further, the catalyst is the hydrogen ion, the additional catalyst being the hydroxyl ion. 
     According to an embodiment of the present invention, the first supply means comprise an additional enclosure designed to receive a solvent containing glucose, the additional enclosure containing glucose oxidase enzymes capable of promoting the oxidation of glucose to provide gluconate ions and hydrogen ions. 
     According to an embodiment of the present invention, the second supply means comprise an additional enclosure designed to receive a solvent containing urea, the additional enclosure containing urease enzymes capable of promoting the oxidation of urea to provide ammonium ions and carbon dioxide. 
     The present invention also provides a solution with an osmolarity which is variable according to the pH comprising a first substance having an amine function and a second substance having an aldehyde function. 
     According to an embodiment of the present invention, the first substance is urea and the second substance is vanillin. 
     According to an embodiment of the present invention, the second substance is a derivative of vanillin. 
     The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  show two steps of the operation of a first embodiment of a motor according to the present invention; 
         FIGS. 2A and 2B  show two steps of the operation of a variation of the first embodiment of the motor according to the present invention; 
         FIGS. 3A to 3C  show three steps of the operation of a second embodiment of the motor according to the present invention; 
         FIGS. 4A to 4C  show three steps of the operation of a third embodiment of the motor according to the present invention; 
         FIGS. 5A to 5D  show four steps of the operation of a fourth embodiment of the motor according to the present invention; 
         FIG. 6  shows a variation of the fourth embodiment; and 
         FIG. 7  shows a variation of the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIGS. 1A and 1B , an osmotic motor  10  according to the present invention comprises an enclosure  12  formed by a bundle of hollow fibers with semi-permeable walls  14 , for example of the type used for dialysis operations. The wall of fibers  14  has a given cut-off threshold, for example, on the order of 200 daltons, that is, it lets through particles having a molecular mass substantially smaller than 200 daltons, that is, than 200 g/mol. Each fiber has, for example, a diameter on the order of 200 μm. Fiber bundle  14  is maintained at a first end by a first junction ring  16 , for example, via a gluing area  18 . Ring  16  comprises an opening  20  closed by a plug  21 . The second end of fiber bundle  14  is maintained by a second junction ring  22 , for example, via a gluing area  24 . A membrane  25 , having a cut-off threshold on the order of 1,000 daltons, closes second ring  22 . 
     Enclosure  12  is attached at the level of second junction ring  22 , at one end of a cylindrical body  28  in which a mobile piston  30  can slide. Mobile piston  30  and the cylindrical body define an expansion chamber  31 . Return means  32 , for example, a spring, exert on piston  30  a pulling force tending to bring it back to its idle position. Cylindrical body  28  comprises a discharge valve  34  communicating with the outside of motor  10 . 
     A catalyst capable of promoting a reaction of synthesis of an osmotically-active substance X from a substance Y, substance Y having a molecular mass smaller than the molecular mass of substance X, is arranged within fibers  14 . The cut-off threshold of fibers  14  is set to prevent the passing of substance X and of the catalyst but allow the passing of substance Y, while the cut-off threshold of membrane  25  is set to allow the passing of substance X but to block the catalyst. In normal operation, motor  10  is placed in a surrounding medium comprising a solvent in which substance Y is dissolved. 
     An operating cycle of motor  10  is then carried out as follows. 
       FIG. 1A  shows motor  10  at the beginning of a cycle. Piston  30  is in its idle position, the volume of expansion chamber  31  being minimum, and discharge valve  34  is closed. The catalyst promotes the forming of substance X from substance Y, which tends to increase the osmotic pressure inside of fiber bundle  14 . The solvent of the surrounding medium penetrates into fibers  14  and into expansion chamber  31 , thus displacing piston  30 . The displacement of piston  30  extends spring  32 , thus enabling storage of mechanical energy. 
     In  FIG. 1B , expansion chamber  31  is shown in maximum expansion. Discharge valve  34  then opens. The pressure inside of expansion chamber  31  equalizes with the pressure of the surrounding medium. Spring  32  brings piston  30  back to its idle position by discharging, through discharge valve  34 , the solvent from expansion chamber  31  into the surrounding medium. The mechanical energy stored in spring  32  is thus recovered. Valve  34  is finally closed, thus ending the motor cycle. 
     Piston  32  may be connected to an external element to which mechanical energy is desired to be transmitted. 
     As an example, substance X is a glucose polymer comprising a high number of glucose molecules, for example, dextrane, and substance Y is a glucose oligomer (which, by definition, comprises a small number of monomer units) of minimum molecular mass equal to 342 g/mol. The catalyst can then be a dextransucrase enzyme, that is, an enzyme promoting the synthesis of dextrane from a glucose oligomer. 
     According to the first embodiment, expansion chamber  31  is formed by a cylindrical body in which a piston slides. According to the desired use of motor  10  according to the present invention, expansion chamber  31  may be formed differently. 
       FIGS. 2A and 2B  show an alternative structure of expansion chamber  31  of motor  10  of the first embodiment. According to this alternative, expansion chamber  31  corresponds to the space defined between an inner envelope  36  and an outer envelope  37 , like an innertube. Inner envelope  36  is deformable and expandable and surrounds a deformable body  38 . Outer envelope  37  is flexible and inextensible. It closes on inner envelope  36  and is connected to junction ring  22  of enclosure  12 . Discharge valve  34  is arranged on junction ring  22 . As an example, in a medical application of osmotic motor  10  according to the present invention, deformable body  38  may be the human heart and the envelopes may define expansion chambers  31  shaped as flanges surrounding the heart. 
     A cycle of motor  10  according to the variation of the first embodiment is the following. 
       FIG. 2A  shows motor  10  at the beginning of a cycle. The volume of expansion chamber  31  is minimum, deformable body  38  being in maximum expansion, which may correspond to a heart in diastole. Discharge valve  34  is then closed. The catalyst promotes the forming of substance X, which causes, by osmosis, the introduction of solvent into expansion chamber  31 . Inner envelope  36  deforms and compresses deformable body  38 . 
     In  FIG. 2B , deformable body  38  is in maximum compression, which may correspond to a heart in systole. On opening of discharge valve  34 , the solvent is discharged from expansion chamber  31 , enabling expansion of deformable body  38 , which ends the cycle. 
     According to another variation of the present invention, the expansion chamber is formed of a resilient envelope enclosing the fibers which are arranged, for example, in a spiral, the two junction rings being tight. On synthesis of the osmotically-active substance, the fibers tend to straighten and to deform the resilient envelope. A discharge valve is provided at the level of a junction ring. On opening of the valve, the pressure inside of the fibers decreases and the envelope tends to recover its initial shape. 
     According to another variation of the present invention, the enclosure may be connected to the expansion chamber by a flexible duct. This enables advantageously arranging the enclosure in a surrounding medium propitious for the supply of solvent in which substance Y is dissolved, and placing the expansion chamber in a location where the mechanical energy is desired to be available. In the case of a medical application, the enclosure may be arranged in a fatty tissue, or on the vascular network. In this last case, the fibers may be arranged to form a hollow tube, leaving at its center a cylindrical space enabling the flowing of a fluid such as blood. The junction rings may be toric and placed against the wall of a blood vessel. One of the toric junction rings communicates with the expansion chamber through the flexible duct which perforates the blood vessel. 
       FIGS. 3A to 3C  show a second embodiment of an osmotic motor according to the present invention. Motor  40  comprises the components of the motor of the first embodiment and the reference numerals associated therewith are kept. 
     Motor  40  comprises a first enclosure  12  of the previously-described type and a second enclosure  42 . Second enclosure  42  comprises a second fiber bundle  44  maintained at its ends by junction rings  45 ,  46  by means of gluing areas  47 ,  48 . Second enclosure  42  is attached on cylindrical body  28  at the level of valve  34 , by junction ring  45  which comprises a membrane  49  separating expansion chamber  31  from second fibers  44 . Second enclosure  42  communicates, at the level of ring  46  via a membrane  52 , with a tight deformable tank  54 . 
     A first catalyst C 1  promoting the forming of a substance Y from a substance X is arranged in first fiber bundle  14 , so that, from an elementary particle of substance X, more than one elementary particle of substance Y is generated. A second catalyst C 2  promoting the forming of substance Y from substance X is arranged in second fiber bundle  44 , so that, to generate one elementary particle of substance X, more than one elementary particle of substance Y is used. 
     According to an example, substance X is a glucose polymer comprising a high number of glucose molecules, for example, dextrane, and substance Y is a glucose oligomer. Second catalyst C 2  can then be a transucrase enzyme, that is, an enzyme which promotes the synthesis of dextrane from a glucose oligomer and first catalyst C 1  can then be a dextranase enzyme, that is, an enzyme which promotes the breakdown of dextrane into glucose or into glucose oligomers. 
     The walls of fiber bundle  14 ,  44  have a cut-off threshold lower than the molecular mass of substances X and Y. As an example, the cut-off threshold is on the order of 100 daltons when substance Y is glucose or a glucose oligomer and substance X is dextrane. Membranes  25 ,  49 , and  52  have cut-off thresholds greater than 1,000 daltons, to let through substances X and Y and maintain the catalysts within respective fiber bundles  14 ,  44 . 
     In normal operation, motor  40  is placed in a solvent. The operating cycle of osmotic motor  40  according to the present invention is the following. 
       FIG. 3A  shows motor  40  at the beginning of the cycle. Valve  34  is closed. The concentrations in substance Y are identical in tank  54  and in expansion chamber  31 , the same applying for the concentrations in substance X. In first fiber bundle  14 , catalyst C 1  promotes the forming of substance Y, which increases the osmotic pressure in expansion chamber  31 . The solvent penetrates into first fiber bundle  14 , then into expansion chamber  31 , thus moving piston  30  and storing mechanical energy by the extension of spring  32 . Meanwhile, in second fiber bundle  44 , catalyst C 2  promotes the forming of substance X, which decreases the osmotic pressure in tank  54 . Tank  54  decreases in volume, without causing any work capacity since nothing opposes this decrease. 
       FIG. 3B  shows motor  10  at the end of the previously-described step, expansion chamber  31  having a maximum volume. 
     Valve  34  then opens. The concentrations in substance X and in substance Y balance in fiber bundles  14 ,  44 , expansion chamber  31 , and tank  54 . Similarly, the osmotic pressures balance in the different compartments. Piston  30  then moves down under the action of spring  32  to reach the position shown in  FIG. 3C . Further, tank  54  expands by filling with liquid, the work required to expand tank  54  being negligible as compared with that provided by spring  32 , the pressures in the surrounding medium being low as compared with those present in expansion chamber  31 . Valve  34  is then closed, which ends the cycle. 
     The second embodiment is particularly advantageous since the exchanges between motor  40  and the surrounding medium are decreased with respect to the first embodiment. Indeed, in the first embodiment, osmotically-active substance X, for example, dextrane, is generated from substance Y, for example, a glucose oligomer, present in the solvent. Further, at the end of a motor cycle, discharge valve  34  is opened and the most part of the formed substance X is released in the surrounding medium. In the case of a medical application, the generated substance X, for example, dextrane, is released into the human body, which may be a problem. In the second embodiment, there only is a solvent transfer between motor  40  and the surrounding medium. 
       FIGS. 4A to 4C  show a third embodiment of osmotic motor  60  according to the present invention. Motor  60  comprises the components of motor  10  of the first embodiment and the references numerals associated therewith are kept. 
     Enclosure  12  is arranged in a tight deformable envelope  61  which closes on junction ring  22  and discharge valve  34 . Envelope  61  is filled with a solvent. Envelope  61  may be arranged in a perforated rigid case  64  so as not to prevent the deformations of envelope  61 . 
     A first catalyst C 1  promoting the forming of a substance Y (for example, a glucose oligomer) from a substance X (for example, dextrane), so that from one elementary particle of X, more than one elementary particle of Y are generated, is arranged in fiber bundle  14  of enclosure  12 . A second catalyst C 2  promoting the forming of substance X from substance Y so that, to generate one elementary particle of substance X, more than one elementary particle of substance Y are used, is arranged in envelope  61 . 
     The membranes of fiber bundle  14  have a cut-off threshold lower than the molecular mass of substances X and Y. As an example, the cut-off threshold is on the order of 100 daltons when substance Y is glucose or a glucose oligomer and substance X is dextrane. 
     An operating cycle of motor  60  according to the third embodiment is the following. 
       FIG. 4A  shows motor  60  at the beginning of a cycle. Valve  34  is closed. Envelope  61  is at its maximum volume. All compartments contain a solvent in which substances X and Y are dissolved. The concentrations in X are substantially balanced between envelope  61  and expansion chamber  31 . First catalyst C 1  promotes the forming of substance Y, which increases the osmotic pressure in bundle  14  and expansion chamber  31 . Second catalyst C 2  promotes the forming of substance X, thus decreasing the osmotic pressure within envelope  61 . Solvent exchanges occur, the solvent passing towards fiber bundle  14  and, from there, into expansion chamber  31 , causing a motion of piston  30 . The piston is in ascending phase. 
       FIG. 4B  shows motor  60  at the end of the previously-described step. The concentration in substance X is minimum inside of fiber bundle  14  and maximum inside of envelope  61 . Conversely, the concentration in substance Y is maximum in fiber bundle  14  and minimum in envelope  61 . 
     Discharge valve  34  is then opened, which directly connects up communication expansion chamber  31  and envelope  61 . The pressure in expansion chamber  31  drops and return spring  32  brings piston  30  back to its initial position, discharging the solvent from expansion chamber  31  into envelope  61 . The piston is said to be in descending phase. Fiber bundle  14  is thus connected up with the inside of envelope  61 . The concentrations in substances X and Y equalize between the two compartments. 
       FIG. 4C  shows motor  60  at the end of the descending phase of piston  30 . Valve  34  is then closed, which ends the cycle. 
     A variation of the third embodiment of the osmotic motor may be used as a motor to drive the wheels of an automobile vehicle. According to this variation, the spring is suppressed and the piston is connected, for example, by a rod, to a crankshaft for driving the wheels similarly to the connection between a piston of a heat engine and the crankshaft. The propelling force corresponds to the ascending phase of the piston, that is, to the expansion phase of the expansion chamber. In descending phase, when the volume of the expansion chamber decreases, the piston only encounters a small resistance, corresponding to the flowing of the solvent through the discharge valve of the expansion chamber. Advantageously, at least two osmotic motors may be placed in parallel to drive the crankshaft so that the expansion chambers of the motors work in opposition, one of them being in expansion phase when the other is in contraction phase. 
     Such a motor may further be used to recover energy on braking of the vehicle. In this case, it comprises an additional valve, called a supply valve, which is arranged between the first fiber bundle and the expansion chamber. On operation of the motor to drive the wheels, the supply valve is opened so that the motor operates as described previously. 
     In the case of a braking, when the piston is in rising phase, the supply valve is closed and the discharge valve is opened so that the expansion chamber fills up with liquid with no significant stress. A great part of the liquid contained in the deformable envelope flows into the expansion chamber without for the concentrations in the different compartments to vary. The supply valve is then opened and the discharge valve is closed. When the piston moves down under the action of the crankshaft driven by the wheels, the liquid contained in the expansion chamber is scoured through the fiber bundle and arrives into the envelope. The work thus generated enables both slowing down the vehicle, and varying the concentrations in substances X and Y. Indeed, the osmolarity of the fiber bundle increases, while the osmolarity in the deformable chamber decreases. 
     Thereby, when the motor operates again as a wheel driving motor, the supply valve being open, the motor cycle resumes with a greater efficiency. Indeed, the concentration differences between the compartments will create a pressure difference due to which the cycle will be carried out faster. 
     Further, enclosures  12  and  42  may be formed otherwise than by a fiber bundle. They may have any shape enabling good exchange of the solutes and of the solvent on either side of the enclosure wall. 
     Further, valve  34  may exhibit any type of known structure. The valve openings and closings may for example be controlled by a device external to the motor, synchronously or not, or be automatically triggered by the very structure of the valve when the pressure in a motor compartment exceeds a determined value. 
       FIGS. 5A to 5D  show a fourth embodiment of osmotic motor  80  according to the present invention. Motor  80  comprises some components of motor  60  of the third embodiment and the reference numerals associated therewith are kept. 
     As compared with motor  60 , enclosure  12  is suppressed. A valve  82  is arranged at the level of rigid case  64  and is capable of connecting up, when opened, the content of envelope  61  with the content of an enclosure  84 . A valve  86  is arranged at the level of rigid case  64  and is capable of connecting up, when opened, the content of envelope  61  with the content of an enclosure  88 . 
     Envelope  61  contains substances U and V, dissolved in a solvent and which are likely to react in the presence of H +  ions, that is, at a sufficiently acid pH, to form a substance Z. Ion H +  plays a role similar to that of a catalyst of the reaction of synthesis of substance Z. The back reaction according to which substance Z is decomposed to give back substances U and V is likely to occur in the presence of ions OH − , that is, at a sufficiently basic pH. Ion OH −  plays a role similar to that of a catalyst of the reaction of breakdown of substance Z. Membrane  25  has a sufficiently low cut-off threshold to block substances U and V which are thus osmotically active. Valves  82 ,  86  are associated with membranes, not shown, which have a cut-off threshold such that they block substances U and V. According to the fourth embodiment, enclosure  84  contains an acid solution and enclosure  88  contains a basic solution. 
     Substances U, V, and Z must be soluble in the solvent and if possible biocompatible in the case of a medical application. As an example, substance U is a molecule comprising a primary amine function (function —NH 2 ) and substance V is a molecule comprising an aldehyde function (function —CHO). Complex-forming reactions from an amine function substance U and an aldehyde function substance V are described in works: “Advanced Organic Chemistry: Reactions, Mechanisms and Structure”, third edition, J. March, John Wilez and Sons, New York (1985), pp. 796-798 and “Advanced Organic Chemistry, Part B: Reactions and Synthesis”, third edition, F. A. Carey and R. J. Sundberg, Plenum Press, New-York (1990), pp. 30-31. As an example, substance U may be urea of molecular mass 60 g/mol and substance V may be vanillin of molecular mass 152 g/mol. Membrane  25  then has a 50-dalton cut-off threshold. Substance Z corresponds to a urea-vanillin complex. In this case, the urea-vanillin complex forming reaction is promoted for a pH lower than 6 while the urea-vanillin complex forming reaction is promoted for a pH on the order of 8. According to another example, compound V is a derivate of vanillin having the following formula: 
                                
where R is a carbonaceous group (R corresponding to the hydroxyl group —OH in the case of vanillin).
 
     Expansion chamber  31  contains an osmotically active substance A, dissolved in the solvent. Membrane  25  has a sufficiently low cut-off threshold to block substance A. As an example, substance A is dextrane. 
     An operating cycle of motor  80  according to the fourth embodiment is the following. 
       FIG. 5A  shows motor  80  at the beginning of a cycle. Envelope  61  is at its maximum volume. The concentrations in substance U and V and the concentration in substance A are initially selected so that the osmotic pressures in expansion chamber  31  and in envelope  61  balance, piston  30  remaining motionless. 
     Valve  86  is closed and valve  82  is then open. Ions H +  spread into envelope  61 , causing a decrease in the pH. When the pH in envelope  61  has decreased down to the desired value, valve  82  is closed. The reaction of forming of substance Z from substances U and V is then promoted. The osmotic pressure in envelope  61  decreases with respect to the osmotic pressure in expansion chamber  31 , which does not vary. A solvent transfer thus occurs, solvent passing from envelope  61  into expansion chamber  31  through membrane  25 , causing the displacement of piston  30 . Piston  30  is in ascending phase. 
       FIG. 5B  shows motor  80  at the end of the ascending phase of piston  30 . 
     In  FIG. 5C , valve  86  has been opened to connect up the content of envelope  61  with the content of enclosure  88 . Ions OH −  spread into envelope  61 , causing a pH increase. When the pH is sufficiently basic, valve  86  is closed. The breakdown reaction of substance Z is then promoted, causing an increase in the number of osmotically active particles in envelope  61 . Since the osmotic pressure increases in envelope  61 , a new solvent transfer occurs, solvent passing from expansion chamber  31  to envelope  61  via membrane  25 . The action of spring  32  promotes the discharge of the solvent from the expansion chamber. However, spring  32  may be omitted, with the piston being moved by suction. Piston  30  is said to be in descending phase. 
       FIG. 5D  shows motor  80  at the end of the descending phase of piston  30 , which ends the cycle. The concentrations in substances U and V in envelope  61  are then substantially identical to the concentrations at the beginning of the cycle. 
     According to a variation of the fourth embodiment, membrane  25  is only impermeable to substance V but is permeable to substance U. In this case, only substances A, Z, and V are osmotically active. Substance U is free to diffuse through membrane  25  and is thus not osmotically active. In the case where substance U is urea and substance V is vanillin, membrane  25  for example has a cut-off threshold on the order of 100 daltons. 
     Substances U and V then have the additional feature that the forming of one particle of substance Z uses, at least in average, more than one particle of substance V. This is verified when substance U is urea and substance V is vanillin. Indeed, a molecule of the urea-vanillin complex is in average obtained from one urea molecule and more than one vanillin molecule (one or two vanillin molecules grafting on the urea according to the steric hindrance). Such a substitution reaction is described in previously mentioned reference “Advanced Organic Chemistry: Reactions, Mechanisms and Structure”, third edition, J. March, John Wilez and Sons, New York (1985), p. 798. 
     In this case, an operating cycle of motor  80  is the following. 
     At the beginning of a cycle ( FIG. 5A ), envelope  61  is at its maximum volume. The concentration in substance V and the concentration in substance A are initially selected so that the osmotic pressures in expansion chamber  31  and in envelope  61  balance, piston  30  remaining motionless. 
     Valve  86  is closed and valve  82  is then opened. Ions H +  spread into envelope  61 , causing a pH decrease in envelope  61 . The reaction of forming of substance Z from substances U and V is then promoted, substance U present in expansion chamber  31  passing into envelope  61  to take part in the reaction. Since the forming of one particle of substance Z requires more than one particle of substance V, the number of osmotically-active particles decreases in envelope  61  with respect to expansion chamber  31 . The osmotic pressure in envelope  61  thus decreases with respect to the osmotic pressure in expansion chamber  31 . A solvent transfer thus occurs, solvent passing from envelope  61  into expansion chamber  31  through membrane  25 , causing a motion of piston  30 . Piston  30  is in ascending phase. 
     When valve  86  is opened, valve  82  being closed ( FIG. 5C ), and the pH in envelope  61  becomes sufficiently basic to promote the breakdown reaction of substance Z, the number of osmotically-active particles in envelope  61  increases (even if part of substance U resulting from the breakdown diffuses into expansion chamber  31 ). As the osmotic pressure increases in envelope  61 , a new solvent transfer occurs, solvent passing from expansion chamber  31  into envelope  61  through membrane  25 . 
     In the previously-described examples, it is necessary to regularly refill enclosures  84 ,  88 , respectively, with acid and basic solutions. 
       FIG. 6  shows an alternative way to obtain the acid solution contained in enclosure  84  adapted to the case where enclosure  84  is bathed in a liquid surrounding medium in which glucose is dissolved, for example, a biological solvent. Enclosure  84  comprises a valve  90  capable of connecting up the content of enclosure  84  with the surrounding medium. According to such an alternative, the reaction of oxidation of glucose (noted R—COH) into gluconate (noted R—COO − ) according to the following reaction is promoted in enclosure  84 :
 
R—COH+½O 2 →RCOO − +H + 
 
     Such a reaction is made possible by providing in enclosure  84  a glucose oxidase enzyme. A membrane  92  between valve  90  and enclosure  84  and a membrane  94  between valve  82  and enclosure  64  are then provided, membranes  92 ,  94  enabling retaining the glucose oxidase enzyme, membrane  92  letting through glucose. 
     To obtain an acid solution in enclosure  84 , valve  82  is closed and valve  90  is opened so that glucose penetrates into enclosure  84 . Valve  90  is then closed. The glucose is then decomposed by generating H +  ions and gluconate. The solution contained in enclosure  84  thus becomes acid and can then be used in an operating cycle of motor  80 . At the next opening of valve  90 , gluconate diffuses outside enclosure  84 . In the case of a medical application, the releasing of gluconate into the human body is harmless since it is naturally discharged by the kidneys. 
       FIG. 6  shows an alternative way to obtain the acid solution contained in enclosure  84  adapted to the case where enclosure  84  is bathed in a liquid surrounding medium in which glucose is dissolved, for example, a biological solvent. 
       FIG. 7  shows an alternative way to obtain the basic solution contained in enclosure  88  adapted to the case where enclosure  88  is bathed in a liquid surrounding medium in which urea is dissolved, for example, a biological solvent. Enclosure  88  comprises a valve  100  capable of connecting up the content of enclosure  88  with the surrounding medium. According to such a variation, the breakdown reaction of urea into ammonia and carbonic acid according to the following reaction is provided in enclosure  88 : 
     
       
                 
         
             
             
         
      
     
     At the physiological pH, carbonic acid dissociates into water and carbon dioxide. The ammonia will balance with water to become the ammonium ion (NH 4   + ), thus resulting in a significant pH increase. The carbon dioxide will be naturally discharged by breathing. 
     Such a reaction is made possible by providing a urease enzyme in enclosure  88 . A membrane  102  is then provided between valve  100  and enclosure  88  and a membrane  104  is provided between valve  86  and enclosure  64 , membranes  102 ,  104  enabling retaining the urease enzyme, membrane  102  letting through the ammonia and the carbon dioxide. 
     According to another alternative embodiment, in the case of a medical application, substances U, V, and Z are such that the breakdown of substance Z which gives back substances U and V is promoted as soon as the pH is lightly basic, for example, on the order of 7.4. Enclosure  88  containing the basic solution and valve  86  are used to directly connect up the content of envelope  61  with the surrounding biological medium. Indeed, the human biological liquid naturally is at a slightly basic pH on the order of 7.4. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, for the second and third embodiments of the motor, the expansion chamber may be formed according to the described variations of the first embodiment. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.