the impactor shown in fig1 is a single component formed from spring steel . it has four main regions , the tail ( 2 ), the spring ( 4 ), the shaft ( 6 ) and the cantilevered arm ( 8 ). for an exemplary total length of ˜ 96 mm , the different regions may comprise ˜ 14 , 38 , 29 , 15 mm respectively . the function of the tail region ( 2 ) is to provide anchorage of the end of the spring region ( 4 ) against rotation during resetting of the impaction mechanism ( storage of energy in the impactor by torsion of the coils of the spring ). as such the tail ( 2 ) is designed to bear on the outer wall of the device body during resetting ( fig2 and 3 ). the tail ( 2 ) or other fixed portion may be curved to prevent it from damaging this plastic wall or from &# 34 ; digging in &# 34 ; and thus altering the setting point , etc . the function of the spring region ( 4 ) is to allow sufficient storage of energy in the impactor . to do this generally requires multiple coils to allow enough elastic potential energy to be stored without leading to undue strains on any length of the wire which might lead to plastic deformation , fatigue fracturing , etc . more turns , however , give rise to a deeper spring and hence lead to a greater space requirement in the device . for many devices , two turns are adequate . the turns must be designed not to tighten on the locating peg ( fig2 and 3 ). the shaft region ( 6 ) of the impactor serves several functions . it allows a greater length of impactor head travel than resetting motion and allows the impactor head motion to be particularly springy , i . e . it can create a whiplash effect as stored potential energy is released during travel , and it serves as the part of the impactor against which the activator component pushes during resetting ( fig2 ) and the part which pushes the activator component out of the way during impactor flight . the impactor cantilevered arm ( 8 ), also referred to herein as the impactor head , is the most critical region of all . the crown ( 10 ) of the cantilevered arm ( 8 ) is curved , e . g . 10 mm radius and it is aligned to strike the centerline of the medicament tape to ensure central impaction . this curvature also serves to give consistent impactions , no matter whether the tape lies at a slight angle to that intended . if the head were straight , then any small misalignments of the tape prior to an impaction would tend to give inconsistent behavior , with the impactor head sometimes striking parallel along its length , sometimes striking the tape with its top corner first , or sometimes striking the tape first with its elbow where it joins the arm region . in addition , the curvature of the head allows the tape to be bent into a three - dimensional surface as it is struck , giving rise to more complex vibrations in the tape and thus serving better to remove and aerosolise powder from it . because only the center of the tape is initially contacted by the impactor head , the vibrations set up are less likely to be damped out by contact with the head . furthermore , the curvature ensures that the end of the impactor wire cannot reach the tape and damage , perforate , or even break it . finally , the curved top of the impactor steers the tape correctly to the front of the impactor head during insertion of a new cassette of tape by the patient , thereby ensuring that the tape cannot be inserted behind the impactor . the end of the cantilevered arm ( 8 ) may be bent further , through 180 °, to reduce the risk of a patient scratching their finger on it . alternatively , the end may be dipped , to form a small spherical head for the same reason . preferably , a tunnel should cover the impactor head to prevent the patient being able to contact the impactor in inhalation devices . the impactor cantilevered arm ( 8 ) should be as low in mass as possible , commensurate with having sufficient strength and rigidity to strike the tape in the correct position and at the correct time . the reason for a low mass requirement is that this characteristic allows maximum transfer of energy to the medicament coated tape . considering the impactor head as an object of mass m h and initial velocity v h ( v h2 after collision ), and a drug particle as an initially stationary object of mass m p and final velocity v p after being struck by the head : in a two body system assuming no losses and where momentum and kinetic and kinetic energy are conserved . if m h = km p then substitution leads to : ## equ1 ## i . e . the maximum possible velocity of the drug particle is twice that of the incoming impactor ( this can be viewed as the drug particle approaching at - v h and bouncing off at + v h in the impactor &# 39 ; s frame of reference ). hence in order to impart as much velocity as possible to the drug particles ( i . e . in order to transfer as much kinetic energy as possible to them ), the velocity of the impactor should be as high as possible , i . e . the impactor head should have as low a mass as possible for a given kinetic energy stored in the spring region of the impactor . for an impactor shown in fig1 typical times of flight of approximately 1 ms are needed to travel ˜ 10 mm to the tape , i . e . mean speed ˜ 10 m / s ; maximum speed just prior to impaction of the rear of the powder coated tape being in excess of this . the impactor is stopped just after striking the tape , because follow - through could cause tearing of the tape , could pull a loop of excess tape off the supply spool , cause the tape to block the airflow channel , etc . fig2 illustrates a dry powder inhaler of the type disclosed in w090 / 13328 , w092 / 08509 and gb 9123953 ; the full details of operation of the inhaler are disclosed in these references , the full disclosures of which are incorporated herein by reference . fig2 shows the inhaler during closure of the mouthpiece cover . the inhaler comprises a housing ( 20 ), mouthpiece ( 22 ) incorporating a deagglomerator ( 24 ) and a mouthpiece cover ( 26 ). the dry powder medicament is carried on an elongate carrier shown in dashed outline ( 27 ) which extends from a supply spool ( 28 ) to a take - up spool ( 30 ). the inhaler comprises an impactor ( 32 ) which may have the form as shown in fig1 and is positioned with the coil of the spring over locating peg ( 34 ). the impactor ( 32 ) is engageable by an activator component ( 36 ) which forms part of a breath - actuated mechanism comprising a vane ( 38 ) which is movable in response to inspiration through the mouthpiece ( 22 ). the impactor ( 32 ) is moved to a primed position during closing of the mouthpiece cover ( 26 ) by action of reset arm ( 40 ) on activator ( 36 ). the impactor ( 32 ) is retained in the primed position by the activator component which is held by catch component ( 37 ) until the patient inspires through the mouthpiece ( 22 ) causing movement of vane ( 38 ) which results in the breath - actuation mechanism moving the catch ( 37 ) releasing the activator component ( 36 ) and the impactor ( 32 ). the impactor springs forward such that the head ( cantilevered arm ( 8 )) hits the rear of the tape in the middle of the 20 mm long impaction region of the tape , thereby releasing powder from the tape into the patient &# 39 ; s inspiratory air flow . the embodiment illustrated in fig3 shows an inhaler ( 102 ) of the present invention , comprising a housing ( 104 , 104a ) defining air inlets ( 106 , 108 ) and a slit deagglomerator ( 110 ) for outlet of the powder aerosol . a mouthpiece ( 140 ) clips onto the housing ( 104 , 104a ), defining a mouthpiece orifice ( 142 ) for insertion into the patient &# 39 ; s mouth . curved surfaces ( 112 , 114 ) guide the powder aerosol towards slit ( 110 ). a desiccant cartridge ( 116 ) may be housed in the housing part ( 104 , 104a ). a tape supply spool , comprising a flange ( not shown ) and a hub ( 118 ) is mounted in the housing , together with a tape take - up spool , comprising a flange ( not shown ) and a hub ( 120 ). a microdimpled tape ( 122 ) coated with micronised drug powder on the side facing the mouthpiece ( 140 ) between corners ( 124 , 126 ) is affixed to the spool hubs ( 118 , 120 ). the length between corners ( 124 , 126 ) is the impacted region of the tape and in a preferred embodiment is 12 mm long . a mouthpiece part ( 140 ) clips removably onto the housing ( 104a ), defining a mouthpiece orifice ( 142 ) through which aerosolised drug powder passes to the patient . the inhaler releases medicament from the impacted region of the tape by striking the rear ( uncoated ) side of it with the head ( shown at ( 138 ), coming out of the plane of the figure ) of a spring steel impactor ( 128 ) which also comprises a two turn coil region ( 134 ), loosely positioned around a peg ( 136 ) in the housing ( 104 ), and a tail ( 130 ) bearing on the inside of the housing wall at ( 132 ). the impactor is generally similar to that shown in fig1 but is cranked as shown in fig3 the additional bend in the arm region of the impactor being provided to allow access to the rear of the tape in the shorter ( 12 mm ) impaction region of the tape . a release component ( 144 ) is pivotally mounted on the housing ( 104 ) at pivot pin ( 152 ). the release component additionally comprises a flexible arm ( 146 ) terminating at a push button ( 148 ) at the top of the inhaler , with barbs ( 150 ) preventing the button being pulled . mouthpiece cover ( 154 ), pivoted at ( 136 ), above the impactor coil ( 134 ), comprises a reset arm ( 156 ) with surface ( 158 ) to push the impactor ( 128 ) into a primed position . the impactor ( 128 ) in turn pushes on surface ( 160 ) of the release component ( 144 ), thereby causing it to rotate clockwise and for surface ( 162 ) to catch the elbow at the bottom of the impactor head ( 138 ) i . e ., the intersection of shaft ( 6 ) and cantilevered arm ( 8 ). in the event of the release component being already in a clockwise position such that it obstructs the impactor during resetting , surface ( 164 ) is contacted by the impactor , causing the release component to rotate anti - clockwise until surface ( 160 ) is contacted to cause it to rotate clockwise . operation of the inhaler is as follows . first , the patient opens the mouthpiece cover ( 154 ). in the simplest embodiment of the device the patient then rotates a knob protruding from take - up spool ( 120 ), to rotate said spool anti - clockwise by one ratchet or detent position ( 60 °). this is sufficient , with a 23 mm diameter hub , to advance 12 mm of tape , equivalent to the length between corners ( 124 , 126 ). after 220 doses of 165 μm thick tape , the take - up spool will have grown to around 36 mm diameter at a typical tape winding tension . the advanced length for a 60 ° turn of the knob will then be nearly 19 mm , but as the drug dose is determined by the impacted region &# 39 ; s length , not by the tape advance length if that is greater , then the drug dose will remain consistent . the supply spool ( 118 ) rotates with a fairly high level of friction , in order to ensure the advanced tape remains tight , and non - reverse means ( not shown ) are provided on the take - up spool ( 120 ). in an alternative embodiment ( not shown ), an additional arm on the mouthpiece cover ( 154 ) pushes on one of a series of radial grooves on an extension protruding from the take - up spool ( 120 ), causing it to rotate 60 ° anti - clockwise upon closing of the mouthpiece cover ( 154 ). in a further embodiment ( not shown ) priming may be effected upon opening the cover . the patient then places mouthpiece ( 140 ) into their mouth , and inhales , causing air to enter the inhaler at inlets ( 106 , 108 ) and pass the tape to reach the deagglomerator slit ( 110 ) and mouthpiece orifice ( 142 ). soon after the start of inhalation the patient pushes button ( 148 ), causing the release block ( 144 ) to rotate anti - clockwise slightly about pivot pin ( 152 ). the causes surface ( 162 ) to release the elbow of the impactor head ( 138 ), causing the impactor head ( 138 ) to travel towards and strike the rear of the impaction region of the tape . this releases a dose of powdered medicament which is entrained in the airstream and passes through the deagglomerator slit ( 110 ) which breaks up agglomerates of micronised drug particles to enable them to reach the patient &# 39 ; s lungs more effectively . the patient then closes the mouthpiece cover ( 154 ), causing the reset arm ( 156 ) to push impactor ( 128 ) back until it contacts surface ( 160 ) of the release component ( 144 ), thereby causing it to rotate clockwise again . this action stores energy in the impactor spring by straining it , tail ( 130 ) being unable to move . the impactor ( 128 ) is thus primed . the next opening of the cover ( 154 ) will then allow the impactor to move forward slightly until the elbow at the bottom of the impactor head ( 138 ) is held by surface ( 162 ) and the operation cycle thus starts to be repeated .