1.1.

Energy-Based Surgical Devices

Suture ligation of blood vessels and tissue structures during open and laparoscopic surgery is both time consuming and skill intensive. The use of energy-based devices enables more rapid and efficient vessel and tissue ligation to maintain hemostasis during surgery than standard sutures and mechanical clips, which leave foreign objects in the body. These energy-based devices can reduce surgical operative times and costs significantly.^1–6 The majority of energy-based devices are powered by ultrasonic (US) or radiofrequency (RF) energy. US devices are indicated for thermal coagulation and cutting of vessels up to 5 mm in diameter.^7,8 RF devices can be used with larger vessels, but in practice, most clinicians are hesitant to use energy-based devices on vessels greater than 5 mm in diameter. A main concern when using energy-based devices is the possibility of unintended thermal damage to adjacent critical tissue structures when performing delicate procedures in confined spaces (e.g., prostatectomies and thyroidectomies). The active jaw of US devices can reach temperatures in excess of 200°C during an application and can take greater than 20 s to cool to usable temperatures before proceeding with further applications. The maximum temperatures on the active jaw of RF devices are lower, less than 100°C, but larger thermal spread is observed.^9,10 These US and RF energy-based devices expedite a number of traditionally labor-intensive surgical procedures, such as lobectomy,¹¹ nephrectomy,³ gastric bypass,¹² splenectomy,¹³ thyroidectomy,¹⁴ hysterectomy,¹⁵ and colectomy.¹⁶ However, both electrosurgical and US devices have limitations, including potential for undesirable charring and unnecessarily large collateral thermal damage zones, with thermal spread beyond the seal zone averaging greater than 1 mm.^17–20

1.2.

Infrared Laser Vessel Sealing

Our laboratory is currently exploring infrared (IR) lasers as an alternative energy modality for vessel sealing, capable of generating less collateral thermal damage.^21,22 Previous studies demonstrated the feasibility of sealing vessels in an open in vivo porcine model using a 1470-nm diode laser.²³ However, the initial prototype was designed for testing in open surgery and featured a plunger style tissue clasping mechanism and 12.7-mm outer-diameter optical components in the light delivery mechanism that were incompatible with standard laparoscopic surgical instruments and size constraints. In this study, an optical laparoscopic prototype with 5-mm-outer-diameter shaft was integrated within a standard Maryland jaw configuration similar to devices currently in surgical use, and performance tests were conducted on sealing of porcine renal blood vessels, ex vivo.

2.1.

Linear Beam Shaping

A standard laparoscopic probe consisting of a 30-cm length, 5-mm outer-diameter shaft, and 20-mm length jaw was chosen as the design constraint. The multimode optical fiber was designed to deliver the laser energy down the shaft to the jaw where beam shaping optics generated a line beam (18-mm length, 1.2-mm width) along the mouth of the jaw, parallel to the fiber and shaft. A sapphire window with dimensions of $19\mathrm{mm}\text{length}\times 2\mathrm{mm}\text{width}\times 0.5\mathrm{mm}$ thick (Meller Optics, Providence, Rhode Island) was added to prevent contamination of the optics from the surrounding fluid environment in the body. The reflective optics selected were designed to transform a high-power fiber output at a 90 deg angle into a long, linear beam with roughly even power distribution within a constraint of $2\times 3\mathrm{mm}$ cross-section, corresponding to the inner volume of the bottom jaw.

The optics was required to handle high laser power (up to 110 W) over a small area, so a metallic mirror was preferred over a beam splitter or diffraction grating. A cylindrical mirror with radius of curvature close to the diameter of the incident collimated beam was used to generate a linear beam directed at an angle of 90 deg. However, use of a single mirror would require too great of a working distance to convert a 1-mm-diameter circular beam to a 20-mm-length linear beam. Therefore, a design consisting of multiple cylindrical mirrors analogous to a “washboard,” with short radii arranged along the length of the jaw, tilted upward like a small angle “staircase,” was chosen to provide the beam length required (Fig. 1).

Fig. 1

(a) Side view diagram of fiber illuminating top half of single cylindrical mirror. (b) Washboard-style, multiple cylindrical mirrors arranged in a staircase, creating a longer linear beam directed at 90 deg. Additionally, the shorter height of the optics meets design constraints of the laparoscopic probe jaw.

2.2.

Simulations

A range of cylindrical diameters (0.5, 0.75, and 1 mm), fiber tilts (5 deg, 0 deg, $-5\mathrm{deg}$), and positions ($H=0.5$, 0.75, 1, 1.25, 1.5 mm) ($D=0$, 2.5, 5 mm) were modeled to determine the power distribution along the line beam, where “$D$” was defined as the diameter of each reflecting cylinder, and “$H$” as the height of the fiber from the bottom of the reflecting “staircase” design, as labeled in Fig. 2.

Fig. 2

Three-dimensional layout of FRED simulation showing (a) fixed and (b) variable parameters tested. The uncoated sapphire window ($L=20$, $W=3$, $H=0.5\mathrm{mm}$) rests on top of a reflecting chamber ($W=1$, $H_{\max }=2\mathrm{mm}$, ${\theta }_{\text{incline}}=5.5\mathrm{deg}$). The 100% reflecting cylinders are arranged along the incline with the origin at the center of curvature of the first cylinder directly below the proximal edge of the window. The $550\text{-}\mu \mathrm{m}$-diameter fiber output is directed toward the distal tip and the side walls of the chamber are 100% reflective.

Secondary simulations were also carried out using a CAD program (SolidWorks, Dassault Systèmes SolidWorks Corporation, Waltham, Massachusetts). An array of alterations to the geometry was simulated due to manufacturing constraints. Material coatings optical properties (98% reflectivity at 1470 nm for gold and 86% transmission for sapphire glass) were incorporated to provide an accurate simulation result. Photographs and diagrams of the final design and its irradiance profile compared to the experimental tests are provided in Fig. 3. There was a discrepancy between the dimensions of the experimental and simulation results, due to the irradiance being analyzed above the sapphire window for the simulation while the imaging plane of the beam profiler setup was closer to the reflecting insert.

Fig. 3

(a, b) Photograph and diagram of reflective insert with “staircase” geometry placed in lower jaw of laparoscopic prototype; (c) simulation of spatial beam profile; (d) experimental results for spatial beam output of probe.

3.1.

Tissue Preparation

Fresh porcine kidney pairs were obtained from a slaughterhouse (Spear Products, Inc., Coopersburg, Pennsylvania) and renal arteries were then dissected, cleaned of fat, and kept hydrated with saline for use within 24 h. The entire vascular tree for each kidney was carefully dissected and surgically exposed in a similar manner to previous studies,^21,22 providing multiple vessels with a wide range of diameters for testing. A total of 69 vessels were tested, ranging from 1.0 to 6.6 mm in diameter and with a mean diameter of $3.3\pm 1.7\mathrm{mm}$.

3.2.

Laser Parameters

A 100-W, continuous-wave, diode laser with a 1470-nm wavelength (BrightLase Ultra-100, QPC Lasers, Sylmar, California) was chosen, based on previous wavelength optimization studies involving seven different laser wavelengths spanning the infrared spectrum.²¹ Since water is a primary absorber of laser radiation in the near- to mid-IR spectrum and soft tissues are composed primarily of water (60% to 80% water content), the optical penetration depth (OPD) can be approximated by the inverse of the water absorption coefficient (${\mu }_{\mathrm{a}}=24{\mathrm{cm}}^{-1}$) at 1470 nm. The OPD is 0.4 mm, which also closely matches the thickness of the blood vessels, once they are compressed within the jaws of the laparoscopic device, thus providing efficient delivery of optical energy to the vessel.

3.3.

Laparoscopic Prototype

A $400\text{-}\mu \mathrm{m}$-core optical fiber (PN315-0033-001, 0.22 NA, QPC Lasers) connected the laser output to a custom high power fiber optic shutter system (SH-200-55-1470-M-0-T-BH-SP, OZ Optics, Ottawa, Canada). A function generator (DS345, Stanford Research Systems, Sunnyvale, California) precisely controlled the laser pulse duration. Laser energy was delivered through a $550\text{-}\mu \mathrm{m}$-core-diameter optical fiber (FG550LEC-Custom, Thorlabs, Newton, New Jersey) with high power proximal SMA connector, from the optical shutter output port to the inside of the laparoscopic probe. The laparoscopic prototype with 5-mm-outer-diameter shaft featured a traditional Maryland jaw configuration (Fig. 4). A reflective insert inside of the lower jaw reflected light at an angle of 90 deg to provide a side-delivery interface, with a beam measuring $18\text{-}\mathrm{mm}\text{-}\text{length}\times 1.2\text{-}\mathrm{mm}\text{-}\text{width}$. The reflective insert consisted of a series of cylinders placed in a “staircase” like arrangement in which light was incident perpendicular to the cylinders and reflected in a line shape. Figure 3 shows the reflective insert and a spatial beam profile, acquired using an infrared beam profiler (Pyrocam III, Spiricon, North Logan, Utah). The beam profile provided a pixel count, and knowledge of the size of each pixel and the magnification used enabled calculation of the beam dimensions. A sapphire window (SCD3955, Meller Optics, Providence, Rhode Island) was placed above the insert, and silicone adhesive was used to create an air and watertight seal, ensuring minimal particle and fluid contamination of the insert. An aluminum insert was also used in the upper jaw to provide a backstop for the light and to compress the tissue sample.

Fig. 4

Prototype device using traditional 5-mm OD Maryland jaw configuration. Insert shows magnified view of upper and lower jaws on the laparoscopic device. Thermal camera was used for noninvasive temperature mapping during the procedure.

3.4.

Vessel Sealing Setup

During experiments, porcine vessels were placed in between the bottom and top jaw, and then compressed to the maximum compression allowed by the probe. Once compressed, vessels were irradiated at 68.3 W for 3 s, with light delivered from the lower jaw. The vessel and jaw temperatures were monitored and recorded by a thermal camera (A65, FLIR, Wilsonville, Oregon). The data were then collected and analyzed by the camera software (ResearchIR, FLIR) on a personal computer. Vessel location on jaw, vessel diameter, and degree of desiccation, sticking, and charring were recorded for each vessel. Photographs were also taken before and after laser irradiation of each tissue sample to measure thermal damage.

3.5.

Burst Pressure Measurements

Vessel burst pressure measurements were used as the primary indicator for measuring vessel strength and success. The standard burst pressure setup consisted of a pressure meter (Model 717 100 G, Fluke, Everett, Washington), infusion pump (78-01000C, Cole Parmer, Vernon Hills, Illinois), and an iris clamp (Fig. 5). The vessel lumen was placed over a cannula attached to the infusion pump. An iris was then closed to seal the vessel onto the cannula. Deionized water was infused at a rate of $100\mathrm{ml}/\mathrm{h}$ and pressure was measured with the meter. Maximum pressure (mmHg) achieved when the vessel seal burst was then recorded. Although there is no universally accepted standard baseline burst pressure for measuring successful seals in the laboratory, for the purposes of this preliminary feasibility study, a seal was considered successful if it exceeded the physiological values for both normal systolic blood pressure (120 mmHg) and malignant hypertension blood pressure (180 mmHg). A total of 69 vessels were tested and the mean and standard deviation of burst pressures was then calculated for different vessel sizes.

Fig. 5

(a) Burst pressure experimental setup; (b) an iris was used to clamp vessels to a needle, which were then filled with deionized water until the seal burst. Pressure before burst was recorded using a pressure meter.

3.6.

Thermal Damage Measurements

Measurements of the seal width, thermal damage width, and lateral thermal spread (defined as total thermal damage zone minus seal width, divided by two) were taken from photographs of gross images of each vessel after laser irradiation, with a clear delineation or transition from native to thermally coagulated zones in the samples, as shown in Fig. 6, and consistent with previous studies.²¹ Such gross images have been shown to correlate well with histologic analysis provided in previous studies.^22,23

Fig. 6

A vessel sample with seal width (1.1 mm), thermal damage (3.8 mm), and lateral thermal spread (1.4 mm) labeled.