WO2013071367A1

WO2013071367A1 - Apparatus and method for surgical insertion of an implantable device

Info

Publication number: WO2013071367A1
Application number: PCT/AU2012/001424
Authority: WO
Inventors: Luke MARSDEN; Matthew Solomon; Brody PAYNE; Micah Atkin; Ramesh RAJAN; Jeffrey V ROSENFELD; Gordon Stewart; Khaled Hejleh; Michael Klass
Original assignee: Monash University
Priority date: 2011-11-18
Filing date: 2012-11-16
Publication date: 2013-05-23

Abstract

A surgical apparatus (200) for insertion of an implantable device within the body of a subject comprises a housing (204) in which an insertion head portion (206) is mounted and adapted to releasably hold an implantable device (102). An electro-mechanical actuator (212) mounted within the housing (204) comprises a shaft (216) movable between first and second stop positions (222, 214) in response to signals applied at an electrical input. A mechanical coupling between the insertion head portion (206) and the shaft (216) is configured such that movement of the shaft from the first stop position (22) to the second stop position (214) applies an insertion force to the insertion head portion. An electronic controller (502) is configured to apply an electrical control signal (506, 508) to the electrical input of the actuator (212), wherein the electrical control signal is adapted to cause application of an insertion force having predetermined characteristics to the insertion head portion (206), for insertion of the implantable device (102). The apparatus enables improved precision of insertion velocity control, which is difficult to achieve with purely mechanical or pneumatic approaches. Furthermore, the control signals generated by the electronic controller may beneficially be varied and adapted to accommodate a range of insertion conditions.

Description

APPARATUS AND METHOD FOR SURGICAL INSERTION OF AN

IMPLANTABLE DEVICE

FIELD OF THE INVENTION

The invention relates to surgical apparatus and procedures for fitting of implantable devices to living subjects, and more particularly to a surgical apparatus and method for insertion of an implantable device having particular application to the implantation of neural microelectrode interfaces within the brain. BACKGROUND OF THE INVENTION

Implantable neural microelectrode interfaces provide a mechanism for coupling neurons, the electrically active cells of the nervous system, with electrical circuitry. Implantable neural interface devices are commonly implemented in medical applications such as cardiac pacemakers, cochlear implants and deep-brain stimulators.

Widespread clinical use of neuroprosthetic devices is impeded by limitations in the reliability of readings obtained from implanted micro-electrode interfaces. In particular, signal degradation may occur in the months following insertion, resulting in inaccuracy and potential complete signal loss. A significant cause of this degradation is biological reactive response to device insertion and ongoing presence of the device. In the brain, this response may culminate in a fibrous and cellular sheath which encapsulates electrodes impeding recording and stimulation of neural activity.

It is known that direct brain injury (DBI) associated with the insertion process exacerbates sheath development. Accordingly, it is anticipated that improvements in chronic functionality will be achieved by reducing the damage occurring at insertion.

It has been demonstrated that fast insertion of sharp electrodes can result in a lower mean effective strain in superficial and middle regions of the cortex. The main structural barrier to electrode insertion is the pia mater. Piercing of the pia mater is accompanied by a sharp drop-off in reactive forces. Fast insertion minimises dimpling of the pia mater, which in turn reduces brain tissue compression, which implies that the rate of insertion may provide a controllable parameter to optimise for minimal brain trauma. While it has been suggested to use an enzymatic solution to break up the collagen network of the pia, in order to weaken the mechanical properties of the pia and possibly reduce the required insertion force, the effectiveness of this technique has not been quantified.

Existing insertion systems, employing mechanical or pneumatic mechanisms for rapid electrode insertion, provide limited control of insertion force and velocity, and are not readily adaptable to different insertion conditions.

Accordingly, it is an object of the present invention to provide an improved surgical apparatus and method for insertion of implantable devices, which enable more-precise control, and are more readily adaptable to different conditions, and which may therefore be used to reduce trauma occurring during insertion.

SUMMARY OF THE INVENTION

In one aspect the invention provides a surgical apparatus for insertion of an implantable device within the body of a subject, the apparatus comprising: a housing;

an insertion head portion mounted within the housing and adapted to releasably hold an implantable device;

an electro-mechanical actuator mounted within the housing, which comprises a shaft movable between first and second stop positions in response to signals applied at an electrical input;

a mechanical coupling between the insertion head portion and the shaft, configured such that movement of the shaft from the first stop position to the second stop position applies an insertion force to the insertion head portion; and an electronic controller configured to apply an electrical control signal to the electrical input of the actuator, wherein the electrical control signal is adapted to cause application of an insertion force having predetermined characteristics to the insertion head portion, for insertion of the implantable device.

Advantageously, embodiments of the invention are able to provide improved precision of velocity control, which is difficult to achieve with purely mechanical or pneumatic approaches. The control signals generated by the electronic controller may beneficially be varied and adapted to accommodate a range of insertion conditions. The use of an electronic controller also opens up the possibility of employing feedback control, based upon sensors positioned within the housing. In one embodiment, the electro-mechanical actuator comprises a solenoid. For example, the electro-mechanical actuator is a linear solenoid, wherein the shaft comprises a cylindrical armature movable along a central axis of the solenoid between the first and second stop positions.

In some embodiments, a location within the housing of the first stop position is controllable. In one embodiment, the apparatus includes a backstop positionable within the housing to define the first stop position. The backstop may comprise a micrometer for precision positioning of the backstop.

In embodiments of the invention the apparatus further comprises biasing means adapted to provide an urging bias acting to urge the armature towards a rest position against the backstop. Exemplary biasing means may include one or more springs.

Embodiments of the invention may further include a position sensor mounted within the housing and adapted to monitor a position of the armature relative to the housing. In an exemplary arrangement, the position sensor is an optical sensor. The position sensor may generate an electrical output signal indicative of the position of the armature relative to the housing, and which may be made available for feedback to the electronic controller.

In an exemplary embodiment, the insertion head portion is fixed by a rigid connection to the armature, to ensure that the motion of the insertion head accurately tracks that of the armature, and to prevent sudden momentum transfer.

In a particular embodiment of the invention, the insertion head portion comprises a vacuum-hold mechanism for releasably holding the implantable device.

In this particular embodiment, the insertion head portion comprises an internal chamber having a first opening at an insertion end of the insertion head, upon which the implantable device may be placed, and a second opening connectable to a vacuum source, such as a pump, for maintaining the internal chamber below an ambient pressure.

Preferably, the second opening communicates with the internal chamber via a valve, such as a solenoid valve, operable to seal the first opening from the internal chamber, and to open the chamber to an exhaust for equalising the pressure within the chamber with an external pressure. The external pressure may be the ambient pressure, or it may be a higher pressure provided by, for example, a compressed air source.

In embodiments employing a solenoid valve, the solenoid valve is operable by an electronic control signal. The electronic controller may be further configured to apply a solenoid valve control signal to an electrical input of the solenoid valve, wherein the solenoid valve control signal is adapted to cause opening of the internal chamber of the insertion head portion to the exhaust at a predetermined time, in order to release the implantable device. The timing and characteristics of the electrical control signal operating the electro-mechanical actuator, and the electrical control signal operating the solenoid valve, may have relative timing and amplitude characteristics adapted to minimise trauma induced in the subject at insertion of the implantable device.

In another aspect, the invention provides a method for controlling the insertion of an implantable device within the body of a subject, utilising a surgical apparatus which is adapted to releasably hold the implantable device for insertion, and which comprises an electro-mechanical actuator operable via an electrical control signal to exert an insertion force for insertion of the implantable device, the method including the steps of:

generating an electrical control signal which is adapted to cause application of an insertion force having predetermined characteristics when applied to the electrical signal input of the electro-mechanical actuator; and

applying the electrical control signal to the electrical signal input of the electro-mechanical actuator.

In embodiments of the invention the characteristics of the electrical control signal comprise a control delay and a control duration. In particular, the electrical control signal may be an electrical pulse, e.g. a voltage or current pulse, commencing after a control delay period, and having a pulse width equal to the control duration.

The control delay and the control duration may be calibrated in order to result in an insertion force having the predetermined characteristics when the control signal is applied to the electro-mechanical actuator. In some embodiments, the surgical apparatus further comprises a position sensor which provides an electrical position sensor output signal, and the method comprises steps of receiving the position sensor output signal during application of the electrical control signal to the electro-mechanical actuator, and adapting the control signal in order to produce the insertion force having the predetermined characteristics, such as a predetermined insertion velocity profile.

In embodiments of the invention, the apparatus further includes an electrically controllable release mechanism for releasing the implantable device, and the method includes the further step of generating an electrical release control signal adapted to cause release of the implantable device at a predetermined time relative to the electrical control signal applied to the electromechanical actuator, and applying the release control signal to the release mechanism.

Further features and advantages of embodiments of the invention will be apparent to the person skilled in the art from the following description of a particular embodiment, which is provided by way of example only, and should not be considered to limit the scope of the invention as defined in any of the foregoing statements, or in the claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which like reference numerals refer to like features, and wherein:

Figure 1 is a series of schematic diagrams illustrating an insertion process according to an embodiment of the invention;

Figure 2 shows cross-sectional and plan views of a microelectrode array

(MEA) insertion tool embodying the invention;

Figure 3 shows timing diagrams illustrating control of the MEA insertion tool of Figure 2;

Figures 4(a) and 4(b) show flowcharts illustrating exemplary control methods according to embodiments of the invention; and

Figure 5 is a block diagram illustrating a feedback control system according to an embodiment of the invention. DETAILED DESCRIPTION

Figure 1 shows a series of schematic diagrams 100 illustrating an insertion process generally embodying the principles of the present invention. In particular, it is an object of the invention to provide controlled insertion of an implantable device, such as a microelectrode array (MEA) tile 102 into living tissue 104, such as the brain. As is well-known, the brain is surrounded by protective membranes, the meninges, including the pia mater 106, which is the innermost membrane layer.

Controlling the process of inserting the MEA tile 102 includes control of the velocity with which the electrodes strike the pia mater 106, the point at which the MEA tile 102 comes to rest, and the timing of the subsequent release of the MEA tile 102.

By precise control of at least these parameters, embodiments of the invention are able to minimise the trauma occurring during insertion of the MEA tile 102 through the pia mater 106 into the brain tissue 104.

Embodiments of the invention comprise a surgical apparatus, which is described in greater detail below with reference to Figure 2, including a driving shaft 108 having an insertion head-end 1 10 which is adapted to hold the tile 102 in place during insertion, and to release the tile 102 in accordance with an external controlling signal.

According to an exemplary insertion process, the shaft 108 and tile 102 initially accelerate to a maximum velocity, as indicated by the segment 1 12 of the illustrated velocity-time graph. Advantageously, the fast insertion of sharp electrodes results in reduced trauma to the pia mater 106 and underlying tissue 104.

After the pia mater has been pierced, the shaft 108 decelerates, as indicated schematically by the segment 1 14 of the velocity-time graph. Preferably, the shaft 108 and MEA tile 102 come to rest at precisely the desired depth of insertion, at which time 1 16 release of the tile by the insertion head 1 10 is triggered.

The shaft 108 then retracts, accelerating away from the inserted tile 102, before coming to rest in a retracted position, as indicated by the segment 1 18 of the velocity-time graph. Precise control is required in the process 100, to ensure that the sharp electrodes strike the pia mater 106 at a sufficient velocity, that the tile comes to rest without impacting excessively against the surface of the pia mater 106, and to ensure that the tile 102 is released from the shaft 108 prior to retraction, so that the tile 102 is not pulled back along with the insertion head 1 10.

Figure 2 shows a plan view 200, and a cross-sectional view 202, of a surgical apparatus for insertion of an implantable device which embodies the invention, and is suitable for use in the insertion process 100.

The insertion tool 200, 202 comprises an exterior housing 204, which may be manufactured as a single part, or assembled from multiple components as in the illustrated embodiment.

Fitted to a front end of the main body of the housing 204, e.g. by screw-thread or other engagement, is an insertion head housing 206. Enclosed within the insertion head housing 206 is an insertion head core 208. The insertion head core 208 has a front insertion head-end 210, which comprises a first opening, the purpose of which is described in greater detail below.

Within the main body of the housing 204 there is an electromechanical actuator in the form of a linear solenoid 212. The actuator 212 comprises the outer solenoid coil 214, and the movable inner solenoid core or armature 216. The armature 216 is movable between a first, retracted, position, in which a solenoid core backplate 220 rests against a backstop 222, and a second, extended, position, at rest against a conical face 218 within the solenoid coil 214.

As shown in Figure 2, the location of the backstop 222 is precisely controllable using a micrometer 224, mounted at one end of the main body of the housing 204.

The insertion head 208 is rigidly fixed to the solenoid armature 216, for example via screw-thread engagement. Accordingly, motion of the armature 216 between the first and second positions results in a direct application of force, resulting in extension of the insertion head-end 210, for insertion of an implantable device, such as the MEA tile 102.

Within the insertion head 208 is a chamber 226, open at the first opening 210 of the insertion head-end, as mentioned above. A valve 228 is provided, in communication with the chamber 226. In the embodiment shown in Figure 2, the valve 228 is a solenoid valve, which is configured to switch a second opening of the chamber 226 between a vacuum source, such as a pump, and an exhaust port, which may be open to ambient pressure, or connected to a higher pressure source, such as compressed air.

The valve 228 is therefore operable to hold and release an implantable device, such as an MEA tile 102, at the insertion head-end 212. When the valve is operated to open the chamber 226 to the vacuum source, the resulting pressure differential with the ambient air pressure may be used to hold a tile 102 in place against the insertion head-end 210. When it is desired to release the tile, the valve 228 is switched to open the chamber 226 to the exhaust port or higher pressure source, resulting in release of the tile 102 from the insertion head-end 210.

The insertion tool 200 further comprises biasing means 230, in the form of one or more springs configured to apply an urging bias causing the solenoid armature 216 to be restored to a retracted rest position against the backstop 222 when the solenoid coil 214 is not energised.

As will be appreciated from the foregoing description of the insertion tool 200, 202, operation of the tool to implement the process 100 may generally be achieved by steps of:

· firstly activating the valve 228 to open the chamber 226 to a vacuum source, and placing an implantable device against the insertion head-end 210, where it is held in place by the pressure differential;

• operating the micrometer 224 to set the desired starting position for insertion of the implantable device;

· energising the solenoid coil 214 at an appropriate voltage/current level, for a suitable duration, to cause the solenoid armature 216 to be projected through the coil 214 from the first position against backstop 222 to the second position against conical surface 218, thereby applying an insertion force projecting the insertion head-end 210, and causing insertion of the implantable device;

• operating the valve 228 to open the chamber 226 to the exhaust port or higher pressure source, in order to release the implantable device at the appropriate time; and • de-energising the solenoid coil 214, resulting in retraction of the armature 216 under the restoring force of the biasing means/springs 230.

A particular benefit of the tool arrangement shown in Figure 2 is the high degree of control available over the various insertion parameters. For example, the initial position of the armature 216, and insertion head-end 210, is precisely adjustable using the micrometer 224. The precise timing and force profile applied to the solenoid armature 216 is controllable via the particular electrical signal waveform applied to energise the solenoid coil 214. Timing of release of the implantable device is also accurately controllable via the operation of the valve 228.

Furthermore, the tool 200, 202 may include a position sensor, such as an optical sensor (not shown in the drawings) which is mounted within the housing and adapted to monitor a position of the armature 216 relative to the housing 204. For example, an optical sensor may be mounted in order to monitor the displacement of the armature backplate 220 relative to the end of the housing 204 at which the micrometer 224 is mounted. The position sensor may provide an output electrical signal which may be monitored in real-time in order to measure position and velocity of the solenoid armature 216.

Advantageously, the tool 200, 202 may be constructed from sterile or sterilisable materials. For example, some components, such as the housing 204, the insertion head 208, the micrometer 224 and the armature 216, may be fabricated from stainless steel. Others, such as the valve 228, may be fabricated from stainless steel and polymer. Polypropylene mounts may be used for mounting components such as the valve 228. Still other components, such as the armature backplate 220, may be fabricated from acetal copolymer. Persons familiar with requirements of surgical sterilisation will appreciate that these and other suitable materials may be employed for all components of the insertion tool 200, 202.

By way of example, Figure 3 illustrates exemplary timing diagrams 300, 302 for operation of the linear solenoid 212 and the valve 228, respectively. Each of the traces represents a corresponding electrical control signal which may be generated by an electronic controller interfaced via suitable wired connections with the insertion tool 200. While simple square pulses are shown in Figure 3, it will be appreciated that, in general, any desired electrical driving waveforms may be used, in order to achieve desired insertion characteristics. In particular, the ultimate objective is to control the insertion velocity and force to have appropriate predetermined characteristics suitable to minimise trauma caused during insertion.

The electronic controller may have a manual activation control, such as a switch or push-button, operated at an initial time 304. After a delay period, at time 306, a control signal is generated energising the solenoid coil 214. As previously described, this results in extension of the insertion head-end 210, for insertion of the implantable device. At a slightly later time 308, a control signal to the valve 228 is activated, switching the valve from the vacuum source to the exhaust port or higher pressure source. This causes release of the implantable device from the insertion head-end 210, and it will therefore be appreciated that the time 308 is determined so as to cause release of the implantable device once it has come to rest, and prior to retraction of the insertion head-end. At time 310 the solenoid coil is de-energised, resulting in retraction of the insertion head-end, after the implantable device has been released. A time 312 the valve 228 is switched back into the initial state, with the chamber 226 in communication with the vacuum source, enabling a further implantable device to be placed and held at the insertion head-end 210.

Figures 4(a) and 4(b) show exemplary flowcharts illustrating possible control methods that may be implemented in an electronic controller in accordance with embodiments of the invention. The flowchart 400 in Figure 4(a) illustrates a simple sequential control method, suitable for implementation using discrete analog or digital electronic components.

According to the method 400, after the operator has set the backstop position at step 402, and pressed the activation button at step 404, a control delay period elapses at step 406. At step 408 the solenoid control pulse is activated. At step 410, a further delay (e.g. corresponding with the difference between times 308 and 306) is implemented. Then, at step 412, the control signal to the valve 228 is activated in order to release the implantable device. At step 414, a further delay is implemented, before the control pulse is terminated at step 416 (e.g. time 310). Step 418 is a further delay, prior to switching of the valve 228 at step 420, corresponding with time 312.

The further flowchart 422 in Figure 4(b) illustrates a more general process, allowing for the use of arbitrary waveforms to control the linear solenoid 212 and the valve 228. Again, the operator first sets the backstop position at step 424 and activates the device at step 426. Generation of the two control waveforms may then occur in parallel, with a solenoid control waveform being generated at step 428, and a device release waveform being generated at step 430. These waveforms are output from the electronic controller via two signal output ports, and thereby applied to the solenoid coil 214 and the valve 228 at steps 432 and 434.

The process illustrated by the flowchart 422 may be implemented, for example, using various analog circuit components, digital circuit components, and/or a microprocessor implementation, enabling the arbitrary generation and adaptation of the control waveforms via programmatic means.

A more sophisticated control system, including feedback from a position sensor, is illustrated by the block diagram 500 shown in Figure 5.

In the feedback control arrangement 500, an electronic controller 502 is provided, which may be implemented in analog or digital electronics, and/or using a programmed microprocessor-based circuit. The controller 502 is interfaced with the insertion tool 504, via the solenoid control signal 506 and valve control signal 508. Additionally, a feedback signal from the insertion tool 504 is provided in the form of the output 510 of the position sensor. The position sensor signal is input to the controller 502, which is operated by an activation control 512, such as a push-button. The activation control generates an activation signal 514 which initiates operation of the insertion tool 504 by the controller 502. A control signal 506 is applied to the solenoid coil 214, and the resulting change in position of the armature 216 monitored via the position signal 510. The control signal 506 is then adapted, using automated control approaches known in the art of signal processing, in order to achieve a desired profile of the feedback position signal, i.e. a controlled velocity and force profile of the insertion head 210. Similarly, the timing of the release control signal 508 may be based upon the feedback position signal 510. According to a further development of the invention (not shown in the drawings) the outer solenoid coil 214 may comprise a plurality of separate windings, each having its own associated electrical terminals. The windings may be arranged along the length of the solenoid 212, in overlapping, contiguous and/or disjoint relation to one another. Successive energisation of the solenoid windings may then be employed to obtain greater control of the forces acting on the armature 216, and therefore its acceleration and velocity, as a function of its position within the housing 204 during the insertion process.

In such embodiments the control signal 506 generated by the controller 502 comprises a corresponding plurality of electrical pulses or other waveforms, each of which is applied to one of the windings. The enhanced ability to control the forces acting on the armature 216 via the plurality of parallel control signals, as a function of its location as measured by the position sensor, enables more precise feedback control of the insertion velocity.

Additionally, a reverse-voltage control signal may be applied to one or more of the windings, or a reverse-direction winding may be provided, in order to apply a retarding force to the armature 216, and timed so as to decelerate the insertion head core 208 after piercing the pia mater, and shortly prior to coming to rest (at time 1 16).

Accordingly, an adaptable and flexible insertion tool is provided, which may be controlled electronically in order to achieve desired insertion force characteristics, thereby enabling trauma caused during insertion of implantable devices, such as MEA tiles, to be reduced.

While particular embodiments of the invention have been described in order to exemplify the general principles and operation of the invention, it will be understood that these are not limiting of the scope of the invention, and that various modifications are possible within the capabilities of persons skilled in the relevant arts. The scope of the invention is as defined in the claims appended hereto.

Claims

CLAIMS:

1 . A surgical apparatus for insertion of an implantable device within the body of a subject, the apparatus comprising:

a housing;

2. The surgical apparatus of claim 1 wherein the electro-mechanical actuator comprises a solenoid.

3. The surgical apparatus of claim 2 wherein the solenoid comprises one or more coils disposed around a central linear axis, and the shaft comprises a cylindrical armature movable along a central axis of the solenoid between the first and second stop positions.

4. The surgical apparatus of claim 1 wherein a location within the housing of the first stop position is controllable via a backstop positionable within the housing to define the first stop position.

5. The surgical apparatus of claim 4 wherein the backstop comprises a micrometer for precision positioning of the backstop.

6. The surgical apparatus of claim 1 which further comprises biasing means adapted to provide an urging bias acting to urge the armature towards a rest position against the backstop.

7. The surgical apparatus of claim 6 wherein the biasing means comprises one or more springs.

8. The surgical apparatus of claim 1 further comprising a position sensor mounted within the housing, which is adapted to generate an electrical output signal indicative of the position of the armature relative to the housing.

9. The surgical apparatus of claim 8 wherein the controller is configured to receive the signal generated by the position sensor, and to apply the received position sensor signal to perform feedback-control of the electrical control signal.

10. The surgical apparatus of claim 1 wherein the insertion head portion comprises a vacuum-hold mechanism for releasably holding the implantable device.

1 1 . The surgical apparatus of claim 10 wherein the insertion head portion comprises an internal chamber having a first opening at an insertion end of the insertion head, upon which the implantable device may be placed, and a second opening connectable to a vacuum source for maintaining the internal chamber below an ambient pressure.

12. The surgical apparatus of claim 1 1 wherein the second opening communicates with the internal chamber via a valve operable to seal the first opening from the internal chamber, and to open the chamber to an exhaust for equalising the pressure within the chamber with an external pressure.

13. The surgical apparatus of claim 12 wherein the valve is a solenoid valve operable by an electronic control signal, and the electronic controller is further configured to apply a solenoid valve control signal to an electrical input of the solenoid valve, wherein the solenoid valve control signal is adapted to cause opening of the internal chamber of the insertion head portion to the exhaust at a predetermined time, in order to release the implantable device.

14. A method for controlling the insertion of an implantable device within the body of a subject, utilising a surgical apparatus which is adapted to releasably hold the implantable device for insertion, and which comprises an electromechanical actuator operable via an electrical control signal to exert an insertion force for insertion of the implantable device, the method including the steps of: generating an electrical control signal which is adapted to cause application of an insertion force having predetermined characteristics when applied to the electrical signal input of the electro-mechanical actuator; and

15. The method of claim 14 wherein the characteristics of the electrical control signal comprise a control delay and a control duration.

16. The method of claim 14 comprising a preliminary step of calibrating the electrical control signal to produce an insertion force having the predetermined characteristics when the control signal is applied to the electro-mechanical actuator.

17. The method of claim 14 wherein the apparatus further comprises an electrically controllable release mechanism for releasing the implantable device, and the method comprises further steps of:

generating an electrical release control signal adapted to cause release of the implantable device at a predetermined time relative to the electrical control signal applied to the electro-mechanical actuator; and

applying the release control signal to the release mechanism.

18. The method of claim 14 wherein the surgical apparatus further comprises a position sensor which provides an electrical position sensor output signal, and the method comprises steps of:

receiving the position sensor output signal during application of the electrical control signal to the electro-mechanical actuator; and

adapting the control signal in order to produce the insertion force having the predetermined characteristics.

19. The method of claim 18 wherein the electrical control signal comprises a plurality of parallel signals, each of which controls operation of the electro- mechanical actuator over a predetermined range of position detectable by the position sensor, and the step of adapting the control signal comprises adapting at least one of the parallel signals selected in accordance with a current position determined from the position sensor output signal.