THE HAND HELD FORCE MAGNIFIER: ENABLING HAPTIC FEEDBACK IN MICROSURGERY

Avin Khera , Cory CizauskasFreshmen Engineering Conference

The Hand-Held Force Magnifier (HHFM) is a surgical tool designed to provide haptic feedback to surgeons performing microsurgery. A prominent issue for surgeons when performing microsurgeries is that they often cannot tactilely assess their movements during operations in which they must manipulate extremely delicate tissue. This lack of somatic sense poses the risk of accidentally rupturing the tissue they are operating upon, and that which surrounds it and bringing potentially permanent harm to a patient’s health. The HHFM provides a solution to this problem by artificially creating forces opposite to the direction of pressure applied by its attachments, thereby sensitizing the surgeon to precise movements. While still in development, the HHFM’s use in future surgeries could significantly reduce the complication rates of microsurgical procedures and improve the local quality of life where it is employed. Such prospects may even increase the sustainability of mIcrosurgical techniques like capsulorhexis by making them simpler for surgeons, and thus more accessible to patients.

Abstract

The Hand-Held Force Magnifier (HHFM)

The design basis of the HHFM is to generate haptic feedback by sending an amplified counter force back to the user’s hand when a pushing and pulling force is applied to a membrane or thin layer of tissue. This effectively increases the sensitivity the user feels when interacting with surfaces that would otherwise be difficult to perceive. The current version of the device utilizes a voice coil and pressure sensor as its primary mechanisms of force production. The pressure sensor at the front of the HHFM records the amount of pressure being delivered to the surface by the user via a mechanical linkage, which reduces recording inaccuracies due to friction by 5 milliNewtons. A pre-amplification chamber near the sensor reduces noise and initiates the sensor before acquiring readings. The proximally located voice coil, which are solenoids commonly used in speakers, generates an electric current by moving across a dipolar ferromagnet.

HHFM Functionality

The sustainability of the HHFM is based on its ability to simplify and increase the efficiency of surgical procedures. This works to enhance the overall quality of life of patients by significantly reducing procedural complications. The current mechanical sustainability of the device is limited by it’s current construction. A shift towards an aluminum or other non-ferromagnetic chassis would improve the operational life of the device. The haptic feedback from the HHFM offers a possible alternative to robotic surgeries and can be employed in third-world countries.

Sustainability

The current model of the HHFM (Model 3) is restricted to amplifying translational forward and backward motion (surging). The next planned model of the HHFM (Model 4) will focus on applying force feedback to pitch rotation [24]. The primary purpose of pitch in surgery applies to the use of scalpels and other vertical pressure-based cutting tools. This makes it an extremely important component of practically all surgical procedures, including CCC. Focusing on application of pitch to the new HHFM will help develop greater understanding of the ergonomic limitations of the device, including handle location and the degree to which forward and backward motion (from Model 3) can be accommodated on future devices.

Future Iterations

To address the problem of tactile insensitivity complications, researchers at the University of Pittsburgh and Carnegie Mellon University engineered a device called the Hand-Held Force Magnifier. It utilizes haptic feedback to increase a surgeon’s sensitivity to the manipulation of membranes and blood vessels and helps prevent accidental ruptures. This means the HHFM could have the potential to evolve capsulorhexis and angioplasty into more risk-free and sustainable surgical techniques. The HHFM has parts that also selectively improve the axial and tactile capabilities of the surgeon. It is held using a brace attached to a rotary bearing and hinge. The surgeon holds the device by slipping their hand through the brace and gripping the plastic handle with their fingers and thumb. The rotary bearing allows for movement in azimuth (horizontal) and the hinge provides altitude (vertical). The brace helps to lessen the need for more inaccurate wrist movements

Figure 1: Curvilinear Capsulorhexis (CCC) being conducted on the lens capsule

Figure 2: Tools used in angioplasty: Guidewire, balloon, and in most cases, a metal stent

One analytical experiment measured the user’s perception of force change. This was conducted by asking participants to freely assign integers or fractions to their perceived intensity of the force stimuli. The force stimuli used for this experiment were 0.1, 0.2, 0.3, and 0.4 newtons. The data gathered from participants was then standardized by dividing each estimation by a participant’s average reported value and then multiplying them by the overall mean for the group.

Psychophysical Experimentation

Force stimuli (N)

Mean Subjective Rating of Force Intensity (0.1 N = 1.0 base unit)

Control (Syringe) HHFM-off HHFM-on

0.1 1.0 1.2 5

0.2 2.5 2.7 8

0.3 3.5 4.5 11.5

0.4 5.1 6.8 13

• Present psychophysical data shows participants underestimatingthe force magnification• The surface texture of the operators hand affects their perception• One concern currently being addressed is how the device will reactonce it has intentionally punctured a membrane• A mechanical failure may result in serious injury

Operational Concerns

Figure 3: The Hand-Held Force Magnifier (HHFM)