GB2107988A

GB2107988A - Wristwatch driven infuser

Info

Publication number: GB2107988A
Application number: GB08127254A
Authority: GB
Inventors: Basil Martin Wright
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK
Priority date: 1981-09-09
Filing date: 1981-09-09
Publication date: 1983-05-11
Also published as: GB2107988B

Abstract

An infuser for drugs such as insulin comprises a syringe having a very short stroke and a relatively large bore 2, e.g. with a ratio of 1:10. The piston 3 of the syringe is driven through gearing and a screw thread mechanism 10-13 by a wristwatch movement 7. This can be employed complete with hands and a dial to give an indication of the operation of the infuser. <IMAGE>

Description

SPECIFICATION Wristwatch driven infuser This invention relates to powered syringe devices whereby drug infusions may be administered. Several such devices exist and examples are described in British Patent Specification No. 1,528,358 and British Patent Application No. 9489/77 (for a patent of addition thereto), and British Patent Application No.

9947/77.

These devices have proved efficacious but experience indicates that for a great many clinical uses, and especially in under-developed countries, there is still a need for devices which are simpler, lighter, smaller and, above all, cheaper. In clinical practice a great many injections are given at a constant speed in one size of syringe and often volumes of only 1-2 ml. per 24 hours are infused. Examples are analgesics, heparin and insulin. The use of an infuser for insulin is, however, unlikely to become widespread unless the device is really small, simple and reliable.The actual injection of such small volumes over such long periods requires only negligible power, even against considerable back pressure, but plastic syringes with rubber pistons become very stiff when rnoved slowly and most of the energy used in present devices is required to move the piston.

According to this invention there is provided an infuser comprising a syringe which has very low power requirements and can be driven by a wristwatch movement.

It has been found that cheap watches, which have more power than more expensive ones, have amply sufficient spare power when a suitable syringe is used.

It being desired to provide an infuser as flat as possible the syringe is made with a very short stroke and a relatively large bore, typically with a ratio of about 1:10.

Syrines using a conventional 'O'-ring-sealed or solid rubber piston are unacceptably stiff because, if the contact between the rubber seal and the syringe wall is firm enough to be perfectly watertight, molecular adhesion develops at slow rates of movement This not only makes it stiff but also causes intermittent stick-slip movement which may be unacceptabie if small volumes of powerful drugs are being given. According to this invention a rolling 'O'-ring seal is therefore used, i.e. the piston of the syringe is sealed with a soft 'O'-ring which is a tight fit between the piston and the cylinder of the syringe, the ring being free to roll when the piston is moved axially, It has been found that, provided the ring is of soft rubber and its thickness is small compared with its diameter, friction and stick-slip are acceptably low.

An infuser in accordance with the invention is described below with reference to the accompanying drawing in which:~ Figure 1 is a sectional elevation of the infuser, Figure 2 is a plan view thereof, Figure 3 is a fragmentary view of a stopping mechanism for the infuser, and Figure 4 is a fragmentary sectional elevation of an optional two-speed drive.

Referring to Figure 1 an infuser comprises a syringe consisting of a flat cylindrical block 1, conveniently made of a tough transparent material such as polycarbonate, having a cylindrical cavity 2 with a major and a minor bore. A piston 3 (shown in the full position), has two corresponding diameters and an 'O'-ring 4 is fitted between the minor diameter of the piston and the major bore of the cylinder. The ring 4 is sufficiently compressed to make a seal. When the syringe is empty the minor diameter of the piston fills the minor bore of the cylinder and the 'O'-ring fills the space between the steps on the cylinder and the piston so that there is no dead space in the syringe. Any tendency of the ring to roll out of true is also corrected each time the syringe is emptied.The 'O'-ring may be moulded, or fabricated from cord, in which case it has little or no set so that it will roll more freely. If it is moulded it will have an elastic resistance to rolling but as it will not normally roll through more than half its circumference, it acts like a spring tending to empty the syringe. As the ring rolls, its inner and outer surfaces are alternately expanded and compressed. This leads to some frictional resistance but it is very much less than that produced by sliding. In the infuser so far developed the volume of the syringe is of the order of 3 ml.Since however the friction from a rolling seal probably varies directly as the radius of the cylinder while its volume varies as the square, it is possible that a substantially larger syringe having for example a volume of 12 or even 24 ml. to be delivered in 24 hours could be operated without requiring a much more powerful drive than that described below.

The cylinder 1 is fitted with a Luer nozzle 5 and the piston has a central blind threaded hole 6. The syringe is secured to the body of the infuser in an easily removable manner, e.g. by a bayonet fitting, so that it can be sterilised or replaced when necessary. It can also be chemically sterilised in situ.

Referring to Figures 1 and 2 the syringe is driven by a wristwatch movement 7 without its case but complete with dial and hands. A pin pallet 12 ligne Roskopf movement has been found to be suitable. It is fitted into a recess in the body of the infuser so that its dial shows through a standard crystal window 8.

As in most modern watches, the toothed rim of the spring barrel 9 is right at the edge of the movement so that power can be taken off by engaging another wheel with it. No modification to the movement is required and another spring barrel can be used as a take-off wheel 10.

The spindle of the take-off wheel 10 carries a pinion 1 1 which engages with a driving wheel 12 through an idler wheel 13 at, in this instance, a 1:7.5 ratio. The take-off wheel 10 is not fast to the pinion 11 but is connected to it through a click mechanism (not shown), which makes it possible to rotate the driving train manually in either direction without disengaging the watch movement. The click mechanism (which may be housed within the spring barrel of the take-off wheel 10) may comprise an eight pointed star wheel so that each click represents 30 minutes of time, or slightly more than 2% of the 24-hour volume of the syringe. Preferably it is arranged that the action of the click mechanism may be both heard and felt by the operator.Since the wheel 10 and pinion 11 normally rotate together, the click does not load the driving mechanism and can be made stiff enough to make the drive positive. The spring barrel makes six revolutions in 24 hours so that the driving wheel 12 makes one in 30 hours. The driving wheel carries a 3 start screw threaded boss 14 which screws into the threaded centre hole 6 of the piston 3. It is convenient to use a syringe with a delivery rate of e.g. O. 1 ml/hr (or some multiple thereof, e.g. .2, .4 or .5) with 25% additional capacity available, giving 2.4 ml in 24 hours and a total volume of 3.0 ml in 30 hours. The volume delivered may be indicated by a scale engraved on the back of the driving wheel 12 and viewed through a window (not shown) in the back of the body of the infuser.

In order to avoid confusion the rotation of the driving wheel 12 is limited to 30/32 of a revolution by a pin which strikes the stopping lever 17 described below. The dial is indexed from 0--30 inclusive so that there can be no doubt whether the syringe is full or empty.

Reverting to the transmission between the watch movement and the driving wheel 12, a second idler wheel 15 is provided which meshes with the idler wheel 13 and carries a knurled thumbwheel 16 of larger diameter which projects through the side of the body of the infuser so that the syringe can be filled and emptied manually using the click mechanism to deliver fixed small doses.

The drive provided by the above described arrangements is positive, so that there is no danger of an infusion running in under gravity, and the relation between the volume delivered and the time indicated by the watch is reasonably constant. The watch is normally set for real time at the beginning of the infusion so that, provided it keeps time, the user can be certain that the infusion has been given, and the second-hand indicates at any time whether it is still running. It is therefore essential that the watch will stop if the flow is obstructed or when the syringe empties.

Owing to the very high gear ratio and slow movement, only about 2.4 mm in 24 hours, the watch takes a long time to build up enough pressure to stop itself. The driving wheel 12 is therefore mounted so as to be capable of a small axial movement which is transmitted to a lever 17 which is pivoted in the plane of the movement and spring loaded against a 450 conical washer 19, see figure 3, housed under the boss 14 of the driving wheel 12. The washer rests on two 450 planes 20 at right angles and on a 450 plane on the lever 17. The other end of the lever carries a fine wire 21 perpendicular to the plane of the movement and touching the rim of the balance wheel 18 when the lever is deflected by the downward movement of the driving wheel 12.

This arrangement has the advantage that the balance wheel is oscillated again when the brake is released. The pivot 22 of the lever 1 7 is eccentrically mounted so that its position can be adjusted by means of an external slotted stud, providing a fine adjustment for the stopping mechanism. The lever stopping mechanism can also be used as an electrical switch to operate a battery operated alarm if one is required.

Normally, for all except intra-arterial injections, little axial force is required apart from that needed to overcome rolling friction, which is almost constant but, if the force rises above a certain preset level the driving wheel is pushed down and stops the balance wheel 18. Because the length of the lever amplifies the movement of the driving wheel and because the movement and force required to stop the balance are both small, the axial movement of the driving wheel can be very small, e.g. 0.02 mm, so that the delay before braking occurs is only a few minutes or at most only about an hour.

For some purposes, especially insulin infusions, it is desirable to have two speeds available. This can be accomplished as indicated in Figure 4 by fitting two pinions 11 a and 11 b to the take-off wheel 10 having for example 12 and 21 teeth respectively and fitting the idler wheel 13 with two sets of teeth, the larger having for example 72 teeth and the smaller 63. The two sets are engaged alternatively but sliding the idler wheel 13 axially, the driving wheel 12, which has 144 teeth, being thickened to keep it in engagement.

With this arrangement alternative drive ratios of 12:1 and 6:1 are thus provided. If these drive ratios are employed in place of the 1:7.5 ratio of the basic arrangement described above then at the faster speed 20% of the volume is available for booster doses and, at the slower speed 60%, which is desirable for many diabetic regimes.

Although a cheap wristwatch is a very efficient, cheap and satisfactory control mechanism and power supply, it may well be possible to use an electronically controlled, battery-powered stepping motor. Such a device would have the advantage of a wide speed-variability and where this is essential the substantial extra cost and the nuisance of battery power might be worthwhile and the rest of the mechanism would be quite suitable.

A substantial advantage of the use of a watch movement complete with dial and hands is that the user can think in time units except when actually filling the syringe. For this purpose it is necessary that the syringe shall have a known volume, and minimum dead space when empty.

Since the volume contained is indicated on the dial on the back of the driving wheel, the user can draw up the required number of units of insulin into the syringe and then fill it up with diluting fluid. Thereafter, the user need think only in time units, whether the infusion is continuous or by boluses. In particular, when giving a bolus, he can check whether there is sufficient liquid left to last until the end of 24 hours.

Claims

1. A device for administering drug infusions comprising a syringe the piston of which is adapted to be driven by a wristwatch movement.

2. A device as claimed in Claim 1 in which the wristwatch movement is a spring driven movement complete with hands and a dial.

3. A device as claimed in Claim 1 or 2 in which the syringe has a short stroke and a relatively large bore such that the ratio of the stroke to the bore is of the order of 1 :10.

4. A device as claimed in Claim 1,2 or 3 in which the piston is sealed by a rolling 0-ring arranged to be trapped and aligned at the completion of each stroke of the piston between complementary abutments on the piston and the cylinder of the syringe.

5. A device as claimed in Claim 4 in which the abutments are so located that when the 0-ring is trapped therebetween no dead space remains within the cylinder of the syringe.

6. A device as claimed in any one of the preceding claims in which means are provided to stop the wristwatch movement in the event of pressure within the syringe rising above a predetermined maximum.

7. A device as claimed in any one of the preceding claims in which the wristwatch movement is arranged to rotate a driving wheel which in turn rotates a screw thread mechanism acting on the piston to drive the same.

8. A device as claimed in Claim 7 in which the driving wheel is mounted for axial movement, against a bias, by pressure within the syringe upon that pressure reaching a predetermined maximum, the axial movement of the driving wheel being arranged to operate a brake acting on the wristwatch movement.

9. A device as claimed in Claim 8 in which the brake includes a pivotted lever arranged to contact the balance wheel of the wristwatch movement.

10. A device as claimed in Claim 6, 8 or 9 in which the wristwatch movement stopping means or brake are arranged upon operation to initiate an alarm.

11. A device as claimed in Claim 7, 8 or 9 in which the driving wheel is arranged to complete a stroke of the piston upon less than one revolution and means are provided to arrest the driving wheel upon completion of the said stroke.

12. A device as claimed in any one of the preceding claims in which the piston is driven via gearing whereby, with the wristwatch movement running at normal speed, a stroke of the piston is completed in approximately twenty-four hours or longer.

13. A device as claimed in any one of the preceding claims in which provision is made to select between at least two gear ratios in a driving train between the wristwatch movement and the piston.

14. A device as claimed in Claim 12 or 13 in which provision is made for manual operation of the gearing.

15. A device for administering drug infusions constructed, arranged and adapted to operate substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.

16. A device for administering drug infusions constructed, arranged and adapted to operate substantially as herein described with reference to Figures 1, 2 and 3 of the accompanying drawings.

17. A device for administering drug infusions constructed, arranged and adapted to operate substantially as herein described with reference to Figures 1, 2 and 4 of the accompanying drawings.