Patent Description:
An example of a device that ejects a liquid chemical to a target region such as an organism includes a needleless injector using no injection needle which has been attracting attention in terms of usability and sanitation, and thus has been actively developed recently. In general, there has been implemented a needleless injector having a configuration in which a liquid chemical pressurized by a driving source such as compressed gas and a spring is ejected to a target region and the liquid chemical is administered to an inside of the target region through use of the kinetic energy of the liquid chemical.

In this context, a needleless injector disclosed in Patent Document <NUM> includes a plunger that presses liquid chemical or the like and has a structure divided into two members along a plunging direction with the two members connected to each other via a bridge. When a driving source for injection is actuated, the bridge is destroyed by the energy provided from the driving source. As a result, one of the members on the upstream side presses the other member on the downstream side, whereby the liquid chemical is pressed. With such a configuration, whether the plunger is used or unused can be easily recognized.

According to the prior art, the driving source provides energy upon being actuated, to deform the plunger from its original shape. Specifically, the bridge is destroyed, and the upstream side member of the plunger thus deformed presses the downstream side member to push the liquid chemical or the like. Thus, the plunger according to the prior art is designed such that desired liquid chemical or the like is ejected by the plunger after the deformation.

On the other hand, with the needleless injectors, the liquid chemical or the like is required to be delivered into a target region by being ejected by the energy applied due to the plunger pushing the liquid chemical. In view of this, general needleless injectors are designed to have components with sufficient strength so that the components are not damaged or destroyed by force applied thereto when the energy is provided for the delivery. Still, the needleless injectors cannot be completely free of risk of an unexpected amount of load being applied on its components due to some reason during the pressurization by the plunger. In particular, a component accommodating the liquid chemical or the like is under risk of such an excessive load because the pressurization operation of the plunger is directly applied to such a component. With a load overwhelming the expected level, the safety of injection using the needleless injector is compromised. Nevertheless, such a risk is mentioned nowhere in the prior art.

In view of the problem described above, an object of the present disclosure is to provide a technique that may guarantee safety of injection using a needleless injector, even when pressurization by a plunger leads to a load overwhelming an expected level.

To solve the above problem, a needleless injector having the features of claim <NUM> is provided. The needleless injector of the present disclosure employs a configuration in which a plunger is intentionally deformed to reduce the moving force of the plunger when force applied to the plunger during the pressurization by the plunger exceeds an expected level. With such a configuration, a component of the needleless injector, particularly a housing part accommodating a substance intended for injection pressurized by the plunger, can be prevented from receiving the excessive force by the plunger.

Specifically, the present disclosure provides a needleless injector configured to eject a substance intended for injection to a target region without using an injection needle, the needleless injector including a housing part that includes an accommodating space that accommodates the substance intended for injection and defines a flow path from the accommodating space so that the substance intended for injection is ejected to the target region, a driving part that imparts ejection energy to eject the substance intended for injection, and a plunger that defines the accommodating space and is disposed to move in the housing part by the ejection energy and pressurize the substance intended for injection. The plunger includes a weakened part that causes a part of the plunger to deform such that, when force applied to the plunger exceeds predefined force while the driving part is imparting the ejection energy with the accommodating space accommodating the substance intended for injection, a distal end of the plunger does not reach a deepest part of the housing part or force received by an inner wall surface of the housing part falls within a predetermined range until the distal end of the plunger reaches the deepest part. Note that the deepest part includes a connection portion between the accommodating space and the flow path in the housing part. The needleless injector has a configuration in which the plunger moves toward the deepest part, thereby pushing the substance intended for injection into the flow path. However, the connection portion with the flow path at the deepest part is not limited to a specific position.

In the needleless injector, the driving part imparts the ejection energy to the substance intended for injection accommodated in the housing part, and thus the substance intended for injection is ejected to the target region. In the present application, "ejection" is achieved by the plunger moving in the housing part through the ejection energy imparted by the driving part and thus pressurizing the substance intended for injection in the housing part, so that the substance intended for injection flows through the flow path in the housing part.

Further, as the substance intended for injection ejected from the needleless injector, predetermined substances including a component expected to have effects in the target region or a component expected to exert a predetermined function in the target region can be exemplified. Thus, as long as at least ejection by the ejection energy described above can be achieved, a physical mode of the substance intended for injection may be in a state of being dissolved in liquid, or may be in a state of simply being mixed without being dissolved in liquid. As one example, the predetermined substance to be sent includes vaccine for intensifying an antibody, a protein for cosmetic enhancement, a cultured cell for hair regeneration, and the like, and is included in a liquid medium in an ejectable manner. The substance intended for injection is formed in this way. Note that the medium is preferably a medium that does not hinder the above-mentioned effect and function of the predetermined substance in a state of being injected into the target region. As another method, the medium may be a medium that exerts the above-mentioned effect and function by acting together with the predetermined substance in the state of being injected into the target region.

The ejected substance intended for injection needs to penetrate the surface of the target region such that the substance intended for injection is ejected from the needleless injector to the target region to be delivered into the inside thereof. Thus, at an ejection initial state, the substance intended for injection needs to be ejected to the target region at a relatively high speed. In view of this point, as an example, the driving part preferably imparts the ejection energy using a combustion product discharged by combustion of an ignition charge. Note that, as the ignition charge, there may be employed any one of an explosive containing zirconium and potassium perchlorate, an explosive containing titanium hydride and potassium perchlorate, an explosive containing titanium and potassium perchlorate, an explosive containing aluminum and potassium perchlorate, an explosive containing aluminum and bismuth oxide, an explosive containing aluminum and molybdenum oxide, an explosive containing aluminum and copper oxide, an explosive containing aluminum and iron oxide, or an explosive composed of a combination of a plurality of the explosives of the above. As characteristics of the above-mentioned ignition charge, the combustion product is gas at a high temperature but does not include a gas component at a room temperature, hence the combustion product is condensed immediately after the ignition. As a result, the driving part can impart the ejection energy in an extremely short period of time. In addition, the driving part may utilize electrical energy such as a piezoelectric element or the like or mechanical energy such as a spring as the ejection energy instead of the ejection energy caused by the combustion of the ignition charge, and may generate the ejection energy by appropriately combining these forms of energy.

Here, the plunger is configured to pressurize the substance intended for injection in the housing part through the ejection energy imparted by the driving part. Thus, during the pressurization by the plunger, relatively large force can be applied to the housing part itself constituting the needleless injector. In particular, due to the pressurization by the plunger, the substance intended for injection will be pushed into the flow path provided in the housing part, so that the housing part will be subjected to force directly from the plunger or through the substance intended for injection until the plunger moving in the housing part toward the deepest part is in direct contact with the deepest part.

Thus, in the needleless injector described above, the plunger includes the weakened part that causes a part of the plunger to deform such that, when force applied to the plunger exceeds predefined force while the substance intended for injection is being pressurized for some reason, the distal end of the plunger does not reach the deepest part of the housing part. Note that the predefined force is a threshold of the force applied to the plunger while undesirably large force is being applied to the housing part. Then, the weakened part causes a part of the plunger to deform when force exceeding predefined force is applied to the plunger for some reason when the driving part is actuated, consuming a part of the ejection energy by the deformation to weaken the moving force of the plunger or to shorten its moving distance, thereby preventing the plunger from reaching the deepest part. Because the deepest part also serves as a portion that receives and stops the moving plunger during normal operation, it is easily subjected to force directly from the plunger. However, by the weakened part causing a part of the plunger to deform when force exceeding predefined force is applied to the plunger as described above, contact beyond predefined force between the plunger and the deepest part is avoided. This prevents excessive force from being applied to the housing part, and the safety of the injection using the needleless injector can be guaranteed. Note that "deformation" by the weakened part in the present application includes any structural change in the plunger in which a part of the ejection energy is consumed, such as damage, destruction, and the like.

The weakened part of the needleless injector described above may cause a part of the plunger to deform to consume a part of the ejection energy such that, when force applied to the plunger exceeds predefined force during the pressurization of the substance intended for injection, the force received by the inner wall surface of the housing part falls within a predetermined range until the distal end of the plunger reaches the deepest part of the housing part. Note that the predetermined range is a range of force applied to the housing part under which the safety of the needleless injector can be suitably guaranteed without damage, deformation, or the like. This deformation of the weakened part can also avoid excessive force being applied to the housing part and guarantee the safety of the injection using the needleless injector. Note that in cases where the weakened part is formed in this manner, the distal end of the plunger may or may not reach the deepest part when the weakened part is deformed. That is, the weakened part deforms when force exceeding predefined force is applied to the plunger and does not substantially deform when force below it is applied thereto. In other words, the weakened part may be a part of the plunger that preferentially deforms when force exceeding predefined force is applied to the plunger and can be formed having lower strength than another part of the plunger.

In the above-described needleless injector, when the force applied to the plunger exceeds the predefined force, the weakened part may be configured to cause a part of the plunger to deform such that frictional force during movement of the plunger in the housing part when a part of the plunger is deformed is greater than frictional force during movement of the plunger in the housing part when the part is not deformed. In the needleless injector according to an alternative aspect, when the force applied to the plunger exceeds the predefined force, the weakened part may be configured to cause a part of the plunger to deform such that a length of the plunger in an axial direction is shortened. These configurations allow a part of the ejection energy, which is a factor causing force exceeding predefined force, to be consumed by deformation of a part of the plunger by the weakened part, an increase in the frictional force during movement, the shortening of the plunger in the axial direction, or a combination of at least two or more of these elements, whereby the plunger can be prevented from reaching the deepest part and the force received by the inner wall surface of the housing part can fall within a predetermined range.

Here, the needleless injector described above may further include a piston disposed to move inside the needleless injector in a predetermined direction by the ejection energy imparted. The plunger may be disposed adjacent to the piston. The plunger may further include a rod part made of resin that receives the ejection energy from the piston, and a stopper portion capable of sealing the substance intended for injection in the accommodating space, the stopper portion being attached to the distal end of the plunger and moving together with the rod part. The weakened part may be disposed in the rod part. That is, by providing the weakened part on the rod part located closer to the piston, between the rod part and the stopper portion constituting the plunger, deformation due to the weakened part can be suitably achieved when the ejection energy is transferred from the piston to the plunger and force exceeding predefined force is applied thereto. In addition, because the stopper portion also serves as a portion that seals the substance intended for injection, it is not preferable that the weakened part is provided to the stopper portion in order to avoid any unfavorable effects working on the sealing state.

Note that four forms of the arrangement of the weakened part in the rod part are illustrated below. In a first embodiment, the weakened part may be one or a plurality of recess portions disposed in a non-uniform manner in a circumferential direction of the rod part. In a second embodiment, the rod part may have a first diameter, and the weakened part may be a reduced diameter portion having a second diameter smaller than the first diameter. In a third embodiment, the weakened part may be a groove portion extending in a circumferential direction over a portion or an entirety of the rod part. In a fourth embodiment, the weakened part may be a through hole or a thin portion formed inside the rod part and extending in a radial direction of the rod part. By configuring the weakened part in these ways, the plunger can be accurately caused to deform when the force applied thereto exceeds predefined force that is a set threshold, thereby effectively slowing down the plunger.

The safety of the injection using the needleless injector can be guaranteed even when pressurization by a plunger results in an unexpected amount of load.

With reference to the drawings, a needleless injector <NUM> according to an embodiment of the present disclosure (herein, simply referred to as "injector") is described below. The injector <NUM> is a needleless injector that implements injection by ejecting an ejection solution, which corresponds to a substance intended for injection in the present application, to a target region through use of a combustion energy of an explosive, that is, a device that injects the ejection solution to the target region without using an injection needle.

Note that each of the configurations, combinations thereof, and the like in each embodiment are an example, and various additions, omissions, substitutions, and other changes may be made as appropriate without departing from the spirit of the present disclosure. The present disclosure is not limited by the embodiments and is limited only by the claims. Note that, in the present embodiment, as terms indicating a relative positional relationship in a longitudinal direction of the injector <NUM>, "distal end side" and "base end side" are used. The "distal end side" indicates a side closer to the distal end of the injector <NUM> described later, that is, a position closer to an ejection port <NUM>, and the "base end side" indicates a side in an opposite direction to the "distal end side" in a longitudinal direction of the injector <NUM>, that is, a direction to an igniter <NUM> side of an injector assembly <NUM> (see <FIG> described later).

Here, <FIG> is a diagram schematically illustrating the appearance of the injector <NUM>. <FIG> is a first cross-sectional view of the injector <NUM>, which is an AA cross section in <FIG>, described below. <FIG> is a second cross-sectional view of injector <NUM>, a BB cross section in <FIG> described below. The BB cross section is orthogonal to the AA cross section. Note that <FIG> is a diagram illustrating a configuration of a housing <NUM> that is a part of the injector <NUM>. Here, the injector <NUM> is formed with the injector assembly <NUM> attached to the housing <NUM>. A power cable <NUM> for supplying drive current to the igniter <NUM> in the injector assembly <NUM> is connected to the housing <NUM>.

Note that, in the following description in the present application, the ejection solution ejected to the target region by the injector <NUM> is formed of a liquid medium including a predetermined substance, which exerts an effect or a function expected in the target region. In the ejection solution, the predetermined substance may be in a state of being dissolved in liquid being a medium, or may be in a state of being simply mixed instead of being dissolved.

For example, examples of the predetermined substance included in the ejection solution include an organism-derived substance and a substance with a desired bioactivity, which can be ejected to the target region being an organism. For example, examples of the organism-derived substance include DNA, RNA, a nucleic acid, an antibody, and a cell. Examples of the substance with a desired bioactivity include various substances exerting pharmacological or therapeutic effects, which are exemplified by medicines composed of low molecular compounds, proteins, peptides, or the like, a vaccine, an inorganic substance such as metal particles for thermotherapy or radiotherapy, and a carrying body functioning as a carrier. Further, the liquid being the medium of the ejection solution is only required to be a substance suitable for administering the predetermined substance exemplified by those substances to the target region, and may be aqueous or oleaginous, which is not limited. Further, viscosity of the liquid being the medium is not particularly limited as long as the predetermined substance can be ejected by the injector <NUM>.

In the injector <NUM>, the injector assembly <NUM> is freely attachable to and detachable from the housing <NUM>. An accommodating space <NUM> (see <FIG>) formed between a container <NUM> and a plunger <NUM> in the injector assembly <NUM> is filled with ejection solution during a preparation stage before the operation of the injector <NUM>. The injector assembly <NUM> is a unit that is replaced each time the ejection solution is ejected. The injector assembly <NUM> will be described in detail below.

On the other hand, the housing <NUM> has a grip portion 2a formed to be gripped by a user of the injector <NUM> in use, and is provided with a plurality of switches for operating the injector <NUM> to eject the ejection solution. Note that the injector <NUM> is configured to be capable of being held and operated by one hand of the user. In this context, the housing <NUM> will be described with reference to <FIG>. In <FIG>, (a) illustrates the outer appearance of the housing <NUM> as viewed from the front side, (b) illustrates the outer appearance of the housing <NUM> as viewed from one side, (c) illustrates the outer appearance of the housing <NUM> as viewed from the back side, and (d) illustrates the outer appearance of the housing <NUM> as viewed from the upper side. Here, "front side" indicates a portion positioned on the distal side of the user holding the housing <NUM>, which is the left side in <FIG>, and "back side" indicates a portion positioned on the proximal side of the user holding the housing <NUM>, which is the right side in <FIG>. Thus, when the user holds the housing <NUM> with one hand, fingertips rest on the front side of the distal housing <NUM> which is the distal side, and the wrist is in the vicinity of the back side of the housing <NUM> which is the proximal side. The "upper side" is a portion of the injector <NUM> on the base end side.

Considering such a way of holding by the user, the grip portion 2a is provided at a front side portion of the housing <NUM> so that the user can easily rest his or her fingertips thereon. The grip portion 2a is provided with a plurality of dimples making the user's fingertips even easier to be rested thereon. Furthermore, the grip portion 2a has gentle recesses and protrusions on the front side of its outer shell (see (b) in <FIG>) so that the user's forefinger and middle finger can be easily rested thereon, for the sake of more stable holding of the housing by the user.

Further, the housing <NUM> is provided with a first switch <NUM> and a second switch <NUM> that are two operating switches for operating the injector <NUM>. A first switch <NUM> and a second switch <NUM> are connected to a control unit, such as a microcomputer (not illustrated). The control unit controls the supply of ignition current to the igniter <NUM> based on a signal from each switch, thereby controlling an operation of the injector <NUM>. The first switch <NUM> is a sliding switch provided on the back side of the housing <NUM>, the sliding direction of which being an upward and downward direction of the housing <NUM> (direction between the distal end and the base end). The first switch <NUM> is constantly biased in the upward direction. The user can achieve a standby state of the injector <NUM> by continuously sliding the first switch <NUM> downward (toward the distal end side) for a predetermined period of time against the biasing force. The standby state is a state in which the injector <NUM> is ready to eject the ejection solution. When a user makes an additional operation in this state, the ejection is implemented.

The second switch <NUM> is a press type switch provided on an inclined surface 2b on the upper side of the housing <NUM>. The user can press the second switch <NUM> in a direction toward the inner side of the housing <NUM>. The control unit is configured to supply an ignition current to the igniter <NUM> in response to the pressing operation on the second switch <NUM> while the injector <NUM> is in the standby state as a result of the operation on the first switch <NUM> described above. A connector <NUM> to which the power cable <NUM> is connected is provided on the front side of the inclined surface 2b on the upper side of the housing <NUM>. In the present embodiment, the connector <NUM> is a USB connector, and the power cable <NUM> is freely attachable to and detachable the housing <NUM>.

Note that, as described above, in the present embodiment, the power for actuating the igniter <NUM> is supplied from the outside through the power cable <NUM>. Alternatively, a battery for supplying such power may be provided inside the housing <NUM>. In this case, the housing <NUM> can be repeatedly used while replacing the injector assembly <NUM>, until the battery runs out. When the battery runs out, the battery may be replaced.

A schematic configuration of the injector assembly <NUM> is illustrated in <FIG>. The injector assembly <NUM> is attached to the housing <NUM> to form the injector <NUM>, as illustrated in <FIG> and <FIG>. Specifically, the injector assembly <NUM> is an assembly including an actuator <NUM>, an attachment <NUM>, the container <NUM>, and the plunger <NUM>. How the injector assembly <NUM> is assembled will be described below.

First of all, the actuator <NUM> will be described with reference to <FIG>. The actuator <NUM> has a body <NUM> formed in a cylindrical shape. The body <NUM> includes a center portion 21a in the center thereof, a distal end portion 21b on the distal end side thereof, and a base end portion 21c on the base end side thereof. The distal end portion 21b, the center portion 21a, and the base end portion 21c of the body <NUM> have their internal spaces in communication with each other. The distal end portion 21b has an opening <NUM> on the distal end side. The igniter <NUM>, which is an electric igniter that generates energy for ejection through combustion of an ignition charge 22a, is attached to the base end portion 21c of the body <NUM> via a cap <NUM>. The igniter <NUM> has an ignition pin 22b to which ignition current is supplied from the outside. The ignition pin 22b is coupled to a socket <NUM> on the side of the housing <NUM> in a state in which the injector assembly <NUM> is attached to the housing <NUM>. The attachment state of the igniter <NUM> to the body <NUM> is determined such that a combustion product generated by the operation of the igniter <NUM> is discharged toward the center portion 21a of the body <NUM>. Specifically, the igniter <NUM> is attached to the base end portion 21c of the body <NUM> to have a discharge surface 22c, from which the combustion product is discharged, directed toward the center portion 21a.

Herein, a combustion energy used in the igniter <NUM> for the ignition charge is an energy for the injector <NUM> to eject the ejection solution to the target region. Note that, examples of the ignition charge include an explosive containing zirconium and potassium perchlorate (ZPP), an explosive containing titanium hydride and potassium perchlorate (THPP), an explosive containing titanium and potassium perchlorate (TiPP), an explosive containing aluminum and potassium perchlorate (APP), an explosive containing aluminum and bismuth oxide (ABO), an explosive containing aluminum and molybdenum oxide (AMO), an explosive containing aluminum and copper oxide (ACO), an explosive containing aluminum and iron oxide (AFO), or an explosive composed of a combination of a plurality of these explosives. These explosives exhibit characteristics that, although the explosives generate high-temperature and high-pressure plasma during combustion immediately after ignition, when the combustion product condenses at room temperature, the explosives do not contain gaseous components and hence the pressure generated decreases abruptly. An explosive other than these may be used as the ignition charge as long as appropriate ejection of the ejection solution can be performed.

The internal space of the center portion 21a of the body <NUM> serves as a combustion chamber 20a into which a combustion product is discharged from the igniter <NUM>. Furthermore, a male thread portion <NUM> is formed in a part of the outer surface of the center portion 21a. The male thread portion <NUM> is configured to mate with a female thread portion <NUM> of the attachment <NUM> described below. The effective lengths of the male thread portion <NUM> and the female thread portion <NUM> are determined to guarantee sufficient coupling force therebetween. The internal space of the distal end portion 21b adjacent to the center portion 21a is formed in a cylindrical shape in which a piston <NUM> is slidably provided and <NUM> rings <NUM> serving as a sealing member are also provided. The piston <NUM> is made of metal, has a shaft member <NUM>, is provided with a first flange <NUM> on the base end side thereof, and is further provided with a second flange <NUM> in the vicinity of the first flange <NUM>, as illustrated in <FIG>. The first flange <NUM> and the second flange <NUM> have a disc shape, and have the same diameter. The O rings <NUM> include one disposed between the first flange <NUM> and the second flange <NUM> and one disposed on another side of the second flange <NUM>. A recess portion <NUM> having a predetermined size is formed in a distal end surface of the shaft member <NUM>. In a state where the piston <NUM> is disposed in the internal space of the distal end portion 21b before the actuation of the actuator <NUM>, the first flange <NUM>, which serves as a surface receiving pressure of the combustion product from the igniter <NUM>, is exposed on side of the combustion chamber 20a, and the distal end of the shaft member <NUM> of the piston <NUM> is inserted into the opening <NUM>.

Then, when the igniter <NUM> is activated and the combustion product is discharged into the combustion chamber 20a and thus the pressure therein rises, the first flange <NUM> receives the pressure, resulting in the piston <NUM> sliding toward the distal end side. Thus, the actuator <NUM> has a mechanism with the igniter <NUM> serving as an actuation source and the piston <NUM> serving as an output unit. Since the second flange <NUM> has a larger diameter than the opening <NUM>, the distance by which the piston <NUM> can slide is limited. Thus, the distance by which the shaft member <NUM> of the piston <NUM> can protrude from the distal end surface of the distal end portion 21b of the body <NUM> is limited. Further, the piston <NUM> may be formed of a resin, and in such case, metal may be used together for a part to which heat resistance and pressure resistance are required.

Additionally, as an alternative mechanism to adjust the pressure applied to the piston <NUM>, the combustion chamber 20a of the actuator <NUM> may be further provided with a gas generating agent that is burned by the combustion product from the igniter <NUM> to produce gas. The agent may be disposed, for example, at a location that may be exposed to the combustion product from the igniter <NUM>. Further, as another method, the gas generating agent may be disposed in the igniter <NUM> as disclosed in <CIT>, <CIT>, and the like. As one example of the gas generating agent, there may be exemplified a single base smokeless explosive formed of <NUM> mass% of nitrocellulose, <NUM> mass% of diphenylamine, and <NUM> mass% of potassium sulfate. Further, various types of gas generating agents used in a gas generator for an air bag and a gas generator for a seat belt pretensioner may be used. A combustion completion time period of the gas generating agent can be changed by adjusting a dimension, a size, a shape, and particularly, a surface shape of the gas generating agent at the time of being disposed in the combustion chamber 20a or the like. With this, the pressure applied to the piston <NUM> can be adjusted to a desired pressure.

Next, the attachment <NUM> will be described based on <FIG>. Note that <FIG> includes the diagram (a) on the left side that is a cross-sectional view of the attachment <NUM>, and the diagram (b) on the right side that is an external view of the attachment <NUM>. The attachment <NUM> is a member for attaching the actuator <NUM>, the plunger <NUM>, and the container <NUM> as illustrated in <FIG>. For a body <NUM> of the attachment <NUM>, nylon <NUM>-<NUM>, polyarylate, polybutylene terephthalate, polyphenylene sulphide, a liquid crystal polymer, or the like, which are publicly known, may be used for example. Further, a filler such as glass fibers and glass filler may be contained in those resins. <NUM> to <NUM> mass% of glass fibers may be contained in polybutylene terephthalate, <NUM> to <NUM> mass% of glass fibers may be contained in polyphenylene sulphide, or <NUM> to <NUM> mass% of minerals may be contained in a liquid crystal polymer.

The internal space of the body <NUM> includes a first region <NUM>, extending from the base end side to the center, where the actuator <NUM> is disposed as illustrated in <FIG>. The first region <NUM> includes a region 33a on the base end side where the base end portion 21c of the actuator <NUM> is generally positioned, and a region 33b on the distal end side of the first region <NUM> where the center portion 21a and the distal end portion 21b of the actuator <NUM> are generally positioned. The region 33b has a smaller diameter than the region 33a. The female thread portion <NUM> is disposed on the inner wall surface at a portion of the region 33b close to the region 33a. The female thread portion <NUM> is formed so as to engage with the male thread portion <NUM> provided on the center portion 21a of the actuator <NUM>.

The internal space of the body <NUM> further includes a second region <NUM> in communication with the first region <NUM>. The second region <NUM> is a region in which the plunger <NUM> is generally disposed as illustrated in <FIG>, and is a hollow region formed in a cylindrical shape extending along the axial direction of the body <NUM>. The second region <NUM> has one end in communication with the region 33b of the first region <NUM>. The second region <NUM> has a diameter smaller that is smaller than the diameter of the region 33b, and enables a sliding movement of the plunger <NUM>. A through hole <NUM> extends from a side outer surface of the attachment <NUM> to the second region <NUM>, to be formed through the body <NUM>. Through the through hole <NUM>, the user can check the status (such as whether the injector assembly <NUM> is before or after being actuated, for example) of the plunger <NUM> in the injector assembly <NUM> from the outside (see <FIG>).

The internal space of the body <NUM> further includes a third region <NUM> in communication with the second region <NUM>. The third region <NUM> is a region in which a part of the container <NUM> is generally disposed as illustrated in <FIG>, and has one end in communication with the second region <NUM>, and has the other end open to the distal end surface of the attachment <NUM>. A female thread portion <NUM> for attachment to the container <NUM> is formed in the third region <NUM>. The female thread portion <NUM> is screwed with a male thread portion <NUM> of the container <NUM> illustrated in <FIG> described below, whereby the attachment <NUM> and the container <NUM> are coupled to each other.

Next, the plunger <NUM> will be described based on <FIG> includes the diagram (a) on the left side that is an external view of a plunger rod <NUM>, which is one of the components of the plunger <NUM>, and a diagram (b) on the right side that is an external view of the plunger <NUM>. The plunger <NUM> is a member that pressurizes the ejection solution by energy received from the piston <NUM>, and a resin material suitable for the pressurization (for example, a resin material similar to that used for the attachment <NUM>) can be used for the plunger rod <NUM>. The plunger rod <NUM> includes a shaft member <NUM>, and has a base end side end surface provided with a protrusion <NUM>. The protrusion <NUM> is shaped and sized to be capable of fitting in the recess portion <NUM> of the shaft member of the piston <NUM> of the actuator <NUM>, when the plunger <NUM> is incorporated in the injector assembly <NUM>. A reduced diameter portion <NUM> that has a diameter smaller than other portions of the shaft member <NUM> is provided in an intermediate portion of the shaft member <NUM> close to the base end. The reduced diameter portion <NUM> is provided with a recess portion <NUM> of a predetermined size which will be described in detail below.

Further, in the plunger rod <NUM>, a protrusion <NUM> is provided to a distal end side of the shaft member <NUM> with a neck portion <NUM> with a smaller diameter than the shaft member <NUM> provided in between. The protrusion <NUM> is shaped like a weight to have a diameter being greater than the diameter of the neck portion <NUM> near a portion to be connected with the neck portion <NUM> and reducing toward the distal end side. The maximum diameter of the protrusion <NUM> is smaller than the diameter of the shaft member <NUM>. A stopper portion <NUM> formed of an elastic member such as rubber is attached to the neck portion <NUM> and the protrusion <NUM>, whereby the plunger <NUM> is formed (see <FIG>). An attachment hole (not illustrated) is formed in the stopper portion <NUM>, and engages with the neck portion <NUM> and the protrusion <NUM>, so that the stopper portion <NUM> is less likely to be detached from the plunger rod <NUM>.

Specific examples of materials of the stopper portion <NUM> include butyl rubber and silicon rubber. Further, there may be exemplified a styrene-based elastomer or a hydrogenated styrene-based elastomer, or a substance obtained by mixing a styrene-based elastomer or a hydrogenated styrene-based elastomer with polyolefin such as polyethylene, polypropylene, polybutene, and an α-olefin copolymer, oil such as liquid paraffin and process oil, or a powder inorganic substance such as talc, cast, and mica. Further, as the material of the stopper portion <NUM>, a polyvinyl chloride-based elastomer, an olefin-based elastomer, a polyester-based elastomer, a polyamide-based elastomer, a polyurethane-based elastomer, various rubber materials (particularly, a vulcanized material) such as natural rubber, isoprene rubber, chloroprene rubber, nitrile butadiene rubber, and styrene butadiene rubber, or a mixture thereof may be employed. Furthermore, the stopper portion <NUM> pressurizes the ejection solution by sliding within the container <NUM> described below. Thus, a surface of the stopper portion <NUM> and an inner wall surface 75a of the accommodating space <NUM> of the container <NUM> may be coated or processed using various matters, to guarantee/adjust slidability between the stopper portion <NUM> and the inner wall surface 75a of the accommodating space <NUM> of the container <NUM>. Examples of the coating agent may include polytetrafluoroethylene (PTFE), silicon oil, diamond-like carbon, nano diamond, and the like.

Next, the container <NUM> will be described based on <FIG>. Note that <FIG> includes the diagram (a) on the left side that is a cross-sectional view of the container <NUM>, and the diagram (b) on the right side that is an external view of the container <NUM>. The container <NUM> is a member containing an ejection solution to be pressurized by the plunger <NUM>, and is a member that defines a flow path for injecting the pressurized ejection solution to the target region. In view of this, a resin material (a resin material of the same type as the attachment <NUM> for example) may be used for forming the container <NUM>.

The container <NUM> includes an accommodating space <NUM>, in which the stopper portion <NUM> of the plunger <NUM> are movable, accommodating the ejection solution, and a nozzle portion <NUM> including a flow path <NUM> connecting the accommodating space <NUM> to the outside of the container <NUM>. The nozzle portion <NUM> has a columnar outer circumference on the distal end side. Note that in the injector assembly <NUM>, as illustrated in <FIG>, a positional relationship between the plunger <NUM> and the container <NUM> is determined so that the stopper portion <NUM> of the plunger <NUM> can slide within the accommodating space <NUM> in a direction toward the nozzle portion <NUM> (direction toward the distal end side). The ejection solution is sealed in a space defined by stopper portion <NUM> of the plunger <NUM> and the container <NUM>. The flow path of the container <NUM> opens in a distal end surface <NUM> of the nozzle portion <NUM>, so that the ejection port <NUM> is formed. Thus, when the plunger <NUM> slides within the accommodating space <NUM>, the ejection solution accommodated in the accommodating space <NUM> is pressurized to be ejected from the ejection port <NUM> through the flow path <NUM>.

The flow path <NUM> provided in the container <NUM> has a diameter smaller than the inner diameter of the accommodating space <NUM>. With this configuration, the ejection solution that has been applied with a high pressure is ejected to the outside through the ejection port <NUM>. The male thread portion <NUM> for attaching the container <NUM> to the attachment <NUM> is formed on the base end side of the container <NUM>. The male thread portion <NUM> is screwed with the female thread portion <NUM> of the attachment <NUM>.

Note that the profile on the distal end side of the stopper portion <NUM> of the plunger <NUM> is shaped to substantially match the profile of the inner wall surface 75a near a portion where the accommodating space <NUM> and the flow path <NUM> are connected to each other (the deepest part of the accommodating space <NUM>). With this configuration, a smallest possible gap can be formed between the stopper portion <NUM> and the inner wall surface 75a of the container <NUM> when the plunger <NUM> slides for ejecting the ejection solution and reaches the deepest part of the accommodating space <NUM>, whereby the ejection solution can be prevented from wastefully remaining in the accommodating space <NUM>. However, the shape of the stopper portion <NUM> is not limited to a particular shape as long as desired effects can be obtained with the injector <NUM> according to the present embodiment.

Now, how the injector assembly <NUM> is assembled will be described. In a state where the stopper portion <NUM> of the plunger <NUM> is inserted to the deepest part of the accommodating space <NUM> of the container <NUM>, the plunger <NUM> is retracted with the ejection port <NUM> of the container <NUM> in communication with the ejection solution. The stopper portion <NUM> and the inner wall surface 75a of the accommodating space <NUM> are suitably in close contact with each other, the retraction action will produce negative pressure in the accommodating space. Thus, the accommodating space <NUM> can be filled with the ejection solution through the ejection port <NUM>. In this process, the plunger <NUM> is retracted to an extent enough for making the part of the plunger <NUM> (plunger rod <NUM>) protruding from the container <NUM> pass through the second region <NUM> to reach the first region <NUM> (the region 33b illustrated in <FIG>), when the container <NUM> is attached to the attachment <NUM> in this state.

After the container <NUM> filled with ejection solution in the accommodating space <NUM> is attached to the attachment <NUM>, the actuator <NUM> is inserted to the attachment <NUM> from the side of the first region <NUM>. The actuator <NUM> is inserted until the distal end surface of its distal end portion 21b comes into contact with a distal end surface 33c of the region 33b of the attachment <NUM> (see <FIG>). Then, in this process, the male thread portion <NUM> provided to the center portion 21a of the actuator <NUM> is screwed with the female thread portion <NUM> of the attachment <NUM>, whereby the actuator <NUM> and the attachment <NUM> are suitably coupled to each other. Furthermore, in this process, the recess portion <NUM> of the shaft member <NUM> of the piston <NUM>, which is incorporated in the actuator <NUM>, engages with the protrusion <NUM> of the shaft member <NUM> of the plunger <NUM>, and the plunger <NUM> is pushed by the piston <NUM> toward the distal end side. Note that, a fixing force of the piston <NUM> in the distal end portion 21b of the actuator <NUM> is set to an extent that the piston <NUM> can slide in the distal end portion 21b in a sufficiently smooth manner by a pressure received from the combustion product produced by the igniter <NUM>, and to an extent that the piston <NUM> can suitably resist force received from the plunger <NUM> so that the position of the piston <NUM> is not displaced when the injector assembly <NUM> is assembled. Alternatively, a stopper may be formed at an intended position of the piston <NUM>, so that the top surface of the first flange <NUM> of the piston <NUM> faces the combustion chamber 20a of the actuator <NUM> and is not displaced toward the combustion chamber 20a as illustrated in <FIG>.

Thus, when the actuator <NUM> is attached to the attachment <NUM> to which the container <NUM> and plunger <NUM> are attached as described above, the plunger <NUM> is pushed to move from the piston <NUM> toward the distal end side, whereby the plunger <NUM> is positioned at a predetermined position within the container <NUM>. Note that, in response to pressing of the plunger <NUM>, a part of the ejection solution is discharged from the ejection port <NUM>.

When the plunger <NUM> is thus positioned at the final position as described above, formation of the injector assembly <NUM> is completed. In this injector assembly <NUM>, the position of the stopper portion <NUM> of the plunger <NUM> in the accommodating space <NUM> of the container <NUM> is mechanically determined. The final position of the stopper portion <NUM> is a position uniquely determined in the injector assembly <NUM>, and hence an amount of the ejection solution that is finally stored in the accommodating space <NUM> in the injector assembly <NUM> can be a predetermined amount determined in advance.

The injector assembly <NUM> thus configured can be loaded into the housing <NUM> with the ignition pin 22b of the igniter <NUM> fitted into the socket <NUM> on the housing <NUM>, whereby the injector <NUM> is prepared to be usable (see <FIG>). The user holds the housing <NUM> of such injector <NUM> with one hand and slides the first switch <NUM> located on the back side of the housing <NUM> for a predetermined period of time, putting the injector <NUM> in the standby state. In this state, when the user presses the second switch <NUM> with the ejection port <NUM> being in contact with the target region, the igniter <NUM> is actuated, and the ejection solution is pressurized via the piston <NUM> and the plunger <NUM>. Thus, the ejection is implemented, and the ejection solution is injected into the target region.

A first embodiment of a safety mechanism of the injector <NUM> will now be described based on <FIG> is a diagram illustrating a schematic configuration of the plunger <NUM>. <FIG> include a diagram (a) on the left side that is an external view of the plunger <NUM> (similar to one illustrated in <FIG>) and a diagram (b) which is an external view illustrated the plunger <NUM> rotated by <NUM> degrees from the state illustrated in the diagram (a). The plunger <NUM> has the reduced diameter portion <NUM> provided with one recess portion <NUM> having a circular opening shape. Thus, the recess portion <NUM> is positioned on one side in the circumferential direction of the reduced diameter portion <NUM>.

Now, the diameter and depth of the recess portion <NUM> will be described based on <FIG>. The diagram (a) on the left side in <FIG> illustrates a state of movement of plunger <NUM> in a case where the force applied to the plunger <NUM> via the piston <NUM> by actuation of igniter <NUM> (hereinafter referred to as "driving force") is equal to or smaller than predefined force. On the other hand, the diagram (b) on the right side in <FIG> illustrates a state of movement of the plunger <NUM> in a case where the driving force applied to the plunger <NUM> exceeds the predefined force. Note that the predefined force in the present application is an upper limit value of the driving force assumed to be safely applicable for pressurizing the ejection solution accommodated in the accommodating space <NUM>. In other words, the injector <NUM> is designed such that injection to the target region can be implemented with the ejection solution safely ejected as long as the driving force applied to the plunger <NUM> is equal to or smaller than the predefined force. Here, the expression "safely" refers to a state in which the ejection solution is suitably ejected from the ejection port <NUM> without inappropriately leaking or scattering due to breakage or the like of the container <NUM> which is a part of the injector <NUM>. Thus, the expression has nothing to do with the biological effect of the ejection solution on the target region or the like. It should also be noted that in <FIG>, the gap between the inner wall surfaces (inner wall surface on the side of the attachment <NUM> and the inner wall surface on the side of the container <NUM>) of a path (hereinafter, referred to as a "movement path") in which the plunger <NUM> moves and the plunger <NUM> is illustrated in an exaggerated manner, for the sake of easier understanding of the behavior of the plunger <NUM>.

The diameter and the depth of the recess portion <NUM> of the plunger <NUM> are determined so that as long as the driving force applied to the plunger <NUM> does not exceed the predefined force, the plunger <NUM> does not substantially deform as illustrated in <FIG>, that is, the shaft member <NUM> in a straight state before the application of the driving force moves in the movement path. In other words, the recess portion <NUM> is designed so that the ejection solution can be suitably pressurized with the plunger <NUM> even when the recess <NUM> is provided, as long as the driving force applied to the plunger <NUM> does not exceed the predefined force.

Further, the recess portion <NUM> of plunger <NUM> is designed to cause intended deformation of the plunger <NUM> as illustrated in <FIG> when the driving force applied to plunger <NUM> exceeds the predefined force (also referred to as "excessive driving force"). Specifically, the diameter and depth of the recess portion <NUM> is determined such that when the driving force exceeds the predefined force, the plunger <NUM> bends near a position P1 of the plunger <NUM> in the axial direction at which the recess <NUM> is provided. With the recess portion <NUM>, the strength against the driving force decreases in the vicinity of the recess <NUM> compared with a portion where the recess <NUM> is not provided, and thus the plunger <NUM> is deformed as described above. Thus, the recess portion <NUM> serves as a weakened part of the plunger <NUM>. Such a deformation results in a part of the energy of the excessive driving force being consumed due to the deformation, and also results in the stopper portion <NUM> of the plunger <NUM> being in contact with one side of the inner wall surface 75a of the container <NUM>. In <FIG>, the stopper portion <NUM> is in contact with the inner wall surface 75a in a right side region R1 but is not in contact with the inner wall surface 75a in a left side region R2. Furthermore, the deformation of the plunger <NUM> in the recess portion <NUM> (the weakened part) leads to stronger or larger contact between the plunger <NUM> and the inner wall surface on the side of the attachment <NUM> and the inner wall surface on the side of the container <NUM>, resulting in relatively large frictional force or resistance against the sliding motion of the plunger <NUM>.

When the one-sided contact state is achieved in this manner, the frictional force during movement, that is, while the plunger <NUM> moves within the accommodating space <NUM>, becomes greater than frictional force during movement without the one-sided contact state achieved (that is, the state illustrated in <FIG>). Thus, the energy of the excessive driving force is further consumed by the frictional force between the stopper portion <NUM> and the container <NUM> and the frictional force between the plunger <NUM> and the attachment <NUM>. As a result, when the excessive driving force is applied to the plunger <NUM>, the distal end of the stopper portion <NUM> can be prevented from reaching the deepest part of the accommodating space <NUM>, or the distal end of the stopper portion <NUM> reaches the inner wall surface but with the magnitude of the force applied to the inner wall surface until the distal end reaches the inner wall surface regulated within a predetermined range enabling the normal shape of the container <NUM> to be maintained. Note that the predetermined range of force not causing breakage of the container <NUM> resulting in scattering or the like of the ejection solution.

In the injector <NUM>, the driving force applied to the plunger <NUM> by the actuation of the igniter <NUM> is designed to fall within a range not exceeding the predefined force, but even with such a configuration, the injector <NUM> cannot be completely free of risk of the driving force exceeding the predefined force due to some unexpected reason. When the plunger <NUM> does not deform by receiving the application of excessive driving force overwhelming the predefined force, the energy of the excessive driving force pressurizes the ejection solution via the plunger <NUM>. Thus, depending on the pressurizing force, the container <NUM> may unintentionally break. In view of this, the injector <NUM> of the present application has the recess portion <NUM> provided to the plunger <NUM>. Thus, even when excessive driving force is applied to the plunger <NUM>, the plunger <NUM> is intentionally deformed so that the energy of the excessive driving force is consumed. As a result, the load on the container <NUM> can be reduced, and thus the container <NUM> can be prevented from breaking.

Thus, the recess portion <NUM> serving as a weakened part of the plunger <NUM> functions as a safety mechanism of the injector <NUM> in a case where the excessive driving force is applied to the plunger <NUM>, whereby the safety of the injection using the injector <NUM> can be guaranteed. Note that while in the embodiment described above, one recess portion <NUM> is provided in the reduced diameter portion <NUM>, a plurality of the recess portions <NUM> may be arranged side by side in the circumferential direction of the reduced diameter portion <NUM>. With this configuration, the plurality of recess portions <NUM> are preferably arranged in a one-sided manner rather than being arranged at an equal interval along the circumferential direction of the reduced diameter portion <NUM>, so that the plunger <NUM> can bend in a certain direction upon receiving excessive driving force. One or a plurality of the recess portions <NUM> may be not only provided in the reduced diameter portion <NUM> but may also be provided on the shaft member <NUM>.

A second embodiment of a safety mechanism of the injector <NUM> will now be described based on <FIG> is a diagram illustrating a schematic configuration of the plunger <NUM>. <FIG> include a diagram (a) on the left side that is an external view of the plunger <NUM> and a diagram (b) which is an external view illustrated the plunger <NUM> rotated by <NUM> degrees from the state illustrated in the diagram (a). The plunger <NUM> has a notch <NUM>, corresponding to a groove portion, formed in the reduced diameter portion <NUM> and extending over a portion thereof in the circumferential direction. The deepest portion in notch <NUM> is defined as a notch bottom portion 82a. The notch <NUM> has a notch depth ND between the surface position of the reduced diameter portion <NUM> and the notch bottom portion 82a, and a notch width NW as an axial direction length of a portion of the plunger <NUM> where the notch <NUM> is formed.

The depth ND and the width NW of the notch <NUM> will now be described based on <FIG>. A diagram (a) on the left side in <FIG> illustrates a state of movement of the plunger <NUM> when the driving force not exceeding the predefined force is applied to the plunger <NUM> due to the actuation of the igniter <NUM>, and a diagram (b) on the right side in <FIG> illustrates a state of movement of the plunger <NUM> when the driving force exceeding the predefined force is applied to the plunger <NUM>. It should be noted that also in <FIG>, the gap between the inner wall surfaces of the movement path of the plunger <NUM> and the plunger <NUM> is illustrated in an exaggerated manner, for the sake of easier understanding of the behavior of the plunger <NUM>, as in <FIG>.

The depth ND and the notch width NW of the notch <NUM> of the plunger <NUM> are determined so that as long as the driving force applied to the plunger <NUM> does not exceed the predefined force, the plunger <NUM> does not substantially deform as illustrated in <FIG>, that is, the shaft member <NUM> in a straight state before the application of the driving force moves in the movement path. In other words, in <FIG>, the notch <NUM> is designed so the notch width NW of the notch <NUM> is maintained to be initially L1 and thus the ejection solution can be suitably pressurized by the plunger <NUM> even when the notch <NUM> is provided, as long as the driving force applied to the plunger <NUM> does not exceed the predefined force.

Furthermore, the notch <NUM> of the plunger <NUM> is designed to make the plunger <NUM> intentionally deform as illustrated in <FIG>, when the excessive driving force is applied to the plunger <NUM>. Specifically, the depth ND and the notch width NW of the notch <NUM> are determined so that when the driving force exceeds the predefined force, the plunger <NUM> is buckled around the notch bottom portion 82a of the notch <NUM> to have the axial direction length reduced. In <FIG>, the notch width NW of the notch <NUM> is reduced to L2 which is shorter than the initial L1. By providing the notch <NUM>, the strength against the driving force decreases in the vicinity thereof compared with a portion where the notch <NUM> is not provided, and thus the plunger <NUM> deforms as described above. Thus, the notch <NUM> serves as a weakened part of the plunger <NUM>. Such a deformation results in a part of the energy of the excessive driving force being consumed due to the deformation, and also results in the plunger <NUM> itself being shorter. As a result, when the excessive driving force is applied to the plunger <NUM>, the distal end of the stopper portion <NUM> can be prevented from reaching the deepest part of the accommodating space <NUM>, or the distal end of the stopper portion <NUM> reaches the inner wall surface but with the magnitude of the force applied to the inner wall surface until the distal end reaches the inner wall surface regulated within a predetermined range enabling the normal shape of the container <NUM> to be maintained. Note that the predetermined range is the same as that in the first embodiment described above.

Thus, also in the injector <NUM> of the second embodiment, even when excessive driving force is applied to the plunger <NUM>, the plunger <NUM> is intentionally deformed due to the notch <NUM> so that the energy of the excessive driving force is consumed. As a result, the load on the container <NUM> can be reduced, and thus the container <NUM> can be prevented from breaking, as in the injector of the first embodiment. Thus, the notch <NUM> serving as a weakened part of the plunger <NUM> functions as a safety mechanism of the injector <NUM> in a case where the excessive driving force is applied to the plunger <NUM>, whereby the safety of the injection using the injector <NUM> can be guaranteed. Note that while in the embodiment described above, the notch <NUM> is provided in the circumferential direction over a portion of the reduced diameter portion <NUM>, the notch <NUM> may be provided in the circumferential direction over an entirety of the reduced diameter portion <NUM>.

A third embodiment of a safety mechanism of the injector <NUM> will now be described based on <FIG> is a diagram illustrating a schematic configuration of the plunger <NUM>. <FIG> include a diagram (a) on the left side that is an external view of the plunger <NUM> and a diagram (b) which is an external view illustrated the plunger <NUM> rotated by <NUM> degrees from the state illustrated in the diagram (a). The plunger <NUM> has a through hole <NUM> formed having a circular opening shape in the reduced diameter portion <NUM>, the through hole <NUM> serving as the weakened part of the plunger <NUM>.

Specifically, as described above in the second embodiment, the diameter of the through hole <NUM> is determined so as not to cause deformation of the plunger <NUM> as long as the driving force applied to the plunger <NUM> does not exceed the predefined force. The diameter is determined to make the through hole <NUM> crushed when the driving force applied to the plunger <NUM> exceeds the predefined force, so that the plunger <NUM> is buckled to have the axial direction length reduced. With the through hole <NUM> thus formed, the through hole <NUM> serving as a weakened part of the plunger <NUM> functions as a safety mechanism of the injector <NUM> in a case where the excessive driving force is applied to the plunger <NUM>, whereby the safety of the injection using the injector <NUM> can be guaranteed.

Other embodiments with the through hole serving as the weakened part is provided as described above will be described based on <FIG> illustrate another schematic configuration of the plunger <NUM> of the present disclosure, and <FIG> illustrates a schematic configuration of a plunger <NUM>' of a comparative example. Note that the plungers <NUM> illustrated <FIG> is the same as the plunger <NUM> illustrated in <FIG>, except for the arrangement of the through hole <NUM>. Specifically, the plunger <NUM> illustrated in <FIG> has the through hole <NUM> provided at substantially the center of the plunger rod <NUM> in the axial direction. The plunger <NUM> illustrated in <FIG> has the through hole <NUM> provided closer to the stopper portion <NUM> than the through hole <NUM> arranged as illustrated in <FIG> is. The plunger <NUM>' illustrated in <FIG> has no configuration corresponding to reduced diameter portion <NUM> provided to a plunger rod <NUM>' nor has the through hole <NUM>. In other words, the plunger <NUM>' only includes the plunger rod <NUM>' and a stopper portion <NUM>'.

Experimentation is performed with <NUM>µl of ejection solution ejected using the injectors <NUM> including the plungers <NUM> and <NUM>' illustrated in <FIG>, with a predetermined amount of ZPP used as the ignition charge. Note that the amount of ZPP used is determined to intentionally apply excessive pressure on the ejection solution when <NUM>µl of ejection solution is ejected.

This ejection experiment clearly indicates that the container <NUM> can be favorably prevented from breaking by providing, at least in the plunger <NUM>, the reduced diameter portion <NUM> and the through hole <NUM> provided at a portion closer to the stopper <NUM> than the center of the plunger rod <NUM>. A more detailed inspection on the modes in <FIG> has revealed that the plunger rod <NUM> has deformed before the stopper portion <NUM> of the plunger <NUM> has reached the deepest part of the accommodating space <NUM> of the container <NUM>. Thus, it can be concluded that with the plungers <NUM> of the modes in <FIG>, the plunger rod <NUM> favorably deforms in response to an excessive pressure to the ejection solution during a process in which the plunger <NUM> moves in the accommodating space <NUM> due to the combustion of the ignition charge, so that application of excessive pressure on the inner wall surface of the container <NUM> can be prevented, and thus the container <NUM> can be prevented from breaking.

The plunger <NUM> may be designed to have the reduced diameter portion <NUM> with the thickness adjusted, so that the reduced diameter portion <NUM> itself buckles when the driving force applied to the plunger <NUM> exceeds the predefined force, instead of providing the notch <NUM> or the through hole <NUM> as described above. The plunger <NUM> thus configured can have the reduced diameter portion <NUM> serving as the weakened part described above.

Claim 1:
A needleless injector (<NUM>) configured to eject a substance intended for injection to a target region without using an injection needle, the needleless injector comprising:
a housing part that includes an accommodating space (<NUM>) that accommodates the substance intended for injection and defines a flow path (<NUM>) from the accommodating space (<NUM>) so that the substance intended for injection is ejected to the target region;
a driving part that imparts ejection energy to eject the substance intended for injection; and
a plunger (<NUM>) that defines the accommodating space (<NUM>) and is disposed to move in the housing part by the ejection energy and pressurize the substance intended for injection, characterized in that
the plunger (<NUM>) includes a weakened part (<NUM>, <NUM>, <NUM>, <NUM>) that causes a part of the plunger (<NUM>) to deform and therefore to consume a part of the ejection energy when the force applied to the plunger exceeds a
predefined force while the driving part is imparting the ejection energy with the accommodating space accommodating the substance intended for injection, such that a distal end of the plunger (<NUM>) does not reach a deepest part of the housing part or such that force received by an inner wall surface of the housing part falls within a predetermined range until the distal end of the plunger reaches the deepest part, the predetermined range being a range of force applied to the housing part without damage to the housing part.