Abstract:
The present invention is about an actuator comprising at least one shape-memory-alloy based converter in a housing and at least one preload spring. The actuator is configured to cause a motion of at least one movable member. The optimal structures along with corresponding methods for improving an actuator are claimed in the independent claims. Preferable embodiments are presented in the dependent claims.

Description:
PUBLIC FUNDING NOTICE 
       [0001]    This document may contain results from work funded by Tekes (The Finnish Funding Agency for Technology and Innovation), Aalto University, and Orton Foundation. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to construction and use of shape-memory-alloy based actuators. 
       BACKGROUND ART 
       [0003]    Actuators utilizing shape change of shape memory alloys or magnetostrictive materials are known from published patent applications WO 2009/115645 A1 and WO 2011/148047 A1. 
       OBJECTIVE OF THE INVENTION 
       [0004]    It is an objective to improve actuators utilizing shape change of shape memory alloys. 
         [0005]    This objective can be fulfilled with an actuator according to claim  1  or  12 , and with the method according to claim  13 . 
         [0006]    The dependent claims describe various advantageous aspects of the actuators. 
       ADVANTAGES OF THE INVENTION 
       [0007]    Actuator according to the invention comprises at least one shape-memory-alloy based converter in a housing and at least one preload spring. The actuator is configured to cause a motion of at least one movable member
       in a first direction, upon the at least one converter undergoing thermally induced phase transition which makes it contracted, and   in a second direction that is opposite to the first direction, upon the at least one converter undergoing phase transition caused by temperature change and enhanced by the preload spring which makes the converter elongated.       
 
         [0010]    Furthermore, the actuator comprises at least one restrictor configured to restrict the strain of the at least one converter caused by the preload spring. In the actuator, the at least one preload spring in its initial state is preloaded with a preloading force and configured to cause the at least one converter in its resting state a tensile force that is limited by the at least one restrictor in such a manner that the tensile force resulting from the tension in the at least one converter is smaller than the force exerted by the at least one preload spring to the converter during phase transition. 
         [0011]    According to a further aspect of the invention, an actuator comprises at least one shape-memory-alloy based converter configured to cause a motion of at least one movable member and located in a housing. Furthermore, the actuator comprises at least one preload spring that is configured to preload the at least one shape-memory-alloy based converter. The improvement comprises at least one restrictor that is configured to restrict strain of the at least one converter caused by the preload spring. 
         [0012]    With the actuators as suggested, the force that can be exerted by the movable member during its movement in the second direction can be increased significantly as compared to the force that is obtainable by the actuator that has no preloading. More precisely, with our actuator, during the movement in the second direction, the force that can be exerted by the movable member is not any more the force of the at least one shape-memory-alloy based converter but the force of the at least one preload spring. 
         [0013]    An advantage that may be obtainable with our actuators is that the amount of movement that can be caused by the actuator during the work phase can be made more predictable than in an actuator in which at least one shape-memory-alloy based converter and a preload spring but no restricting of the preload force exerted by the preload spring is employed, as suggested by our results shown in  FIG. 6 . This enables a much more simpler construction as compared with the one-degree-of-freedom positioning system proposed by Azfal Khan et al. in their poster SHAPE MEMORY ALLOY WIRES FOR ACTUATING POSITIONING SYSTEMS WITH ELASTIC BEARINGS, at the time of writing electronically retrievable under http://www.aspe.net/publications/Annual 2005/POSTERS/2EQUIP/3DTEST/1815.PDF. 
         [0014]    A further advantage we consider potentially very relevant for certain uses is that our actuators may be capable for more cycles before failure. The main purpose of the restrictor is to avoid such a high preloading force to damage the at least one shape-memory-alloy based converter. 
         [0015]    If the actuator further comprises at least one preloading compression-to-tension transformator connector for passing force exerted by the at least one preload spring to the converter, the configuration of the actuator can be carried out by a relatively simple mechanical structure. 
         [0016]    If in the actuator the at least one converter comprises at least one bundle of individual wires or rods made of or consisting shape-memory-alloy in such a manner that in the bundle the individual wires are electrically connected to each other in series or in parallel and mechanically arranged in parallel, we can ensure that the contractive force obtainable from the at least one converter via its phase transition is high enough since arranging wires or rods in parallel sums the contractive force of each individual wire or rod together. Optionally, if there are more than one bundle, the bundles may be connected to each other electrically in series. In this manner, we can ensure that the heating of all wires or rods in the bundle can be carried out simultaneously in order to avoid damaging the actuator by non-uniform deformation that could result in bending of the actuator, for example. In addition, the bundles may preferably be connected in parallel to increase force or to achieve desired form of movement. 
         [0017]    If the actuator further comprises at least two connectors electrically connected to said bundle for feeding electrical energy received from a power source, such as from a wirelessly switchable battery or through at least one inductive coil in said actuator to the bundle, the actuator can be used in a wireless manner. This is particularly important in such use situations of the actuator when the actuator is designed as an actuator of an orthopedic treatment device that is implemented in a patient for a treatment of several days, weeks or even months, since thanks to the wireless useability, the energy can be supplied in a wireless manner to the actuator and therefore the need to have open wounds in the patient during the treatment may be removed. 
         [0018]    If in the actuator comprising at least one bundle of wires or rods, the electrical and mechanical connection between the individual wires of the bundle is implemented within at least one connecting unit and wherein the restrictor is configured to restrict the tensile force of the bundle by restricting the movement of the at least one connecting unit or a connection to force transmission in the work direction of the at least one preloading spring, the at least one connecting unit can be used to absorb much or even all of the difference to cut the higher preload stress of the preload spring to a lower force that acts as the tensile force on the at least one converter. 
         [0019]    If in the actuator the preload spring is arranged around the converter in such a manner that contraction of the converter pulls the movable member compressing the preload spring, the actuator can be made smaller. Additionally to this, or as an alternative, it can be ensured in a relatively simple manner that the relative spatial location of the preload spring and converter to each other remains practically unchanged during the operation of the actuator. 
         [0020]    If the housing of the actuator is encapsulated in a biocompatible material or consists of biocompatible material, the actuator can be used as an actuator in medical appliances or similar that may come into connection with bodily fluids or bodily tissues. 
         [0021]    Preferably, the actuator is joined to a transformer transforming reciprocating motion of the movable member to a unidirectional motion. In this manner, the actuator can be used as the actuator of a device that converts the movement of the actuator to distracting movement (by extending movement or movement outwards) or to contracting movement (by shortening movement or movement outwards). Devices producing distracting or contracting movement are widely used in the field of medicine and in particular in orthopedics. 
         [0022]    Preferably, the shape-memory-alloy based converter is or comprises one or more NiTi elements. Particularly advantageously, the stress imposed on some or all NiTi elements by the preloading force is in the range of 250-450 MPa during actuation and the tensile stress caused on the NiTi elements in it&#39;s martensite state is 20-90 MPa. In this manner we can ensure that the forces that can be produced by the actuator are high enough for many practical purposes, in particular in the field of orthopedics and in particular for osteodistraction or scoliosis treatment devices, such as but not limited to devices used in connection with mandible, metacarpals, metatarsals, cranial vault, mid-face, long tubular bones or other bones. Within the presented range of the preloading forces and tensile stress, the strain of the NiTi elements is with a high probability better predictable. In addition, the actuator may have an extended lifetime i.e. it can be used to undergo more cycles under load. 
         [0023]    Preferably, the actuator is suitable for use as an actuator in an implantable treatment device fixed or interlinked to bone. 
         [0024]    The actuator may in particular comprise a plurality (i.e. at least two) actuators assembled next to each other in the housing. This kind of configuration is in particular suitable for a scoliosis treatment device or an internal osteodistraction device. 
         [0025]    Alternatively, the actuator may comprise a number of converters arranged around an axis. This kind of configuration is in particular suitable for a bone distraction actuator of an internal osteodistraction device. 
         [0026]    Another actuator according to the invention comprises at least one shape-memory-alloy based converter in a housing and at least one preload spring. The actuator is configured to cause a motion of at least one movable member
       in a first direction, upon the at least one converter undergoing thermally induced phase transition which makes it contracted, and   in a second direction that is opposite to the first direction, upon the at least one converter undergoing phase transition caused by temperature change and enhanced by the preload spring which makes the converter elongated.       
 
         [0029]    Furthermore, the actuator comprises rod connecting to the said movable member and to a spring. The spring may be a compression spring stack placed around the spring compression rod. The spring may also be a tension spring connecting to the end of the rod. In the actuator, the rod is pulled to the optimal prestress for the converter such that a fixation part can be added. The added fixation part holds the optimal prestress. 
         [0030]    The same embodiments as for the previous actuator can be utilized for this another actuator. 
     
    
     
       LIST OF DRAWINGS 
         [0031]    In the following, the actuators are described in more detail by reference to the examples shown in the attached drawings in  FIGS. 1 to 28 , in which: 
           [0032]      FIG. 1  illustrates the concept of actuator based on the testing system presented; 
           [0033]      FIG. 2  illustrates theoretical stress-strain behaviour of actuator; 
           [0034]      FIG. 3  schematic of the testing system and the samples used; 
           [0035]      FIGS. 4(   a ) and ( b ) illustrate the stress-strain behaviour of the NiTi wire in the test system, (a) shows the test at 30 MPa and (b) the test at 69 MPa, the higher stress level on each curve is recorded during heating and the lower level during cooling; 
           [0036]      FIG. 5  illustrates strain behaviour of NiTi during actuation at a prestress of 69 MPa and stress level of 250 MPa, the solid line is the mean of the actuation cycles of the test and the dotted represent standard deviations of the test; 
           [0037]      FIG. 6  illustrates the strain versus cycle number under a load of 300 MPa; 
           [0038]      FIGS. 7(   a ) and ( b ) illustrate the strain behaviour of the NiTi elements under different load conditions: (a) shows the performance under a prestress of 30 MPa and (b) under a prestress of 69 MPa; 
           [0039]      FIGS. 8(   a ) and ( b ) in (a) is the achieved net stress illustrated as a function of the achieved strain and in (b) the achieved net stress is illustrated as a function of the achieved fatigue life for all of the tested stress-prestress combinations: the black symbols denote a prestress of 69 MPa and the open symbols a prestress of 30 MPa; 
           [0040]      FIG. 9  schematic drawing of a second embodiment of the actuator presented; 
           [0041]      FIG. 10  schematic drawing of the first embodiment of the actuator presented in  FIG. 1 ; 
           [0042]      FIG. 11  a section of a first actuator according to the second embodiment; 
           [0043]      FIG. 12  a section of a second actuator according to the second embodiment; 
           [0044]      FIG. 13  a section of a second actuator according to the second embodiment in a contracted state; 
           [0045]      FIG. 14  a section the actuator according to the first embodiment shown in  FIGS. 1 and 10 ; 
           [0046]      FIGS. 15-17  illustrate the cycle of an actuator according to the second embodiment, from initial phase of the actuator, through movement in the first direction to the contracted phase and through movement in the second direction back to the initial phase; 
           [0047]      FIG. 18  illustrates certain components of an actuator according to a third embodiment that is an actuator that comprises at least two actuators assembled next to each other in the housing; 
           [0048]      FIG. 19  illustrates certain components of an actuator according to a fourth embodiment that is an actuator that comprises a number of converters arranged around an axis; 
           [0049]      FIGS. 20 ,  23  and  24  illustrate a bone distraction actuator; 
           [0050]      FIG. 21  section A-A of the bone distraction actuator; 
           [0051]      FIG. 22  is a zoomed view C of spring stack end of the bone distraction actuator as illustrated in  FIG. 23 ; and 
           [0052]      FIG. 25  illustrates sections D-D and E-E at locations illustrated in  FIG. 24 . 
           [0053]      FIG. 26  illustrates the parts of the actuator 
           [0054]      FIG. 27(   a ) illustrates general view of the actuator 
           [0055]      FIGS. 27(   b ) and ( c ) illustrate the section views of the critical points of the actuator of  FIG. 27(   a ). 
           [0056]      FIG. 28  illustrates the positions of the actuator parts 
       
    
    
       [0057]    The same reference numerals refer to the same technical features in all drawings. 
       DETAILED DESCRIPTION 
     1. INTRODUCTION 
       [0058]    We have invented a shape memory alloy (SMA) actuator that produces a predictable output performance and tested it. 
         [0059]    There are several application areas for SMA actuators with tight space limitations, where the control of the actuator by utilizing sensors is difficult to realize. 
         [0060]    The test system for the actuator concept allowed the performance evaluation of NiTi against a constant load with different prestresses. Commercially available NiTi elements, Flexinol® (trade mark of Dynalloy, Inc.) wires, were tested in this system against high constant load levels of 250, 300, 350 and 400 MPa at two different prestress values 30 MPa and 69 MPa. The strain output and fatigue life of the NiTi wires under these conditions were measured. Increasing the stress level was found to decrease the fatigue life as expected. In addition, increasing the prestress from 30 MPa to 69 MPa improved the strain output at all stress levels. We found out that different stress-prestress combinations can lead to the same net output force from the material but their maximum strain output and fatigue life are different. According to the results, the actuator concept is feasible and can be realized with predictable output performance. 
         [0061]    Shape memory alloys are used in a wide variety of applications ranging from medical devices and implants (Aalsma 1997, Ryhänen 1999) to aerospace applications (Chau 2006) and robotics (Kheirikhah 2011). Most of these applications utilize binary nickel-titanium alloys because of their superior mechanical and shape memory properties. Generally, in typical actuator applications, nickel-titanium (NiTi) elements are designed to last for hundreds of thousands of cycles, which in turn limit the stress level to under 200 MPa (Dynalloy 2012, Mertmann 2009). 
         [0062]    Fumagalli et al. (2009) presented the common reset mechanisms used in SMA actuator applications. In all of these reset mechanisms, the NiTi element works directly against the external load. Similar designs have been proposed by others (Aalsma 1998, Elwaleed 2007, Kim 2008). However the strain and fatigue life performance of NiTi has been shown to depend greatly on the stress imposed on it (Lagoudas 2009, Mammano and Dragoni 2012, Bertacchini 2009). This leads to unpredictable actuation behaviour, and accurate control requires sensors and feedback loops. 
         [0063]    NiTi alloys are particularly lucrative for use in medical implants due to their established biocompatibility (Ryhänen 1999, Shabalovskaya 2002) and high power density (Reynaerts 1998). In many of these applications, for example in the field of orthopaedics, large forces are needed but the space is limited. The space limitations also limit the possibility of using sensors for actuation control. Therefore, it would be a great advantage if an actuator could be constructed in such a way that behaves predictably under various loading conditions. In addition, the amount of actuation cycles needed in orthopaedic applications are usually fairly low. This allows the utilization of SMAs at high stress levels, which has not been extensively studied. 
         [0064]    Shape memory alloys are also introduced in Dahlgren 2009. In the application, the NiTi elements are movable on each pulse and change their position in the implant. This means that the external load exerted on the NiTi elements is not well known and will change according to the location of the elements. 
         [0065]    Additionally, a device for moving two bodies relative to each other is presented in Soubeiran 2003. The application utilizes a spring. The power of the device is reduced by increasing the number of actuations, as the compression of the spring decreases. This would, in practice, lead to the device to become stuck. It doesn&#39;t give any hint about utilizing the shape memory alloys or any other intelligent materials. 
         [0066]    Helsinki University of Technology 2009 introduces the shape memory alloy but not the application of it. In the application, the spring is only the preload spring of the magnetostrictive material and it does not apply any external work. 
         [0067]    Further, the publication Olympus Corp. 2012 introduces an actuator that a shape memory alloy wire, that contracts when heated and expands when cooled. The device consists of a hollow member, a movable element, an elastic member and an insulation member. The shape memory alloy wire is prevented from being in electrical contact with the elastic member. Thereby, the NiTi element is isolated from the preload spring. However, the isolation members also carry the load, which in practice would cause problems as the typical materials are not strong enough. 
         [0068]    In this communication, an actuator concept that utilizes NiTi in generating a predictable strain and force output is presented. The concept is evaluated using a testing system that simulates the behaviour of NiTi in loading conditions similar to that of the proposed actuator. The performance of the NiTi actuator wires is evaluated under various high stress levels with two different prestress values. Particularly the evolution of the strain in the martensitic and austenitic phase and the fatigue life of the material are studied. These results can be utilized to realize the proposed actuator concept and to optimize it for various stress levels. 
         [0069]    It is still another advantage of the present actuator concept, that the NiTi elements stay in one position inside the implant, and thereby, the external loads can be minimized because the knowledge of the exact position can be used to direct the loads, e.g. bending, away from this area. The advantages are gained by stabilizing the external work of the device, by utilizing small isolative members that are free of loads. 
         [0070]    Horst et.al. 2013 discusses the medical device including an artificial contractile structure having at least two contractile elements adapted to contract an organ. The NiTi element of the publication is used for squeezing a tubular body part. The publication does not give any hint about separating the ends of the NiTi element such that the compression of the NiTi element would cause device pats diverging from each other. 
         [0071]    Belson 2013 discusses an apparatus an a method for endoscopic colectomy.The publication does not give any hint for utilizing a preload spring or transferring its load to a NiTi element. The NiTi element is used for turning around the endoscopic end of the device. It does not give any hint for diverging device parts from each other. 
         [0072]    It is an advantage of the another actuator of the invention that the NiTi elements can be used for separating or diverging the device parts from each other. 
       2. MATERIALS AND METHODS 
       [0073]    2.1. Actuator Concept 
         [0074]    A schematic representation of actuator  10  that actuates against a constant and known load is presented in  FIG. 1 . The actuator comprises a NiTi element, a spring, and the housing (a tube, for example). The spring is preloaded to the stress level that the actuator is expected to produce. The NiTi element has to be fitted into the tube in such a way that its elongation in the martensitic phase causes the desired prestress to be exerted on it. Now when the NiTi element starts to elongate it will do work against the predetermined spring load. When the NiTi is allowed to cool, the spring returns it to the original position while a net force output is generated. The net force output can be defined as, 
         [0000]        F   net =(σ s −σ p ) A    (1)
 
         [0000]    where σ s  is the stress imposed on the NiTi by the spring when actuating, σ p  is the stress acting on the NiTi in the martensitic phase, and A is the area of the NiTi element. 
         [0075]    The required martensitic strain to cause the desired prestress in the NiTi element can be determined by using Hookes law, 
         [0000]      ε M =σ p   /E    (2)
 
         [0000]    where ε M  is the strain required to cause the prestress and E is the Young&#39;s modulus of NiTi in the martensitic phase. The shape memory strain of the actuator is calculated from the martensitic strain, ε M , and austenitic strain, εA. The shape memory strain is 
         [0000]      ε SME =ε M −ε A    (3)
 
         [0076]    The theoretical stress-strain output of actuator  10  is shown in  FIG. 2 . When actuator  10  is being assembled, the strain in the martensitic phase is determined by Equation (2). When actuator  10  starts to actuate, the stress jumps straight to the stress level σ S . At this point actuator  10  starts to generate a strain output, and the maximum stress σ m  at least one NiTi element  11  works against, is determined by the preloaded stress of preload spring  12  and its spring factor k: 
         [0000]      σ m =σ S   +kε   SME   /A    (4)
 
         [0077]    Once the at least one NiTi element  11  is allowed to cool, the energy stored in the preload spring  12  during heating is released and strain and force are generated from actuator output that is configured as movable member  13  (cf.  FIG. 14 ) and/or that can be transmitted through openening  1113  for movable member. 
         [0078]    An advantage of an actuator  10  is that the at least one NiTi element  11  always actuates against a known load. As long as the load defined by preload spring  12  selected is not exceeded, actuator  10  performs in a predictable way. However, because the prestress exerted on the at least one NiTi element  11  is determined by the attachment of the at least one NiTi element  11  in the actuator  10 , it is important to know the evolution of the plastic strain in the martensitic phase. If an excess amount of plastic strain accumulates, the prestress imposed on the element changes and leads into changes in performance. 
         [0079]    2.2. Evaluation of the Actuator Concept 
         [0080]    In order to evaluate the feasibility of the actuator concept, a testing system  30  was designed and built. The NiTi wires  302  used were commercial grade Flexinol® acquired from Dynalloy, Inc. The nominal diameter of the NiTi wires  302  was 0.381 mm. The ends of the NiTi wires  302  were press-fitted with stainless steel barrel crimps  301  to allow easy mounting to the testing system. The length of the NiTi wire  302  between the crimps  301  was set to 20.0±0.1 mm. A schematic illustration of sample  300  is shown in the lower part of  FIG. 3 . 
         [0081]    The test system  30  is shown in  FIG. 3 . The right side shows preload spring  12 , having a small spring constant, inside housing  14  (preferably implemented as stainless steel tube). Rod  31  goes through preload spring  12  and is attached to sample  300 , as shown on the left in  FIG. 3 . The other end of rod  31  is attached to steel block  17  that compresses the spring against the left side of the housing  14 . Prior to attaching sample  300  to test system  30 , the preload spring  12  was tightened to the desired level by using a material testing machine (MTS® 858 Table Top System, equipped with FlexTest® controller and MTS TestSuite™ Multi Purpose Elite software, all trade marks of MTS Systems Corporation). At the desired stress level, the preload spring  12  locking clamp  34  was tightened in place to hold the preload spring  12  at the load. In this way, the NiTi wire  302  was subjected to the full stress as soon as it started actuating as in the proposed actuator concept. 
         [0082]    After the preloading of the spring, the NiTi wire  302  was attached to the preload spring  12  compression rod  31 . At this point, sample  300  holding block  39  shown on the left in  FIG. 3  was attached to the load cell connection  32  of the material testing machine and the load cell value zeroed. After the zero adjustment of the load cell, the sample  300  was attached to sample holding block  39  and the extensometer  37  was secured in place to record the strain. 
         [0083]    Electric current was used to resistively heat the NiTi wire  302 . Prior to activation of the current, the NiTi wire  302  was prestressed to a desired prestress value by using the hydraulic actuator of the material testing machine via the connection  35  to the hydraulic machine, to pull the NiTi wire  302 . After the desired prestress value was achieved, the hydraulic actuator was held in place in displacement control mode while the current was applied and NiTi wire  302  started to actuate against the load of preload spring  12 . 
         [0084]    Actuation was achieved by supplying an electric current of 2.25 to 3.5 A for a duration of 2.25 to 5 seconds to NiTi wire  302 . This was achieved using a power source in current limited voltage control mode, which was coupled to the NiTi wire  302  through a solid-state-relay. The operation of this relay was controlled by a program built for material testing machine. The optimal current and actuation time were selected in order to achieve as complete a transformation as possible into the austenite phase. This was done by observing the strain behaviour of the NiTi wire  302 ; and as soon as the strain started levelling to saturation, the current was switched off. No direct measurement of the temperature of the NiTi wire  302  was conducted. After the actuation, the samples  300  were cooled down by using forced air convection. This cycle of prestress-actuation-cooling was repeated until the samples failed. 
         [0085]    Full force and strain data were recorded at 102.4 Hz from every tenth cycle. The maximum and minimum values of force and strain were collected from the remaining cycles. The strain was not corrected for strain caused by the stress or by the thermal expansion due to their limited effect compared to the shape memory effect. 
       3. RESULTS 
       [0086]      FIG. 4  shows stress-strain curves for NiTi wire  302  samples at different stress and prestress levels. It is evident that the test setup behaves as desired and NiTi wire  302  sample is subjected to the full stress almost immediately at the start of the actuation. However the raise in the stress is not instantaneous, as shown in the theoretical performance shown in  FIG. 2 . This is due to the spring locking clamp  34  also acting as a spring, although with a very high spring coefficient. As expected, the stress level is not fully constant during actuation, but rather increases according to the spring constant of the load spring, as described in Equation 4. The area inside the stress-strain loop at the top in  FIG. 4(   a ) shows that there is some friction in the test system that causes the stress level during heating and cooling to be slightly different. 
         [0087]    However, in most of the tests conducted, the friction caused by the test system itself is minimal. The results show that lower stress levels, in general, lead to higher achieved strains. On all stress levels, the NiTi wires  302  were able to achieve higher strains at the 69 MPa prestress. 
         [0088]      FIG. 5  shows the typical strain behaviour of a NiTi wire  302  under actuation.  FIG. 5  shows the mean of the actuation cycles and the standard deviation at a 95% confidence level. The width of the standard deviation is due to the diminishing strain as the material is cycled. 
         [0089]    The strain accumulates rapidly at the beginning of the actuation but slows down when full transformation is approached. 80% of the maximum strain is achieved during the first 0.7 seconds of the 2.5 s actuation. The following 1.8 seconds (or 72%) of the actuation time only causes an additional 20% of strain. Therefore, the energy efficiency of the material can be improved by sacrificing some of the maximum strain. It has also been confirmed in multiple studies that the lifetime of SMA alloys increases if the material is only partially transformed (Lagoudas et al. 2009). 
         [0090]      FIG. 6  shows the strain of the samples as a function of the cycle number under a stress level of 300 MPa. Increasing the nominal prestress from 30 MPa to 69 MPa increases the strain significantly. Similar behaviour was observable in all of the tested stress levels. However, it seems that increasing the prestress lowers the maximum amount of cycles before failure. More tests would be needed in order to confirm this behaviour, as no repeated test runs were performed in this study. The net force output is smaller at a higher prestress due to the fact that a greater portion of the force is required to overcome the prestress. 
         [0091]      FIGS. 7(   a ) and ( b ) show the effect of increasing the stress level the material has to work against while keeping the prestress constant. It seems that increasing the stress level NiTi wire  302  has to work against decreases the achieved shape memory strain. This becomes more clearly visible in  FIG. 7(   b ). In fact, in the tests shown in  FIG. 7(   a ), the 250 MPa stress gave lower strains than the test at 310 MPa. The amount of actuation cycles the material can exhibit before failure also seems to decrease with increasing stress. The test of 350 MPa at a prestress of 69 MPa differs from this trend. Further tests are needed to conclude how the increasing stress affects fatigue life. 
         [0092]    It is evident that the maximum achievable strain diminishes quickly during the first actuation cycles. The effect is more pronounced at higher stress levels. However, between 1000 and 2000 cycles, the achievable strain stabilizes and the speed at which strain capability is lost decreases. From this point on, the material exhibits a stable, although slowly decreasing, strain until failure. 
         [0093]    Table 1 summarizes all the results in this study. The values in Table 1 have been calculated from the points marked by symbols in  FIG. 7 . The first and second point on each curve represents the points used for the calculation of the shape memory strain diminishment in the beginning of the actuation cycles. Points three and four are used for the calculation of the diminishment of strain, dε sme , in the stable region, and the last point is also the strain at failure. Again it seems that the number of cycles the material can withstand decreases as the stress level is increased. Increasing the prestress has a similar effect. Similarly, the fast drop of strain at the beginning of the actuation cycles and its increase at higher stress levels can be seen. At all stress levels, the strain increases when going from 30 MPa to 69 MPa prestress. The rate at which strain is lost after achieving the stable strain region at 2000 cycles is negligible and the behaviour of the material remains predictable until failure. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Performance data of the NiTi samples 
               
             
          
           
               
                   
                   
                 Net 
                   
                 ε SME  lost at 
                 ε SME  at 
                   
               
               
                 Stress 
                 Prestress 
                 stress 
                 Max 
                 2000 cycles 
                 failure 
                 dε SME   
               
               
                 (MPa) 
                 (MPa) 
                 (MPa) 
                 cycles 
                 (%-p) 
                 (%) 
                 (%-p) 
               
               
                   
               
               
                 235 
                 68 
                 160 
                 6151 
                 0.37 
                 4.63 
                 0.06 
               
               
                 249 
                 30 
                 215 
                 7701 
                 0.48 
                 2.83 
                 0.07 
               
               
                 305 
                 27 
                 276 
                 7091 
                 0.65 
                 3.13 
                 0.10 
               
               
                 316 
                 67 
                 195 
                 6481 
                 0.74 
                 3.65 
                 0.11 
               
               
                 349 
                 27 
                 324 
                 7371 
                 0.67 
                 2.67 
                 0.08 
               
               
                 350 
                 65 
                 282 
                 7601 
                 0.84 
                 3.14 
                 0.11 
               
               
                 379 
                 67 
                 310 
                 4961 
                 0.98 
                 3.01 
                 0.10 
               
               
                 401 
                 29 
                 342 
                 6011 
                 1.04 
                 2.30 
                 0.10 
               
               
                   
               
             
          
         
       
     
         [0094]      FIG. 8(   a ) shows the achieved strain and net stress for the different stress and prestress levels. The correlation between net output stress and strain is not straightforward. It depends greatly on the combination of stress and prestress that results in the net output. For example, an approximate net output of 320 MPa can be achieved by using two combinations from the conducted tests. When using a prestress of 27 MPa and a stress level of 349 MPa, the strain produced is 2.7%. Nearly the same output stress can be achieved with a prestress of 67 MPa and a stress level of 379 MPa, but in this case, a strain of 3% is produced. 
         [0095]    The effect the correct combination is even more pronounced when comparing the two combinations that produce a net stress output of roughly 200 MPa. The combination with the lower prestress yields a strain of under 3% while the combination with the 69 MPa prestress yields a strain of almost 4%. Then again at the 270 MPa net stress level, no significant difference can be made between the combination with lower prestress and the combination with higher prestress. On the other hand,  FIG. 8(   b ) shows that the higher stress-prestress combinations lead to lower fatigue life. 
       4. DISCUSSION 
       [0096]    The testing system presented in this work depicts a feasible way to test the performance of NiTi wire  302  under constant load while allowing the adjustment of prestress as an independent variable. Therefore, the testing system  30  can be used to simulate the performance of the actuator concept proposed earlier under various possible configurations. Further improvements to the testing system  30  may be made to minimize the generation of friction in the load spring assembly. All samples  300  failed with a fracture at the free length of the NiTi wires  302 , which indicates that the sample fixation worked as intended and no excessive stress was generated at the crimping sites. 
         [0097]    The results obtained with the testing system  30  can be compared to published studies that evaluate the fatigue life of NiTi under constant load. Even though the alloy studied by Lagoudas et al. (2009) was NiTiCu instead of binary NiTi, the comparison can be made. Their results showed a similar evolution of the strain as the material is cycled. Especially, the sharp decline of the strain at the beginning of cycling is evident. Their results also showed that increasing the stress level leads to reduced fatigue life of the samples. 
         [0098]    Comparing the results to those of Mammano and Dragoni (2012), who studied the functional fatigue of NiTi under various loading conditions, their constant stress cycling test does not show as sharp a drop in the beginning of the test. Their results, however, confirm that increasing the stress the NiTi element is subjected to decreases the fatigue life of the samples. At a stress level of 200 MPa, Mammano and Dragoni observed a fatigue life of 3,509-4,940 cycles, which is similar to the present results at a stress level of 400 MPa. However, in their case, the full stress of 200 MPa also acted as a prestress to the material, and in the present study the prestress was adjusted separately. This supports the hypothesis that increasing the prestress the NiTi is subjected to decreases the fatigue life. 
         [0099]    The results of this study have several implications for realizing the actuator as discussed in more detail in the following. Applications in which the heating efficiency is important, for example in medical implant devices, the use of partial transformation is preferred. Heating the material until full transformation is achieved is inefficient because the accumulation of strain slows considerably when approaching the point of full transformation. This leads to generation of excess heat while achieving a small change in the maximum strain. As pointed out earlier, partial transformation has also been found to increase the fatigue life of the material. 
         [0100]    The rapid decrease of strain during the beginning of actuation cycles on a new NiTi element would lead to poor predictability of actuator behaviour unless it is taken into account. When building an actuator, especially if high stress levels are employed, it is beneficial to age the elements by cycling before use. After the initial sharp drop in the achieved strain, the material behaves predictably. This is especially important in constructing an actuator according to the concept presented, as the accumulation of plastic martensitic strain would lead to a change in the prestress the NiTi is subjected to. If the accumulation of this plastic strain is sufficiently large, the actuator will seize working. 
         [0101]    There are many combinations of prestress and stress levels that can achieve the same force output from NiTi. It is clear that the selection of the combination affects the achieved performance. From the tested prestress values of 69 MPa and 30 MPa, the 69 MPa prestress led to higher strain output at all stress levels. It is possible to reach higher strains at a certain output force level by utilizing a higher stress and prestress. However, at the same time, the maximum cycle life of the material has to be sacrificed. 
         [0102]    Further studies may be necessary to test the performance of the NiTi at different stress and prestress values in order to find the optimal stress-prestress combinations for different applications. 
       5. CONCLUSIONS 
       [0103]    We have implemented a testing system  30  that allows the testing of NiTi wire  302  under constant stress with the free adjustment of prestress as an independent variable. Then we have used testing system  30  to study the performance of NiTi wires  302  under high stress levels from 250 MPa to 400 MPa at two different prestress values of 30 MPa and 69 MPa. From the results, we conclude that 69 MPa prestress leads to higher strains at all stress levels compared to the prestress of 30 MPa. In addition, we can show that utilizing a partial transformation may increase the energy efficiency of the material if some of the maximum strain can be sacrificed. We have also found that in order to generate a predefined force output from the material, several different stress-prestress combinations can be used. The selection of the combination has profound implications on the achieved maximum strain and fatigue life. Further studies may be carried out to find the optimal combinations for different strain and fatigue life requirements. 
         [0104]    The actuator concept presented is beneficial in many applications, where a high force output and predictable strain output are needed. The test results show that NiTi could be utilized in the manner the concept describes. However, in order to fully evaluate the concept, the actuator should be realized and tested separately. This is mainly due to the fact that the proposed concept does not allow the adjusting of the prestress during testing, unlike the testing system used in this work. 
       Exemplary Embodiments of our Actuators 
     1) First Embodiment (FIGS.  1 ,  10  and  14 ) 
       [0105]    Certain details of actuator  10  illustrated in section in  FIG. 1  and  FIG. 14  and schematically in  FIG. 10  have already been discussed above. NiTi element  11  comprising a crimps  161 ,  162  at both of its ends is arranged to run through a preload spring  12 . Crimps  161 ,  162  may be pressed around the NiTi element  11 , or they can be integrally formed, especially by suitable processing, e.g. by turning or milling or by using a suitable chip removing process. 
         [0106]    The preload spring  12  acts as stress-to-tension transformator  18 . Preload spring  12  and NiTi element  11  are confined in housing  14  having as actuator output one or at least one movable member  13 . In  FIG. 1  only opening  1113  for movable member  13  is shown, the movable member  13  is in place in  FIG. 14 . 
         [0107]    Instead of NiTi element  11  that is preferaby of Flexinol® but that in general may be replaced by one or more converters of material that exhibits shape-memory-alloy performance, but here we discuss our actuator  10  with the help of the exemplary NiTi element  11  for the sake of clarity. 
         [0108]    NiTi element  11  configured to cause a motion of the at least one movable member  13  in a first direction, upon NiTi element  11  undergoing thermally induced phase transition from martensite to austenite state which makes it contracted, and in a second direction that is opposite to the first direction, upon NiTi element  11  undergoing phase transition from austenite to martensite state caused by temperature change and enhanced by preload spring  12  which makes NiTi element  11  elongated. 
         [0109]    It must be understood that housing  14  of actuator  10  acts as restrictor that has been configured to restrict the movement of NiTi element  11 . 
         [0110]    Preload spring  12  in its initial state is preloaded with a preloading force that preferably lies in the range of 250 to 450 MPa (in particular in the range between 300 and 350 MPa) during actuation and the tensile stress caused on the NiTi elements in its martensite state is in the range from 20 to 90 MPa (in particular in the range between 67 to 71 MPa), and limited by the housing  14  (more generally, any restricting arrangement could be used) in such a manner that the tension resulting from the tensile force in the NiTi element  11  is smaller than the preloading force of the preload spring  12 . 
       2) Second Embodiment (FIGS.  9 ,  11 ,  12 ,  13 , and  15  to  17 ) 
       [0111]      FIG. 9  is a schematic drawing of actuator  110 ,  120  according to the second embodiment of our actuator. 
         [0112]      FIG. 11  shows a section of a first actuator  110  according to the second embodiment and  FIG. 12  a section of a second actuator  120  according to the second embodiment.  FIG. 13  shows a section of a further actuator  130  and illustrates how the restrictor can be located in such a manner that it limits the movement of the spring and not the movement of the NiTi element  11 . 
         [0113]      FIGS. 15 to 17  illustrate the cycle from initial phase of the actuator  120 , through movement in the first direction to the contracted phase and through movement in the second direction back to the initial phase. It should be understood that the movement cycle of actuator  110  is same to that of actuator  120 . 
         [0114]    In  FIG. 11  we see that actuator  110  comprises NiTI element  11  that is confined in housing  14 . It should be understood that housing  14  continues beyond the first crimp  161  but that part of the housing  14  has been omitted to improve clarity. 
         [0115]    Actuator  110  has in the housing  14 , around NiTi element  11  a pre-tensioning tube  112  that is connected to restrictor  111  that works as inhibiting unit. 
         [0116]    Actuator  110  further comprises a movable member  13  that is configured to work as preloading compression-to-tension transformator arranged to transform the preloading force of the preload spring  12  to tensile force of the NiTi element  11 . As movable member  13  we have used a bar that extends through preload spring  12 . 
         [0117]    We see in  FIGS. 11 and 12  that while in the actuator  10  according to the first embodiment the preload spring  12  was configured to transform the preloading force of the preload spring  12  to tensile force of the NiTi element  11 , in actuator  110 ,  120  the transformation is carried out by the preloading compression-to-tension transformator (movable member  13 ) that connects the preload spring  12  and crimp  162  of NiTi element  11 . 
         [0118]    We also see in  FIGS. 11 and 12  that while in the actuator  10  according to the first embodiment the housing  14  was configured to restrict the elongation of the NiTi element  11  and in this manner to restrict the tension caused by the tensile force, in actuator  110 ,  120  the restriction is implemented by restrictor  111  or by restrictor  121  that is an intermediate wall. 
         [0119]      FIG. 13  illustrates actuator  130 . Now instead of using intermediate wall  1319  as restrictor, the end of housing  14  acts as restrictor  19 . In other words, restrictor  19  is located in such a manner that it limits the movement of spring  12  and not the movement of the NiTi element  11 . 
         [0120]    In  FIG. 15 , actuator  120  is shown in its initial state. This means that preload spring  12  is also in its initial state. 
         [0121]      FIG. 16  illustrates actuator  120  after the first phase transition of NiTi element  11 , i.e. after movable member  13  has been pulled in a first direction. Between  FIGS. 15 and 16  the phase transition of NiTi element  11  has occurred: NiTi element  11  has been heated, which has caused a thermally induced phase transition from martensite to austenite state which has made NiTi element  11  contracted. 
         [0122]    NiTi element  11  has contracted by distance Δk and movable member  13  has been displaced by distance Δm. Usually, Δk=Δm but this is not necessary. If Δk≠Δm this relate from load conditions of the actuator  120  to which movable member  13  may be exposed. 
         [0123]    In  FIG. 17  actuator  120  has returned from state illustrated in  FIG. 16  to its initial state. This means that the movable member  13  has been pushed in a second direction that is opposite to the first direction. NiTi element  11  has undergone phase transition from austenite to martensite state caused by temperature change and enhanced by preload spring  12  making NiTi element  11  elongated. 
         [0124]    We see that the work done by actuator  120  through movable member  13  in the second transition (i.e. between  FIGS. 16 and 17 ) is, under load conditions, done by movable member  13  exerting force that is at the beginning effectively the preloading force of preload spring  12  minus tension force of NiTi element  11 . During the transition, the preloading force is slightly reduced (depending on Δm at each instance) and the tension force is also slightly reduced (depending on Δk at each instance). 
         [0125]    By suitably selecting the preloading force and tension force, basically any desired force output from actuator  10 ,  110 ,  120  may be obtained. 
         [0126]    However, we have found out that NiTi elements  11  behave individually. Also preload spring  12  material we have used seems to exhibit large variations in its elasticity and therefore we have observed the spring constant of preload springs  12  to vary strongly even between individual units of actuators  10 ,  110 ,  120  we have tried to manufacture in series. 
         [0127]    So we have invented a method for compensating for manufacturing or material tolerances of actuators  10 ,  110 ,  120 , and improved our actuators  10 ,  110 ,  120  so that the manufacturing or material tolerances are compensable in an assembled unit. 
         [0128]    We can utilize restrictors  121 ,  111  to even out or to compensate for manufacturing tolerances. 
         [0129]    The restrictor  121  as shown in actuator  120  can be assembled in housing  14  after the characteristics of preload spring  12  and/or NiTi element  11  have been measured. Easiest, the restrictor  121  is implemented by a welded dot that is welded in housing at appropriate distance to produce a suitable combination of preloading force of preload spring  12  and tensile force on NiTi element  11 . If the preload stress is too small, the dot  121  is placed from a reference point towards the end of actuator  120  that increases the preload stress, and in the opposite case the dot is placed from the reference point to the opposite direction. 
         [0130]    Assembly  110  is even more easy to manufacture. Restrictor  111  that can be implemented as inhibiting unit such as a plate is brought in place and attached to pre-tensioning tube  112 . We can supply the pre-tensioning tube  112  with a thread. If the restrictor  111  is threaded, we can by rotating adjust the preload stress (and simultaneously the tensile force). The thread ensures that the restrictor  111  is held in place by a form-locking mechanism. 
         [0131]    It may be possible to delay the exertion of pushing force by actuator  10 ,  110 ,  120  by arranging movable member  13  in such a manner that at the beginning of the second transition (i.e. situation according to  FIG. 16 ) the movable member  13  is retracted in housing  14 . During the transition, movable member  13  protrudes from housing the distance Δl. Difference Δm−Δl may be adjusted by changing the length or positioning of movable member  13  to dimensions of housing  14 . 
         [0132]    Common to actuators  10 ,  110 ,  120 ,  130  according to the first and the second embodiment is that NiTi element  11  has been fixed to housing  14  also from the end of housing  14  that is opposite to the side of movable member  13 , or restricted in such a manner from the opposite than when the NiTi element  11  contracts, its side that is opposite to the side of movable member  13  cannot substantially move. 
         [0133]    The purpose of the fixing or the restricting is to ensure that when NiTi element  11  contracts, the NiTi element  11  shall substantially move only from the side of movable member  13 . The fixing can be carried out by welding NiTi element  11  and/or crimp  161  to housing  14 , or alternatively or in addition to welding by glueing, by wedging, by compressing or by any other suitable means. Restricting, which may be in place in addition to or instead of the fixing, can be carried out with restrictor  183 , for example. 
         [0134]    In the first and second embodiments, instead of using one NiTi element  11 , a bundle of NiTi elements can be used as well. Then crimps  161 ,  162  most preferably are arranged within connecting units  1818 ,  1819 , the structure of which is discussed below. 
       3) Third Embodiment (FIG.  18 ) 
       [0135]      FIG. 18  illustrates certain components of actuator  180  that comprises two individual actuators assembled next to each other in the housing. Generally, the number of individual actuators may also be larger than two. 
         [0136]    Actuator  180  shown in  FIG. 18  has been implemented by using actuators  130  previously discussed. However, with only minor modifications, actuators  130  can be replaced with any of actuators  10 ,  110 ,  120 . 
         [0137]    Actuator  180  may in particular be used as actuator of a scoliosis treatment device. 
         [0138]    Instead of one NiTi element  11  we now preferably have a bundle  1811  of wires or rods made of or consisting shape-memory-alloy, such any number larger than one of NiTi elements. In practice, NiTi elements  11  of the above kind or other options as already discussed can be used as the elements in the bundle. 
         [0139]    The individual elements in the bundle  1811  are electrically connected to each other in series or parallel and mechanically arranged in parallel as bundle  1811 . This is exactly the same approach as chosen in the variant of the first and second embodiments discussed earlier where a bundle is used comprising a number of NiTi elements  11 . 
         [0140]    With the bundle  1811 , much larger mechanical contractive forces can be exerted than with a single element alone. An “individual element” referred to here may, of course, encompass a number of NiTi wires that are in parallel both mechanically and electrically. The bundles  1811  are most preferably connected electrically in series, alternatively, they can be connected electrically in parallel. 
         [0141]    The electrical connection between the elements in bundle  1811  is implemented by connecting units  1818 ,  1819  located in the respective crimp  162 ,  161 . Restrictors  121  are configured to restrict the tensile force of the bundle  1811  by restricting the movement of the connecting unit  1818 ,  1819  in the work direction of the preloading spring  12 . Restrictors  121  can be in pairs so that the movable member  13  can go between the individual restrictors  182 . Restrictors  183  keep the bundles  1811  in place. 
         [0142]    The individual actuators  130  in actuator  180  are electrically cross-connected to each other via cross-connect member  181  that most preferably is of biocompatible material. In this manner, the bundles  1811  can be connected electrically in series, or if desired, in parallel. 
         [0143]    In addition, the actuator  180  comprises at least two connecting terminals  186  electrically connected to the bundle  1811  for feeding electrical energy received by at least one inductive coil in the actuator  180  to the bundle  1811 . So, when the coil (not shown in  FIG. 18 ) is energized, all elements in both bundles  1811  get warmed up and the phase transition takes place as already explained. 
         [0144]    Preferably, restrictors  121  are implemented pair-wise in such a manner that movable member  13  is confined between restrictors  182 . Cups  184 ,  185  are preferably implemented as one part that advantageously joins them through base part  1899 . 
       4) Fourth Embodiment (FIGS.  19 - 25 ) 
       [0145]      FIG. 19  illustrates actuator  190  that comprises a number of converters around an axis. Also here, the converters have been implemented by using actuators  130  previously discussed. However, with only minor modifications, actuators  130  can be replaced with any of actuators  10 ,  110 ,  120 . 
         [0146]    Actuator  190  is particularly advantageous as actuator of a bone distraction actuator i.e. as an internal osteodistraction device. 
         [0147]    Actuator  190  may comprise any number of converters, most preferably there are between 2 and 8 such converters. 
         [0148]    Bundles  1811  are again cross-connected in the manner explained with reference to actuator  180 , except that now the connecting between the bundles of upper and lower individual actuators are electrically cross-connected by conducting plates  2013 ,  2014  and series connectors  2018 ,  2019  that most preferably is of biocompatible material. 
         [0149]    Restrictor  111  preferably has at least one opening in the middle such that the movable member  13  can move back and forth through the opening. 
         [0150]    Connecting cable from the inductive coil (as explained with reference to  FIG. 18  already) is most preferably fed via cable recess  1992 . The cable recess  1992  can be located in part  199  of the actuator  190  that is to be connected to implants&#39; outer shell  197 . The outher shell  197  is required in the current implementation because the restricor  111  is a part of it. However, we can replace this construction with any other previously implemented manner to move restrictor. 
         [0151]    Actuator  190  may be connected to force transmission via connection  198  to force transmission, in particular via a thread, and be connected from its other end via connection thread  1991 . 
         [0152]      FIGS. 20 ,  23  and  24  illustrate the bone distraction actuator  190  and  FIG. 21  section A-A of it.  FIG. 22  is a zoomed view C of spring stack  3  end of the distraction actuator  190 .  FIG. 25  illustrates sections D-D and E-E at locations illustrated in  FIG. 24 . 
         [0153]    When bone distraction actuator  190  is assembled, lower NiTi holder  6  is attached to end block  5  in such a way that they are galvanically isolated from each other. End block  5  attaches to outer shell  197  that may be a tube in such a way that they form a non-separable body. One or more insulating parts  8 ,  9 ,  10  are attached to lower NiTi holder  6 . The function of insulating parts  8 ,  9 ,  10  is to insulate NiTi bundles  102 ,  103  from NiTi holder  6 . Two conducting plates  2013 ,  2014  are in contact with lower NiTi holder  6 . The function of conducting plates  2013 ,  2014  is to connect NiTi bundles  100 ,  101  to holder  6  in such a way that electric current can flow between NiTi bundles  100 ,  101  and NiTi holder  6 . 
         [0154]    NiTi bundles  102 ,  103  are attached to coil connectors  2011 ,  2012  which are used to connect NiTi bundles  102 ,  103  to receiving coil assembly  2 . Receiving coil assembly  2  comprises a coil housing  21 , coil wire  23  and ferrite  22  or any other suitable material to focus the external magnetic field. However, receiving coil assembly  2  does not need to contain all these parts and only coil wire  23  is needed in the most basic case. Receiving coil assembly  2  may be replaced with a battery and a remote controllable switch, for example. 
         [0155]    The other end of NiTi bundles  100 ,  101 ,  102 ,  103  is connected to upper NiTi holder  7 . Between the ends of NiTi bundles  100 ,  101 ,  102 ,  103  are series connectors  2018 ,  2019  (e.g. connection plates) which form the electrical connection between the adjacent bundles. Below series connectors  2018 ,  2019  there are again insulations  2015 ,  2016  that galvanically isolate upper NiTi holder  7  from NiTi bundles  100 ,  101 ,  102 ,  103 . Insulating part  17  is also used for this purpose. 
         [0156]    The electrical connection is implemented as follows: Coil wire  23  connects from one end to coil connector  2011 . Coil connector  2011  connects to NiTi bundle  103 . NiTi bundle  103  connects to series connector  2019 . Series connector  2019  connects to NiTi bundle  100 . NiTi bundle  100  connects to conduction plate  2013 . Conduction plate  2013  connects to lower NiTi holder  6 . Lower NiTi holder  6  connects to conduction plate  2014 . Conduction plate  2014  connects to NiTi bundle  101 . NiTi bundle  101  connects to series connector  2018 . Series connector  2018  connects to NiTi bundle  102 . NiTi bundle  102  connects to coil connector  2012 . Coil connector  2012  connects to the other end of the coil wire  23  and the connection path for full series connection of the NiTi bundles  100 ,  101 ,  102 ,  103  is complete. 
         [0157]    Upper NiTi holder  7  is connected to a spring guide  4  through suitable means, for example thread or welding in order to achieve a rigid construction, or more preferably, a form-locking construction. The end of the spring guide  4  not connected to lower NiTi holder  7  compresses spring stack  3  against outer shell  197 . Spring stack  3  may consist of disc springs or it can be a compression spring or any other construction, assembly or material that provides similar functions to a spring. 
         [0158]    When assembling the bone distraction actuator  190 , when NiTi bundles  100 ,  101 ,  102 ,  103  are under no tension, there exists a small gap between restrictor  111  (which preferably is implemented as middle wall of outer shell  197  especially if the outer shell  197  is implemented as a tube or tube-like structure) and upper NiTi holder  7 . When NiTi bundles  100 ,  101 ,  102 ,  103  are tensioned by placing spring stack  3  into the bone distraction actuator  190  and tensioning the spring stack  3 , NiTi bundles  100 ,  101 ,  102 ,  103  come under tension according to Hooke&#39;s law. The lenght of the gap in the non-tensioned state therefore determines the amount of stress (tension) that each NiTi bundle  100 ,  101 ,  102 ,  103  will be under when the bone distraction actuator  190  is assembled. 
         [0159]    It should be noted that no matter what the load the spring stack  3  is tensioned to, NiTi bundles  100 ,  101 ,  102 ,  103  will only be tensioned to the amount defined by the gap. Therefore it is possible to design the bone distraction actuator  190  so that NiTi bundles  100 ,  101 ,  102 ,  103  are always under optimal tension (or at least close to it) prior to actuation and once they start to actuate (lower NiTi holder  7  separates from restrictor  111 ) it will start generating some other predetermined, preferably higher, force. 
         [0160]    The actuation cycle of the bone distraction actuator  190  proceeds as follows: an external magnetic field induces a voltage to coil wire  23 , or, alternatively, if no receiving coil assembly  2  is used, the battery is switched on. 
         [0161]    This voltage is applied over NiTi bundles  100 ,  101 ,  102 ,  103  as described earlier in the electrical connection pathway. This voltage generates a current according to Ohm&#39;s law that will run through NiTi bundles  100 ,  101 ,  102 ,  103 . The power dissipated in NiTi bundles  100 ,  101 ,  102 ,  103  then heats the NiTi bundles  100 ,  101 ,  102 ,  103  resistively. The heating causes a phase transition from the martensite phase to the austenite phase in NiTi bundles  100 ,  101 ,  102 ,  103 . The phase transition causes NiTi bundles  100 ,  101 ,  102 ,  103  to contract (their length gets shorter). When this happens, NiTi bundles  100 ,  101 ,  102 ,  103  pull upper NiTi holder  7  towards lower NiTi holder  6  and upper holder  7  separates from restrictor  111 . This will in turn pull spring guide  4  towards lower NiTi holder  6 . This will cause spring stack  3  to compress (its length gets shorter) because the gap between outer shell  197  and spring guide  4  shortens. 
         [0162]    Now, when the external magnetic field is switched off (or, alternatively, the battery is switched off) there is no current flowing in NiTi bundles  100 ,  101 ,  102 ,  103 . This allows NiTi bundles  100 ,  101 ,  102 ,  103  to cool down and transform from the austenite phase back to the martensite phase, assisted by spring stack  3 . At the same time spring guide  4  moves back to its original position and spring stack  3  compression returns to the the original level. At the same time the bone distraction actuator  190  provides an output force if measured from the left-hand end of spring guide  4  in  FIG. 20  i.e. at connection  198  to force transmission. Spring guide  4  can be connected from connection  198  to force transmission to suitable power transmission methods in order to generate movement from the repeating contraction pulses of the power transfer mechanism. In particular, this may be any such power transfer mechanisms disclosed in 2009/115645 A1 and WO 2011/148047 A1. Especially we here mean the mechanisms described in any of the referred documents for converting a reciprocal movement to extending movement in one direction. 
         [0163]    The many features and advantages of the present invention are apparent from the written description. Further, since numerous modifications and changes will be readily apparent to those skilled in the art, the invention should not be limited to the exact construction and operation as illustrated and described. Hence, all suitable modifications and equivalents may be resorted to as falling within the scope of the invention. 
       5) An Alternative Construction of the Actuator (FIG.  26 - 28 ) 
       [0164]      FIGS. 26 and 27  show an alternative construction of the actuator. The actuator contains four bundles of NiTi wires ( 3001 ) that are electrically connected in series with a receiving power transfer cable ( 3017 ), that may be a receiving coil via cable. The electrical circuit of the actuator is completed with several elements, that are described below. 
         [0165]    The inductive power transfer cable ( 3017 ) connects to the inductive power transfer coil (not shown). 
         [0166]    The other end of the cable ( 3017 ) connects to the NiTi element ( 3111 ). The contact may be done by crimping. The contact may also be done by soldering. Further, the contact may be done by welding. The NiTi element ( 3111 ) is resting on a lifting plate ( 3015 ) whose function is to make the NiTi elements ( 3111 - 3114 ) sit on equal heights such that they distribute the load on the small crimp on the underlying insulation more evenly. 
         [0167]    The NiTi element ( 3111 ) connects to the NiTi element ( 3112 ) through the series connection plate ( 3014 ). 
         [0168]    The NiTi element ( 3112 ) further connects to the NiTi element ( 3113 ) through the series connection plate ( 3016 ). 
         [0169]    The NiTi element ( 3113 ) further connects the the NiTi element ( 3114 ) through the series connection plate ( 3141 ). 
         [0170]    The NiTi element ( 3114 ) further connects to the other pole of the inductive power transfer coil through identical wires to the ones of the power transfer cable ( 3017 ). 
         [0171]    The parts ( 3010 ), ( 3011 ), ( 3012 ) and ( 3013 ) act as insulations that insulate the metallic parts of the power source from the electrical circuit. They are also for preventing the short circuits. The insulation may be made of any electrically insulating material. The insulation may ge e.g. of polymer or ceramic material. The insulation ( 3013 ) further acts as a heat barrier that slows the transmission of heat from inside the power source to the power source housing tube ( 3004 ). 
         [0172]    The NiTi elements ( 3111 - 3114 ) are placed between the movable member ( 3002 ) and a fixed support structure ( 3003 ). The fixation of the support structure ( 3003 )is achieved through the actuators housing tube ( 3004 ), spring support plate ( 3005 ) and actuator support tube ( 3006 ). The free end (left side in  FIG. 26 ) of the support tube ( 3006 ) is fixed in place. A spring compression rod ( 3007 ) which may also be in more general called as a rod connects to the movable member ( 3002 ). These can also be manufactured as a single part. A compression spring stack ( 3009 ) is placed around the spring compression rod ( 3007 ) so that they are resting against the spring support plate ( 3005 ). The spring compression rod ( 3007 ) is pulled to the optimal prestress for the NiTi elements and a fixation part ( 3008 ) is added. The added fixation part ( 3008 )then preferably holds the optimal prestress. This can be fastened by any means, e.g. by thread or welding etc. 
         [0173]    Alternatively, instead of fixing the support tube ( 3006  on its place, the support tube  3006  can be excluded. In that case, the actuator should be fixed with the support structure ( 3003 ), housing tube ( 3004 ) or spring support plate ( 3005 ). 
         [0174]    As the compression spring stack ( 3009 )tries to elongate and is blocked by the fixation part ( 3008 ), the force is transferred through the spring compression rod ( 3007 ) to the movable member ( 3002 ). The movable member ( 3002 ) further tries to move to the left in  FIG. 26  causing the force to be transferred to the NiTi elements ( 3111 - 3114 ). The other end of the NiTi elements ( 3111 - 3114 ) rests on the support structure ( 3003 ) which takes the load and transfers it through the tubes ( 3004 ,  3006 ) and the spring support plate ( 3005 ) to the fixation point at the left end of the part ( 3006 ) in  FIG. 26 . 
         [0175]    Alternatively, instead of a compression spring stack ( 3009 ) a normal compression spring can be used. Alternatively instead of a compression spring stack ( 3009 ) or compression spring a tension spring can be used. When using a tension spring the tension spring is connected to the end of the rod ( 3007 ). When pulling the tension spring from the other end, the spring causes the rod ( 3009 ) to move and to exert stress on the NiTi elements ( 3111 - 3114 ). When the tension spring is pulled to optimal prestress it can be fixed to the part ( 3006 ) using any kind of fixation means. 
         [0176]    The operation cycle of the actuator is explained by  FIG. 28 . In resting state the measure L 1  may be e.g. 19.70-19.90 cm, preferably 19.80 cm, measure L 2  may be e.g. 19.90-20.10 cm, preferably 20.00 cm and the measure L 3  may be e.g. 22.90-23.10 cm, preferably 23.00 cm. 
         [0177]    When the external magnetic field is activated, a voltage is induced in the inductive power transfer coil. The power transfer coil further leads to a current going through the inductive power transfer cables ( 3017 ) and the series electrical circuit of the actuator containing NiTi elements ( 3111 - 3114 ). The NiTi elements are further heated through resistive heating. This causes the NiTi elements to go through a phase change from martensite to austenite phase. This causes the NiTi elements to contract. The contraction moves the movable member ( 3002 ) to the right by the amount of the contraction. The contraction may be subtracted by any elastic deformations in the structures. The contraction also causes the spring compression rod ( 3007 ) to move to the right by the same amount. This in turn causes the compression spring stack ( 9 ) to compress further. 
         [0178]    Now the actuator has fulfilled its actuation step. Now, the measure L 1  may be e.g. 18.70-18.90 cm, preferably 18.80 cm, measure L 2  may be e.g. 18.90-19.10 cm, preferably 19.00 cm and the measure L 3  may be e.g. 23.90-24.10 cm, preferably 24.00 cm. 
         [0179]    As the left side of the support tube ( 3006 ) is fixed, the actuator is able to produce a displacement and force output through the right end of the movable member ( 3002 ). Subsequently, the magnetic field is turned off and the NiTi elements ( 3111 - 3114 ) cool down. As a consequence, the compression spring stack ( 3009 ) pulls the NiTi elements to the original position. Also the movable member ( 3002 ) moves back to the original position. The actuator is ready for a new actuation cycle. 
         [0180]    By connecting the actuator to a one way movement permitting element a distraction osteogenesis device extendable with aid of the magnetic fields can be realized. 
       LIST OF REFERENCE NUMERALS USED 
       [0000]    
       
         
           
               2  receiving coil assembly 
               3  spring stack 
               4  spring guide (acts as movable member  13 ) 
               5  end block 
               6  NiTi holder 
               7  NiTi holder 
               8 ,  9 ,  10  insulating part 
               10  actuator 
               11  NiTi element 
               12  preload spring 
               13  movable member 
               14  housing 
               16  empty volume 
               17  insulating part 
               18  preloading force-tension transformer 
               19  restrictor (end wall) 
               21  coil housing 
               22  ferrite 
               23  coil wire 
               30  testing system 
               31  compression rod 
               32  connection to load cell 
               33  electrical connection 
               34  spring locking clamp 
               35  connection to hydraulic actuator 
               37  extensometer for strain measurement 
               100 ,  101 ,  102 ,  103  NiTi bundles 
               110  actuator 
               111  restrictor (inhibiting unit) 
               112  pre-tensioning tube 
               120  actuator 
               121  restrictor (intermediate wall) 
               130  actuator 
               161  crimp 
               162  crimp 
               180  scoliosis treatment actuator 
               181  cross-connect member 
               183  restrictor 
               184 ,  185  cup in which crimp is set during assembly 
               186  connecting terminal 
               190  bone distraction actuator 
               195  connector 
               197  implant&#39;s outer shell, optional part of the actuator  190   
               198  connection to force transmission 
               199  part of the actuator to be connected to implant&#39;s outer shell 
               300  test sample 
               301  crimp 
               302  NiTi wire 
               1113  opening for movable member 
               1811  bundle 
               1818  connecting unit 
               1819  connecting unit 
               1840  mechanical connection piece 
               1899  base part 
               1991  connection thread 
               1992  cable recess 
               2011 ,  2012  coil connectors 
               2013 ,  2014  conducting plate 
               2015 ,  2016  insulation 
               2018  series connectors 
               2019  series connectors 
               3000  actuator 
               3001  NiTi wire 
               3002  movable member 
               3003  support structure 
               3004  housing tube 
               3005  spring support plate 
               3006  actuator support tube 
               3007  rod 
               3008  fixation part 
               3009  compression spring stack 
               3010 ,  3011 ,  3012 ,  3013  insulating parts 
               3014 ,  3016  series connection plates 
               3015  lifting plate 
               3017  power transfer cable 
               3111 ,  3112 ,  3113 ,  3114  NiTi elements 
               3141  series connection plate 
             L 1  length of the spring stack 
             L 2  dislocation parameter of the compression rod 
             L 3  dislocation parameter of the movable member 
           
         
       
     
       CITED LITERATURE 
       [0261]    Aalsma A M M, Hekman E E G, Stapert, J W J L, Grootenboer H J 1997 The Design of A TiNi actuator in an intramedullary leg lengthening device J. PHYS. IV FRANCE 7 C5 627-631 
         [0262]    Aalsma A M M, Hekman E E G, Staper J, Grootenboer H 1998 A completely intramedullary leg lengthening device Proc. of the 20th Ann. Int. Conf. of the IEEE Engineering in Medicine and Biology Society 20-5 2710-2713 
         [0263]    Belson A 2013, patent publication U.S. Pat. No. 8,361,090 B2, Apparatus and method for endoscopic colectomy 
         [0264]    Bertacchini O W, Lagouds D C, Patoor E 2009 Thermomechanical transformation fatigue of TiNiCu SMA actuators under a corrosive environment—Part I: Experimental results Int J Fatigue 1571-1578 
         [0265]    Chau E T F, Friend C M, Allen D M, Hora J, Webster J R 2006 A technical and economic appraisal of shape memory alloys for aerospace applications Mater. Sci. Eng., A 438-440 589-592 
         [0266]    Dynalloy Inc. 2012 Technical charactreristics of Flexinol® actuator wires manufacturers datasheet, retrieved online Oct. 10, 2012 from http://www.dynalloy.com/pdfs/TCF1140.pdf 
         [0267]    Dahlgren J M 2009, patent application publication US 2009076597 A1, System for mechanical adjustment of medical implants 
         [0268]    Elwaleed A K, Mohamed N A, Nor M J, Mustafa M M 2007 A new concept of a linear smart actuator Sens. Actuators, A 135 244-249 
         [0269]    Fumagalli L, Butera F, Coda A 2009 SmartFlex NiTi wires for shape memory actuators J. Mater. Eng. Perform. 18 691-695 
         [0270]    Helsinki University of Technology 2009, patent application publication WO 2009115645 A1, Internal osteodistraction device 
         [0271]    Horst M, Hayoz D, Borghi E, Tozzi P 2013, patent application publication US 20130096586 A1, Medical device comprising an artificial contractile structure 
         [0272]    Kheirikhah M M, Rabiee S, Edalata M E 2011 A Review of shape memory alloy actuators in robotics Lecture Notes in Computer Science 6556 206-217 
         [0273]    Kim H, Yoo Y, Lee J 2008 Development of a NiTi actuator using a two-way shape memory effect induced by compressive loading cycles Sens. Actuators, A 148 437-442 
         [0274]    Lagoudas D C, Miller D A, Rong L, Kumar P K 2009 Thermomechanical fatigue of shape memory alloys Smart Mater. Struct. 18 
         [0275]    Mammano G S, Dragoni E 2012 Functional fatigue of Ni—Ti shape memory wires under various loading conditions Int J Fatigue http://dx.doi.org/10.1016/j.ijfatigue.2012.03.004 
         [0276]    Mertmann M, Vergani G 2008 Design and application of shape memory actuators Eur. Phys. J. Special Topics 158 221-230 
         [0277]    Olympus Corp. 2012, patent application publication EP 2133566 A2, Shape memory alloy actuator 
         [0278]    Ryhänen J 1999 Biocompatibility evaluation of nickel-titanium shape memory alloy Dissertation for University of Oulu ISBN 951-42-5221-7 
         [0279]    Soubeiran A A 2003, patent application publication US 2003032958 A1, Device for relative displacement of two bodies