Minimally invasive surgery (MIS) is a relatively modern technique that enables surgeons to perform operations through small incisions. Compared to traditional, ‘open’ surgery, MIS presents clear advantages for limiting trauma to the patient and encouraging faster recovery, but it is more intricate to perform and requires further surgical training. Minimally invasive robotic surgery (MIRS) systems offer solutions to minimize or eliminate many of the shortcomings associated with traditional MIS techniques. Since 2000, when FDA granted approval to the first such system, MIRS system developers have continued to advance the capabilities of the technology, in part through the incorporation of force sensors that improve the haptic feedback provided to users.
To find out more about the use of advanced force sensors in robotic surgery applications, MDB recently spoke with Ebenezer Ferreira, PMP, MBA, project manager for medtech programs at FUTEK Advanced Sensor Technology Inc. (Irvine, CA). FUTEK specializes in creating innovative custom force and torque sensor solutions for today’s leading tech innovators.
MDB: MIS techniques and advanced MIRS systems have been evolving for more than two decades. What have been found to be their key strengths and weaknesses?
Ebenezer Ferreira, PMP, MBA: Early MIS techniques offered many benefits, but also had a number of inherent drawbacks, including the limited range of motion permitted by straight laparoscopic instruments; limited fixation enforced by the small incision; impaired vision of the surgical site as a result of dependence on two-dimensional imaging; amplification of a surgeon’s tremors by long instruments; poor ergonomics imposed on the surgeon; and loss of haptic feedback, which is distorted by friction on the instrument and reactionary forces from the abdominal wall.
FDA’s approval of the first MIRS platform—the Da Vinci surgical system (Intuitive Surgical, Sunnyvale, CA)—marked a milestone in the history of surgical technologies. The ability of MIRS systems to leverage the advantages of MIS while augmenting surgeons’ dexterity and visualization, and eliminating the ergonomic discomfort of long-lasting surgeries, has made them an essential technology with benefits for patients, surgeons, and hospitals alike.
Despite all the improvements brought about by commercially available MIRS systems, however, haptic feedback is still a major limitation reported by surgeons. Since the interventionist no longer manipulates the instrument directly, MIRS systems eliminate the natural haptic feedback provided in traditional open surgery or first-generation MIS.
MDB: What is haptic feedback, and why is it important in robotic surgery?
Ferreira: Haptic feedback is produced by a conjunction of touch sensations involving both kinesthetic (form and shape of muscles, tissues, and joints) and tactile (cutaneous texture and fine detail) perceptions. It is a combination of many physical variables, including force, distributed pressure, temperature, and vibration. Benefits of being able to sense interaction forces at the surgical end-effector include improved organic tissue characterization and manipulation, easier assessment of anatomical structures, reduced breakage of surgical sutures, and an overall increase for the ‘feel’ of robotic-assisted surgery.
During surgical training for MIRS procedures, haptic feedback can shorten the learning curve by providing valuable data about instrument-tissue interaction forces. Further, novel technologies such as haptic feedback force sensors can improve surgeons' hand-eye coordination, thereby preserving dexterity and extending the careers of surgeons, and ultimately increasing the number of ‘surgeon-hours’ available to support societal medical needs.
MDB: Beyond the benefits to surgeons, how do haptics improve MIRS outcomes?
Ferreira: Haptics offer benefits for all stakeholders. For patients, haptics can result in less trauma to internal organs and tissues, reducing recovery time in the hospital or other inpatient settings. For hospitals, haptic feedback can leverage quicker patient turnover rates to free up resources, provide more-accurate surgical procedure data for medical training, and reduce costs per procedure. And for payers, haptics can potentially lower medical costs and bring about improved member satisfaction.
Haptic feedback also offers the benefit of accurate real-time direct force measurements during surgical procedures. The data collected from haptics sensors can be used to produce accurate tissue and organ models for surgical simulators used in MIS training.
MDB: What kinds of challenges are involved in the application of haptics to MIRS systems?
Ferreira: FUTEK’s involvement in the application of haptics to MIRS systems can be part of creating a brand-new system, or retrofitting haptics components into an existing system. In general, the biggest challenge when developing a brand-new system has to do with the design timeline, which can sometimes be more aggressive than should be permitted under design control requirements.
Retrofitting haptics components into an existing system generally presents more complications to be considered, especially when the desired performance improvements involve the application of multiple haptics components, each with their own requirements. It is possible, for example, to engineer an autoclavable sensor, to design a flexure to withstand extraneous loads, or to implement a framework for integrated system electronics. But accomplishing all of these feats together is very challenging. Nevertheless, in some of our recent projects, we’ve accomplished all of these and many more.
MDB: When seeking to incorporate haptics sensors into a MIRS system, what options are available to product developers?
Ferreira: Early in the process of designing a MIRS system—and well before any sensors enter the design or manufacturing phases—the engineers and scientists seeking to apply the sensors must first address the important issue of where the sensors are to be located.
The technologies involved in measuring haptics are inherently complex, and the location of a sensing element can significantly influence the consistency of its measurements. MIRS designers must therefore decide whether to use ‘indirect force sensing’—an approach in which the sensor is placed outside the abdominal wall, near the actuation mechanism driving the end-effector—or ‘direct force sensing’—an approach in which the sensor is embedded on the end-effector at the tip of the MIRS instrument that enters the abdominal cavity. Each of these approaches is associated with different levels of measurement accuracy as well as different size restrictions and requirements for sterilization and biocompatibility (Table 1).
For most MIRS procedures, it is critical that the surgeon be able to sense the interaction between the MIRS instrument and the human organs and tissues it encounters. This need typically leads the developers of MIRS systems to conclude that direct force sensing is necessary, and that placing the sensing element near the end-effector is the more appropriate approach. However, this novel approach raises a number of design and manufacturing challenges that can run up against economic constraints on the development and use of such systems.
Reusability is an important strategy for reducing the costs associated with using a MIRS system. Commercially available MIRS systems that are modular in design permit instruments to be reused approximately 12 to 20 times. However, placing the sensing element near the end-effector invariably increases the cost of the instrument, and requires system developers to give serious consideration to strategies that can further enhance sensor reusability.
To improve the reusability of a MIRS system, appropriate electronic components, strain measurement methods, and electrical connections must all be able to survive a high pH washing and withstand additional autoclave cycles. Satisfying such special design requirements increases the unit cost per sensor. However, increasing the number of permissible sterilization cycles can extend the lifespan of a MIRS system, consequently reducing the cost per cycle and making the direct force sensing approach more economical.
MDB: Does increased reusability make it easier for hospitals to consider adopting a MIRS system?
Ferreira: In the current environment, capital expenditures do not represent a major problem for most hospitals. Across both established and emerging capital markets, the cost of capital is currently at an all-time low, and one has seen even negative interest rates. So hospitals and other medical institutions have sufficient access to cheap capital to support the purchase of MIRS systems.
The real issue for hospitals is the operational expenditures—or cost per procedure—associated with implementation of a MIRS system. Hospitals must be able to justify such expenditures so that insurance carriers and other payers will consider the technology acceptable from both a medical and business perspective. That’s where reusable, autoclavable sensors fit in. They improve the reusability of MIRS instruments and drive the costs of such systems down to acceptable levels.
MDB: What other characteristics are desirable in haptics feedback sensors being applied to MIRS systems?
Ferreira: In addition to meeting the requirements essential for any high-performance force measurement device, haptics sensors used in surgical environments must also satisfy unique requirements for miniaturization, biocompatibility, autoclavability, and reusability.
When high-precision subminiature force-sensing elements are used in intraabdominal direct force sensing, it is essential that they be hermetically sealed. The conventional approach to sealing such electronic components is the use of conformal coatings, which are extensively utilized in submersible devices. While such an approach is practical for consumer electronics that may be exposed to low-pressure water submersion environments, however, coating protection is not sufficiently hermetic for medical applications intended to remain highly reliable after many cycles of reuse and sterilization. Another conventional approach to achieving hermeticity is to weld a header interface onto the sensor. But in miniaturized sensors, size constraints often present an insurmountable obstacle to the use of welding. A novel and robust approach to overcoming the limitations of coating and welding technologies is to design a monolithic sensor that uses high-temperature fused isolator technology to feed electrical conductors through the walls of the sensing element. Such sensors can be custom formulated to be chemically neutral; thermally compatible, with a matched coefficient of thermal expansion for all components to prevent failure and limit thermal drifts; and reliable after hundreds to thousands of autoclave cycles.
Another area of concern is compensating for extraneous signal loads (crosstalk), in order to provide optimal resistance to off-axis loads, thereby minimizing reading errors and maximizing the sensor’s operating life. Force and torque sensors are engineered to capture forces along the Cartesian axes, typically X, Y, and Z. From these three orthogonal axes, one to six measurement channels derives three force channels (Fx, Fy, and Fz) and three torque or moment channels (Mx, My, and Mz). Theoretically, a load applied along one of the axes should not produce a measurement in any of the other channels—but this is not always the case. For a majority of force sensors, undesirable cross-channel interference may be between 1% and 5% of the total signal load. Considering that a single channel can capture extraneous loads from five other channels, total crosstalk could be as high as 5% to 25%. When intended for MIRS applications, force sensors must be designed to negate crosstalk loads, which may include friction between the end-effector instrument and trocar, reactionary forces from the abdominal wall, or the gravitational effects of mass along the instrument axis. In some instances, miniaturized sensors are very limited in space and have to compensate for side loads using such alternative methods as electronic or algorithmic compensation.
Thermal effects also represent a major challenge in strain measurement. Temperature variations cause material expansion, gage factor coefficient variation, and other undesirable effects on measurement results. Temperature compensation is needed to ensure the accuracy and long-term stability of the sensors, even when exposed to severe ambient temperature oscillations. Measures to counteract temperature effects on sensor readings include the use of high-quality custom and self-compensated strain gages compatible with the thermal expansion coefficient of the sensing element material; use of half or full Wheatstone bridge circuit configuration installed in both load directions (tension and compression) to correct for temperature drift; and full internal temperature compensation of the zero balance and output range, without the use of external conditioning circuitry. In some special cases, the use of custom strain gages with reduced solder connections can help to reduce the temperature effects of solder joints. A regular force sensor with four individual strain gages typically has upward of 16 solder joints, but custom strain elements can reduce the number of joints to fewer than six. As each solder joint represents an opportunity for failure, reducing the number of such joints can greatly improve reliability.
Sensors used in medical applications are rarely required to resist strenuous structural stresses, but they must be able to withstand a high number of reuse and sterilization cycles. For MIRS sensors in particular, overload and fatigue testing must be performed in conjunction with sterilization testing in an intercalated process. The ability to survive hundreds of overload cycles while maintaining hermeticity, translates into a failure-free, high-reliability sensor with lower mean time between failure and a more competitive total cost of ownership.
MDB: How do system developers define high performance with regard to MIRS applications?
Ferreira: From a sensor design perspective, autoclavability, hermeticity, high-volume manufacturability, and thermal effect compensation are all characteristics that are sought after in a MIRS force sensor. In addition, sensors must also exhibit high-level mechanical performance, as measured by the following factors:
- Creep. This measurement represents the change in transducer output occurring over time, while under load, and with all environmental conditions and other variables remaining constant.
- Hysteresis. The maximum difference between transducer output readings for the same applied load; one reading is obtained by increasing the load from zero and the other by decreasing the load from the rated output. Hysteresis is usually measured at half the rated output and is expressed as a percentage of the rated output. Measurements should be taken as rapidly as possible to minimize creep.
- Linearity (or nonlinearity). Few force sensors have a completely linear characteristic curve, meaning that their output sensitivity (slope) changes at a different rate across the measurement range. Some sensors are linear enough over the desired range and do not deviate significantly from the straight line (theoretical), but others require more complex calculations to linearize their output. So, force sensor nonlinearity is the maximum deviation of the actual calibration curve from an ideal straight line drawn between the no-load and rated load outputs, expressed as a percentage of the rated output.
- Long-term zero stability. This value is the degree to which the transducer maintains its zero balance with all environmental conditions and other variables remaining constant.
- Repeatability (or nonrepeatability). This value is the maximum difference between transducer output readings for repeated loadings under identical loading and environmental conditions. It represents the force sensor’s ability to maintain consistent output when identical loads are repeatedly applied.
- Temperature shift span and zero. This measurement represents the change in sensor output span or zero, respectively, due to a change in transducer temperature.
MDB: FUTEK has developed an extensive line of sensors suitable for MIRS applications. Which of these components is used most often, and what performance characteristics do they offer for MIRS systems?
Ferreira: FUTEK has been investing in research and development for miniature custom sensor solutions for three decades, advancing medtech innovation in areas such as haptic feedback in robotic surgery and less-invasive tools for laparoscopy. FUTEK sensors combine subminiature package design; precision measurement; and consistent sensitivity, stability, and repeatability in sterile environments, paving the way for smarter and more reliable healthcare solutions.
The majority of FUTEK’s robotic surgery components and MIRS force sensors are produced for original equipment manufacturers under nondisclosure agreements. However, we also offer a number of standard commercial off-the-shelf force sensors for use in medical devices:
- The QLA414 Nano ultraminiature force transducer is the smallest unit of its kind (Figure 1). It offers a high capacity to size ratio, and is particularly well suited for robotic surgery applications, as it allows force measurement to be pushed to the very tip of robotic surgery tools, laparoscopy instruments, and other medical devices where sensor size is critical.1
- The LSB205 is an adaptable load cell that boasts high power in a small package. It is capable of measuring tension and compression forces from 1 to 100 lb, and features formidable fatigue life and off-center loading capabilities. The unit is suitable for monitoring tension forces on the strings or rods used to control the motion of MIRS tools (Figure 2).2
- The QTA141 miniature torque sensor provides highly accurate strain gauge and torque measurements in a package that measures just 0.86 inches in diameter and 0.39 inches in height (Figure 3). The sensor was developed to provide a faster and more accurate torque feedback system, closing the loop and improving the 5% to 10% accuracy of the current loop to 0.25% or better. Such feedback is especially beneficial for surgeries that require swift movement and consistent torque to minimize trauma, such as automated thoracic retractor procedures.3
MDB: You mentioned FUTEK’s work to extend the autoclavability of its sensors. What other performance advances are being incorporated into the company’s latest generation of sensors?
Ferreira: An important challenge in supplying micro autoclavable sensors is the ability to meet customer needs for product ramp-up and high-volume manufacturing. In that regard, FUTEK is well versed in working with top players in the robotic surgery market, and is capable of delivering thousands of the force sensors that go inside MIRS systems.
Although it is easily passed over, the manufacturability of sensors for MIRS applications is extremely important. When design engineers are engaged in developing a robotic armature, they devote most of their attention to arm design, motor and gearbox specifications, joint mating parts, the physician-robot gimble interface, software development, and system integration. The manufacturability of a force sensor can easily be neglected, in turn resulting in over-constraining critical features of the flexure element, or not utilizing the most economical strain gage or cable for the sensor.
Another area critical for MIRS force sensor design is machinability. While working on OEM projects, we have encountered a number of opportunities to improve on the customer’s design in order to achieve better sensor performance, or to optimize geometric dimensioning and tolerancing in order to improve machinability. Since machining occupies the bulk of the schedule for sensor manufacturing, it has a tremendous impact on production yields. Working hand-in-hand with a sensor design specialist during the conceptual phase of robotic joint design can save both time and money for the MIRS company.
MDB: Are there other areas in which FUTEK is working to respond to the needs of the MIRS marketplace?
Ferreira: Although it is often taken for granted, conditioning of the force sensor’s output signal for integration with the robot controller’s motherboard is an area of great importance. Successful integration requires the combined efforts of mechanical and electronic engineers in an iterative process that is typically more productive when performed in the very early stages of the robotic system’s design. FUTEK always prescribes a full turnkey force-measurement solution that incorporates a force-sensing element as well as a digital signal conditioner, ultimately delivering a filtered and amplified digital output to the robot controller. Adopting such a turnkey solution reduces the design effort required of the engineering team, which can then invest its time in other areas of the robot’s design.
MDB: How would you characterize the current strength of the market for MIRS technologies?
Ferreira: It’s been estimated that more than 1 million robotic surgeries are performed in the United States each year, and that number continues to grow. The US market is currently valued at roughly $3 billion in annual industry revenue, and of course that number is also increasing. Silicon Valley investors alone have provided some $5 billion in funding to support the growth of the robotic surgery industry.
Although the COVID-19 pandemic has disrupted the ways that we interact in our professional lives—as well as our efforts to balance all work-life elements—the adoption of remote and work-from-home solutions remains a strong market trend. In the short term, the pandemic has created an economic roadblock to expansion of the robotic surgery market because hospitals have had to postpone elective procedures in order to free up hospital resources to handle infection cases.
But in the long run, we believe the pandemic will accelerate the adoption of robotic surgery procedures and related technologies and, most important, will help to overcome any residual psychological barriers to broader acceptance of this novel technology. Already, the pandemic has brought about significant changes in the ways that patients are engaging with their healthcare providers, including rapid adoption of remote and telehealth applications for many situations. Ultimately, we expect that these trends will expedite broader acceptance of MIRS procedures and technologies, not only by physicians but also by patients.
MDB: What do you see as the key opportunities for future growth of the MIRS market? How will FUTEK continue to support achievements in these areas?
Ferreira: FUTEK CEO Javad Mokhbery is fond of reminding us that “the future belongs to those who dig wells before getting too thirsty.” FUTEK continuously fosters this kind of thinking with cross-functional engineering teams that support customer requirements for miniaturization, autoclavability, and turnkey force-sensing solutions. In addition, the company maintains a supply chain based on long-established vendor relationships, making it possible to allocate significant manufacturing resources in support of fast turnaround for high-volume OEM robotic surgery applications. FUTEK’s marketing and sales team is especially attentive to these customers, making sure that internal resources are marshaled and aligned as necessary to meet the needs of the robotic surgery market.
- Introducing the Nano miniature force sensor [online]. Irvine, CA: FUTEK, 2020. Available at: https://go.FUTEK.com/nano-platform. Accessed August 13, 2020.
- Miniature S-beam Jr. load cell 2.0 [online]. Irvine, CA: FUTEK, 2020. Available at: https://www.futek.com/store/load-cells/s-beam-load-cells/miniature-s-beam-jr-LSB205. Accessed August 13, 2020.
- Micro reaction torque sensor [online]. Irvine, CA: FUTEK, 2020. Available at: www.FUTEK.com/store/custom-sensors-and-instruments/micro-reaction-torque/micro-reaction-torque-sensor-QTA141. Accessed August 13, 2020.