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A drive mechanism for delivery of infusion medium a coil capable of being electrically activated to provide an electromagnetic field. The coil surrounds a piston channel extending in an axial direction. An armature is located adjacent the coil, on one side of the axial channel. The armature is movea

A drive mechanism for delivery of infusion medium a coil capable of being electrically activated to provide an electromagnetic field. The coil surrounds a piston channel extending in an axial direction. An armature is located adjacent the coil, on one side of the axial channel. The armature is moveable toward a forward position, in response to the electromagnetic field produced by activation of the coil. A piston is located within the piston channel and is moveable axially within the channel to a forward position, in response to movement of the armature to its forward position. The armature and piston are moved toward a retracted position, when the coil is not energized. The armature may be configured with a reduced diameter by including a coil cup for supporting the coil including a shelf portion defining at least a portion of a pole surface of the coil cup.

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1. A coil cup composed of a magnetizable material, the coil cup comprising:a generally annular inner wall having one end defining an inner pole surface of the coil cup; a generally annular outer wall having a first inner diameter and a first outer diameter, the outer wall having a generally annular

1. A coil cup composed of a magnetizable material, the coil cup comprising:a generally annular inner wall having one end defining an inner pole surface of the coil cup; a generally annular outer wall having a first inner diameter and a first outer diameter, the outer wall having a generally annular shelf portion extending from the outer wall towards the inner wall, the shelf portion having a second inner diameter and a second outer diameter, the second inner diameter being smaller than the first inner diameter, the shelf portion having a second end defined by the second inner and outer diameters, the second end defining at least a portion of an outer pole surface of the coil cup; and a generally annular interior between the inner and outer walls, the annular interior containing a coil, wherein the shelf portion has an angled edge. 2. A coil cup composed of a magnetizable material, the coil cup comprising:a generally annular inner wall having one end defining an inner pole surface of the coil cup; a generally annular outer wall having a first inner diameter and a first outer diameter, the outer wall having a generally annular shelf portion extending from the outer wall towards the inner wall, the shelf portion having a second inner diameter and a second outer diameter, the second inner diameter being smaller than the first inner diameter, the shelf portion having a second end defined by the second inner and outer diameters, the second end defining at least a portion of an outer pole surface of the coil cup; and a generally annular interior between the inner and outer walls, the annular interior containing a coil, wherein the second outer diameter is equal to the first outer diameter. 3. A coil cup composed of a magnetizable material, the coil cup comprising:a generally annular outer wall having one end defining an outer pole surface of the coil cup; a generally annular inner wall having a first inner diameter and a first outer diameter, the inner wall having a generally annular shelf portion extending from the inner wall towards the outer wall, the shelf portion having a second inner diameter and a second outer diameter, the second outer diameter,being greater than the first outer diameter, the shelf portion having a second end defining at least a portion of an inner pole surface of the coil cup; and a generally annular interior between the outer and inner walls, the annular interior containing a coil. 4. The coil cup recited in claim 3, wherein the shelf portion has an angled edge.5. The coil cup recited in claim 3, wherein the second inner diameter is equal to the first inner diameter.6. A coil cup composed of a magnetizable material, the coil cup comprising:a generally annular outer wall having a first inner diameter and a first outer diameter, the outer wall having a generally annular outer shelf portion extending from the outer wall towards a generally annular inner wall, the outer shelf portion having a second inner diameter and a second outer diameter, the second inner diameter being smaller than the first inner diameter, the outer shelf portion having one end defined by the second inner and outer diameters, the one end defining at least a portion of an outer pole surface of the coil cup; a generally annular inner wall having a third inner diameter and a third outer diameter, the inner wall having the generally annular inner shelf portion extending from the inner wall towards the outer wall, the inner shelf portion having a fourth inner diameter and a fourth outer diameter, the fourth outer diameter being greater than the third outer diameter, the inner shelf portion having a second end defining at least a portion of an inner pole surface of the coil cup; and a generally annular interior between the outer and inner walls, the annular interior containing a coil. 7. The coil cup recited in claim 6, wherein the outer shelf portion has an angled edge.8. The coil cup recited in claim 6, wherein the inner shelf portion has an angled edge.9. The coil cup recited in claim 6, wherein the second outer diameter is equal to the first outer diameter.10. The coil cup recited in claim 6, wherein the fourth inner diameter is equal to the third inner diameter.11. A drive mechanism comprising:a piston channel; a coil surrounding the piston channel; a piston located within the piston channel and moveable axially within the piston channel; a coil cup supporting the coil, the coil cup including: a generally annular inner wall having one end defining an inner pole surface of the coil cup; a generally annular outer wall having a first inner diameter and a first outer diameter, the outer wall having a generally annular outer shelf portion extending from the outer wall towards the inner wall, the outer shelf portion having a second inner diameter and a second outer diameter, the second inner diameter being smaller than the first inner diameter, the outer shelf portion having a second end defined by the second inner and outer diameters, the second end defining at least a portion of an outer pole surface of the coil cup; and a generally annular interior between the inner and outer walls, the annular interior containing the coil; and an armature disposed adjacent the coil on one side of the piston channel the armature having inner pole and outer pole surfaces for acting with the inner and outer pole surfaces of the coil cup to provide an electromagnetic flux path upon electrical activation of the coil, the electromagnetic flux path for drawing the armature towards the coil cup and for moving the piston axially within the piston channel. 12. The drive mechanism recited in claim 11, wherein the inner wall has a third inner diameter and a third outer diameter, the inner wall having a generally annular inner shelf portion extending from the inner wall towards the outer wall, the inner shelf portion having a fourth inner diameter and a fourth outer diameter, the fourth outer diameter being greater than the third outer diameter, the fourth inner and outer diameters defining a at least a portion of inner pole surface of the coil cup.13. The drive mechanism as recited in claim 11, wherein the inner pole and outer pole surfaces of the armature have a generally annular shape, the inner and outer pole surfaces of the armature each being made of a magnetizable material, wherein the inner pole surface of the armature faces the inner pole surface of the inner wall of the coil cup, and the outer pole surface of the armature faces the outer pole surface of the coil cup.14. The drive mechanism recited in claim 13, wherein the armature has a generally annular shape and has an outer diameter smaller than the first outer diameter of the outer wall of the coil cup.15. The drive mechanism recited in claim 13, wherein the armature has a generally annular shape and has an outer diameter smaller than the first inner diameter of the outer wall of the coil cup.16. The drive mechanism recited in claim 13, wherein the armature has a generally annular shape and has an outer diameter smaller than the second outer diameter of the outer shelf portion of the coil cup.17. The drive mechanism recited in claim 13, wherein the outer pole surface of the armature faces only a portion of the outer shelf, the portion of the outer shelf being the portion having a diameter smaller than the first inner diameter of the outer wall of the coil cup.18. The drive mechanism recited in claim 13, wherein the outer pole surface of the armature faces a portion of the outer shelf having a diameter smaller than the first inner diameter of the outer wall of the coil cup and greater than the second inner diameter of the outer shelf portion.19. The drive mechanism recited in claim 11, wherein the outer shelf portion has an angled edge.20. The drive mechanism recited in claim 12, wherein the inner shelf portion has an angled edge.21. The drive mechanism recited in claim 11, wherein the piston channel is surrounded by the inner wall and the piston channel is substantially coaxial with the coil.22. The drive mechanism as recited in claim 11, wherein the piston and armature are composed of a single, unitary structure.23. The drive mechanism as recited in claim 11, wherein the piston and armature are composed of separable structures and are moveable independent of each other.24. A drive mechanism as recited in claim 11, further including an outlet chamber adjacent to the piston channel and means for urging the armature and piston to move in an axial direction away from the outlet chamber.25. A drive mechanism as recited in claim 24, wherein the means for urging includes a spring.26. A drive mechanism as recited in claim 24, wherein the means for urging includes a magnet.27. A drive mechanism as recited in claim 11, further including:an outlet chamber adjacent to the piston channel; a valve member moveable between open and closed positions to selectively open and close one end of the piston channel to the outlet chamber; and a valve spring for urging the valve member in the closed position; wherein the valve member and valve spring are located within the outlet chamber. 28. A drive mechanism as recited in claim 11, further including:an outlet chamber adjacent to the piston channel; a housing having a cavity containing the coil cup, the housing having a central channel that defines the piston channel, the housing having a further cavity disposed on one end of the piston channel, wherein the further cavity defines the outlet chamber; a valve member moveable between open and closed positions to selectively open and close one end of the piston channel to the outlet chamber; and a valve spring for urging the valve member in the closed position; wherein the valve member and valve spring are located within the further cavity defining the outlet chamber. 29. A drive mechanism as recited in claim 28, wherein the piston is moveable in the axial direction of the piston channel between a retracted position and a forward position, wherein the valve member is located in the closed position when the piston is in the retracted position and wherein the valve member is moved to the open position when the piston is moved to the forward position.30. The drive mechanism as recited in claim 11, wherein the armature includes a magnetically permeable material.31. The drive mechanism as recited in claim 11, wherein the armature includes at least one of a material having no magnetic permeability and a material having a relatively low magnetic permeability, wherein the armature includes a cavity for holding another material having a relatively high magnetic permeability.32. The drive mechanism as recited in claim 31, wherein the at least one of a material having no magnetic permeability and a material having a relatively low magnetic permeability is selected from the group consisting of titanium, stainless steel, biocompatible plastic, ceramic, and glass.33. The drive mechanism as recited in claim 31, wherein the relatively high magnetic permeability material includes a ferrous material.34. The drive mechanism as recited in claim 31, wherein the cavity having the relatively high magnetic permeability material held therein is enclosed by a cover.35. The drive mechanism as recited in claim 31, wherein the cover includes a foil material.36. A drive mechanism for delivery of infusion medium comprising:an inlet for receiving infusion medium; a piston channel; a coil surrounding the piston channel; a piston located within the piston channel, the piston having a central channel extending through an axial length of the piston and having openings at both ends of the piston, the central channel for conveying infusion medium received by the inlet, the piston being moveable axially within the piston channel to drive infusion medium into the central channel; an armature disposed adjacent the coil, on one side of the piston channel; an outlet chamber disposed adjacent the coil, on the opposite side of the piston channel relative to the armature for receiving infusion medium from the central channel; and an outlet in flow communication with the outlet chamber, for discharging infusion medium from the outlet chamber. 37. The drive mechanism recited in claim 36, wherein the armature includes a central channel for conveying infusion medium received by the inlet, the central channel of the armature being in fluid communication with the central channel of the piston.38. The drive mechanism recited in claim 36, further including at least one valve structure in fluid communication with the central channel of the piston, the at least one valve structure for controlling a flow of infusion medium from the central channel of the piston to the outlet chamber.39. The drive mechanism recited in claim 38, wherein the at least one valve structure is configured to close during a forward stroke of the piston and open during a reverse stroke of the piston.40. The drive mechanism recited in claim 38, wherein the at least one valve structure is configured to inhibit a reverse flow of infusion medium from the outlet chamber to the central channel of the piston.41. The drive mechanism recited in claim 38, wherein at least a portion of the at least one valve structure is within the central channel of the piston.42. The drive mechanism recited in claim 38, wherein at least a portion of the at least one valve structure is adjacent one of the openings of the central channel of the piston.43. The drive mechanism recited in claim 38, wherein the at least one valve structure includes a plug and a tapered volume for selectively inhibiting the reverse flow of infusion medium.44. The drive mechanism recited in claim 43, wherein the plug is seated in the tapered volume.45. The drive mechanism recited in claim 43, wherein the plug is in the central channel of the piston and wherein at least one of the openings is shaped to include the tapered volume.46. The drive mechanism recited in claim 43, wherein the plug is a ball-shaped plug.47. The drive mechanism recited in claim 43, wherein the tapered volume is a cone-shaped tapered volume and wherein the plug is a cone-shaped plug.48. The drive mechanism recited in claim 38, wherein the valve structure includes:a seat having a central channel extending through an axial length of the seat and having openings at both ends of the seat, the central channel of the seat for conveying infusion medium, one end of the seat being coupled to one end of the piston such that one of the openings of the seat is in fluid communication with one of the openings of the piston; a seal having a first side for moveably contacting the other one of the ends of the seat to inhibit flow of infusion medium from the other one of the openings of the seat; and a cap slideably connected to the one end of the piston, the cap having a first side for contacting a second side of the seal, the cap being slideable on the one end of the piston between a first position wherein the seal contacts the seat and a second position wherein the seal does not contact the seat. 49. The drive mechanism recited in claim 48, wherein the cap includes a catch for slideably securing the cap to the one end of the piston and wherein the piston includes an indented area for receiving the catch.50. The drive mechanism recited in claim 48, wherein the cap is in the first position when the piston is moving in one direction and in the second position when the piston is moving in the opposite direction.51. The drive mechanism recited in claim 48, wherein the cap is slideably connected to the end of the piston closest to the outlet chamber.52. The drive mechanism recited in claim 51, wherein the cap moves to the first position as a result of positive pressure in the outlet chamber.53. The drive mechanism recited in claim 51, wherein cap moves to the second position as a result of a vacuum in the outlet chamber.54. The drive mechanism recited in claim 48, wherein the cap includes an indentation for seating the seal.55. The drive mechanism recited in claim 48, wherein the seat has a generally frusto-conical shape.56. The drive mechanism recited in claim 48, wherein the cap is a plastic cap.57. The drive mechanism recited in claim 56, wherein the plastic cap is a polysulfone cap.58. The drive mechanism recited in claim 48, wherein the cap is a metal cap.59. The drive mechanism recited in claim 48, wherein the seal is a washer.60. The drive mechanism recited in claim 48, wherein the seal is a silicone rubber seal.61. The drive mechanism recited in claim 36, further including a coil cup composed of a magnetizable material, the coil cup including:a generally annular inner wall having one end defining an inner pole surface of the coil cup; a generally annular outer wall having a first inner diameter and a first outer diameter, the outer wall having a generally annular shelf portion extending from the outer wall towards the inner wall, the shelf portion having a second inner diameter and a second outer diameter, the second inner diameter being smaller than the first inner diameter, the shelf portion having a second end defined by the second inner and outer diameters, the second end defining at least a portion of an outer pole surface of the coil cup; and a generally annular interior between the inner and outer walls, the annular interior containing the coil. 62. The drive mechanism recited in claim 36, further including an annular space between a wall of the piston channel and the piston located within the piston channel, the annular space for conveying infusion medium received by the inlet.