Skin-like material can ‘sense’ if surgeons get off track

Experimental setup for calibrating the sensing skin. Each sensor pad is lowered onto the indentation surface placed on a weight-measuring scale. Changes in the channel resistance are processed through analog signal conditioning and a DAC board, and recorded on a computer. (J Neurosurgery: Pediatrics)
Experimental setup for calibrating the sensing skin. Each sensor pad is lowered onto the indentation surface placed on a weight-measuring scale. Changes in the channel resistance are processed through analog signal conditioning and a DAC board, and recorded on a computer. (Images: J Neurosurgery: Pediatrics)
When surgeons perform image-guided minimally invasive procedures using an endoscope, some aspects of visualization and image quality are typically compromised as compared with open surgeries in which the physician can peer into the body. However, a new pressure-sensing material, placed over an endoscope, may someday provide surgeons with additional guidance and protect healthy tissue during these procedures.

“Neurosurgeons, especially pediatric neurosurgeons, are increasingly using neuroendoscopy to perform minimally invasive brain and spine surgery,” notes Patrick Codd, MD, from the Department of Neurosurgery at Boston Children’s Hospital, who was the lead author on a study evaluating this new material.

“Whenever you move to image-guided minimally invasive surgery, there is typically a tradeoff between the resolution of the image and the field of view,” where you have one but not the other, says Pierre Dupont, PhD, chief of Pediatric Cardiac Bioengineering at Boston Children’s and senior author on the study. In endoscopic procedures, you have optical view at the tip of the scope, but there is no field of view across the length of the instrument.

The capability to monitor the pressures applied along the surface of a surgical instrument “could provide critical feedback to the surgeon about impending collateral damage and so enable avoidance of this damage as the operation is carried out,” write Dupont and colleagues in the January issue of Journal of Neurosurgery: Pediatrics.

In their proof-of-concept study, they describe results with a novel pressure-sensing polymer skin that provides touch/pressure-sensing feedback, designed for use during intracranial endoscopy. They placed the skin over a standard rigid endoscope introducer sheath, performed a series of calibration experiments and then conducted two experiments in adult pig brains.

“In our experimental phase, we recognized the value in the fact that if you can’t see it, you can still feel it,” says Dupont. “In assessing how much pressure is ‘bad,’ we estimated that damage will begin to occur to tissue at about 30 mm of mercury, based on a review of the modest literature available on the topic.”

Animal testing of the sensing operating sheath. Here, a single sensor was examined at increasing degrees of lateral displacement (a to c). The darkest red warning threshold is reached at an applied force of 30mmHg.
Animal testing of the sensing operating sheath. Here, a single sensor was examined at increasing degrees of lateral displacement (a to c). The darkest red warning threshold is reached at an applied force of 30mmHg.
To validate the pressure sensor in their animal model, Dupont, Codd and colleagues first manually applied the endoscope operating sheath, bearing a 3×3 array of calibrated sensors, to the surface of the cortex with gradually increasing pressures. During repeated applications, they received consistent, intuitive, color-coded feedback on the severity of applied force. In the second test, they introduced the same sheath, bearing the calibrated sensing skin, roughly perpendicular through the brain surface to simulate the trajectory of a typical transcortical endoscopic procedure. Again, gradual lateral pressure was manually applied.

The two experiments allowed the researchers to compare the visually observed magnitude of cortical deformation by the endoscope (often the current method of feedback available to the surgeon) with the pressure measurements of the sensing sheath. In both assessments, Codd and colleagues report, the red warnings triggered by high pressure readings indeed indicated tissue deformations that would be worrisome for damage. “This work represents the first application of this promising technology to neurosurgery as a means to provide feedback to the surgeon,” they write.

Before this pressure-sensing polymer skin is ready for prime time, the researchers will be assessing biocompatible alternatives to the sensing material, optimizing the size and number of pressure sensors per unit of sensing skin and enhancing the sensitivity of the recording equipment.

Also, the researchers are exploring how information is fed back to the surgeons. Currently, color-coded visual cues provide a graded feedback proportional to the recorded pressures. Alternative feedback modalities such as auditory or tactile feedback may reduce the cognitive load on the clinician and improve performance, uncluttering an already busy assortment of visual cues, according to Dupont.

“This development is an important stepping stone to applying this technology to more complex procedures and imaging devices with greater curvature,” says Dupont.

The National Institutes of Health and the Wyss Institute for Biologically Inspired Engineering supported this research.