Pulsed Electrochemical Machining in the Surgical Robotics Industry

With a custom cathode, PECM is capable of machining unique features, including complex gear profiles, with high repeatability.

Introduction

Robot-assisted surgery (RAS) techniques are growing in popularity as surgeons and engineers collaborate to optimize the dexterity and precision of surgical equipment to improve patient safety. RAS can reduce trauma and improve patient recovery in a variety of ways, including, but not limited to:  

• Minimally-invasive-surgical (MIS) techniques via RAS reduces the quantity and size of incisions
• The high dexterity of RAS can be gentler on sensitive tissue
• Robotic arms can hold or position other equipment, allowing surgical assistants to perform other crucial tasks

The collaboration between surgeons and engineers developing RAS equipment is sometimes called “surgineering”, which will be a recurring phrase central to this article’s premise.

At its core, the purpose of surgineering is to optimize the symbiotic relationship between human and machine—to effectively translate the intentions and actions of the surgeon to the equipment and, simultaneously, relay all necessary information back to the operator.

Broadly speaking, RAS can achieve the objective of surgeon-robot symmetry in four distinct ways:

• Optimize control ergonomics and display for surgeon comfort and tremor minimization
• Maximize the dexterity of robotic arms and end-effectors to improve functionality
• Ensure the robotic equipment receives and executes information quickly and accurately
• Effectively relay information back to surgeon

Out of these methods, one is very unlike the others. While the control ergonomics, information processing, and information execution all loosely fall under a software programming paradigm, maximizing robotic dexterity is an issue closer to mechanical engineering. In other words, robotic dexterity is ultimately driven by the mechanics in the joints and end-effectors, as opposed to relying on the programming or information processing.

Mechanical engineering challenges associated with robotic dexterity are, in turn, manufacturing challenges with the objective of producing optimal components that allow the robot maximum dexterity and precision. We will explain several ways of how this can be achieved, such as with manufacturers producing tight-tolerance gear components with minimum backlash, or by reducing friction in moving parts that may otherwise impair the surgeon’s capabilities, or even by producing miniaturized components (such as smaller-diameter surgical instrumentation affixed to the end-effectors) that minimize necessary incisions or apply reduced pressure to sensitive tissue.

Producing high-quality, higher-volume precision parts for the surgical robotic industry with the goal of improving dexterity and precision is an engineering and financial challenge, particularly for conventional manufacturing methods. Fortunately, a technology called pulsed electrochemical machining (PECM) may alleviate many of these technical and economic hurdles.

The purpose of this article will be to explain the various engineering and financial challenges of producing critical components in the surgical robotics industry, explain how PECM works, and discuss the applicability of PECM to produce these components.

PECM fundamentals

Before we discuss the engineering challenges afflicting the surgical robotics manufacturing industry (and PECM’s potential role) it is important to establish a framework by first explaining how PECM works.

How PECM Works

Pulsed electrochemical machining, or PECM, is a non-thermal, non-contact material removal method capable of producing small features, superfinished surfaces, and high repeatability on metallic parts. It is an improved version of electrochemical machining (ECM) that enables higher resolution and better surface finish.

In PECM a custom tool electrode is used to dissolve the workpiece material atom-by-atom using a charged electrolytic fluid in microscopic gap between the tool and workpiece.

To understand the process best, there are four key terms to know:

The cathode, or tool, is a custom-designed part shaped as the inverse of the desired geometry to be machined on the workpiece.

The anode, or workpiece, could be a 3D printed part, wrought stock, a near-net shape, or take on many other forms. However, it is crucial the anode must be a conductive material. PECM will not work on non-conductive parts, including any polymers or plastics.

The Electrolytic Fluid is a salt-based electrolyte that is flushed between the tool and workpiece serves two crucial purposes: the fluid both acts as the conductor for the electrochemical reaction to take place and as the flushing agent that removes the dissolved material.

The Inter-Electrode Gap (IEG) is the microscopic space between the cathode and electrode, where the electrolytic fluid flows. The size of this gap is a crucial variable affecting the precision of the process; as PECM continues to advance, this gap becomes smaller, improving the accuracy of the process. Currently, this gap can be as small as 10-100um Ra (.0004-.004in).

Other PECM Applications

Outside of the applications we’ll discuss in this article, there are many more uses for the technology in critical environments. For instance, PECM can be utilized to machine critical aerospace components used in high pressure, high temperature-flux environments that require tight tolerances, high surface quality, and tough alloys. Examples include Inconel turbine vanes or microchannel heat exchangers within aircraft turbofan engines.  

Within medical device manufacturing, PECM can be used to machine parts that require superfinished surfaces, biocompatibility, and/or anti-corrosive reasons. Examples include nitinol bone fixture devices, molybdenum x-ray components, and orthopedic joints.

Now that we’ve established a framework by understanding the both the fundamentals of PECM and its applicability outside of RAS, let’s return to discussing surgineering challenges affecting the industry. Primarily, there are three common engineering challenges we’ll discuss: utilizing advanced materials, miniaturizing components, and superfinishing surfaces.

Let’s review how these engineering challenges affect critical components within two distinct parts of the robots: within the end-effectors, and inside the robotic arm joints.

An example of how PECM is capable of machining thin-walled features that may otherwise be sensitive to tool vibration or thermal distortion.

End-Effectors

 A primary surgineering objective is to miniaturize components—reducing the size of critical parts while simultaneously maintaining (or even improving) its original capabilities. There are a variety of incentives for miniaturizing end-effector components, including:

• Reducing weight, allowing additional movement and improved dexterity
• More space within the part for alternative uses such as sensors, cameras, or joints
• Smaller surgical tools which can perform different or more advanced surgical procedures or minimize patient impact
  

Miniaturization in End-Effectors

Let’s focus on this final point, as reducing the size of the end-effectors, and the surgical equipment attached, has the potential to not only change how the RAS performs a given surgery, but what types of surgery it can perform.

A primary benefit of using robotic equipment is how it generally achieves higher positioning accuracy when directly compared to freehand methods for many surgical procedures—such as pedicle screw placement for spinal surgery. The capabilities of the end-effector (and especially the grasp of the instrument) is the final axis of motion and significantly influences the overall positioning accuracy and dexterity of the robotic equipment. Put another way, consider how the dexterity, experience, and talent of an artist would be rendered useless if their fingers were incapable of holding their paintbrush properly.

Miniaturizing surgical robotic end-effectors can play a significant role in improving the dexterity, accuracy, and position repeatability of the entire robot, by lightweighting and allowing additional room for more components- including joints, pulleys, or sensors. Most importantly, smaller components can also facilitate safer, less invasive surgical procedures. Let’s briefly review an example.

Miniaturized End-Effectors and Hysterectomies

Hysterectomies, the surgical removal of the uterus, were traditionally performed via a large, six-to-eight-inch abdominal incision. As medical technology advanced, this version of the procedure was only performed in more serious cases, such as on an enlarged uterus. Today, however, with the help of miniaturized robotic surgery equipment, a majority of hysterectomies are performed in a significantly safer and more effective manner.

With the assistance of robotic instruments, the patient’s uterus can be dissected and removed during a hysterectomy via several tiny incisions made by the surgeon (sometimes just a few millimeters in length!) which allow cameras and other robotic equipment to be inserted, providing a 3D rendering of the surgical site for the surgeon to dissect and subsequently remove the uterus through the vagina.

Smaller, more dexterous surgical end-effectors are the key to this safer iteration of hysterectomies. While the 3D rendering of the surgical site provided by RAS allows surgeons to avoid dangerous errors by providing additional depth of field, this information can only be fully realized with smaller end-effectors that can navigate sensitive areas without posing unnecessary trauma. Smaller end-effectors can also allow the surgeon to manually create smaller initial incisions, which can significantly reduce patient recovery times from several months down to several weeks.

Challenges Associated with Miniaturization

However, despite the significant benefits of miniaturizing robotic surgery components, producing more dexterous and sensitive end-effectors poses significant design and manufacturing challenges.

For example, bipolar forceps require a wide range of motion: up and down the shaft, pitch, yaw and roll along a joint, and sometimes an additional dimension of pitch, yaw and roll via the wrist, and, finally, the grasping motion of the forceps themselves.

As each individual range of motion in the instrument requires a mechanical system of joints and pulleys, and potentially an additional system of sensors and/or cameras, producing smaller components in surgical end-effectors can pose significant engineering challenges.

A primary reason for these difficulties lies in the materials and unique geometries required for these components, sometimes just a few millimeters in size. For example, a robotic end-effector with monopolar curved scissors, of which can be around 8mm in length, may require even smaller components within the joints, but these parts must still require durable, corrosion-resistant materials which may be difficult to machine. While tight tolerances and small features are necessary for these small components to function optimally, surface quality is another crucial aspect of these parts that poses yet another engineering challenge.

Furthermore, the complex features and materials found within these components often require expensive, time-consuming manufacturing methods; economically producing miniaturized robotic surgery components can be a particular challenge for conventional manufacturing methods.

Surface Quality and End-Effectors

Improving surface quality on critical end-effector components provides numerous advantages: 

• Surface quality improves corrosion resistance, improving durability and safety
• Good finish on sliding or rotating features on end-effectors diminish unwanted force feedback to the surgeon
• Superfinishing surgical instruments also improves sterility by removing microcracks that could otherwise house harmful bacteria
• Certain critical parts cannot perform their function without smooth surfaces

To elaborate on this second point, consider the number of moving parts required for a single robotic instrument to properly function. As the purpose of the robotic arm is to replicate the movements and intentions of the surgeon’s own hands, a variety of robotic joints must function perfectly to mirror the movements of the surgeon. As haptic feedback reduction is a crucial objective for this equipment, these devices also require surgical tools with low sliding friction, with the objective of making any feedback representative of tissue interaction, rather than friction stemming from the device itself.

A direct example of the final point mentioned can be found within the anvils of robotic surgical stapler end-effectors. The geometry and smoothness of the surgical stapler anvil’s pockets are directly correlated with the success of the procedure; insufficient surface quality (>0.1umRa) can create a misdirected, deformed, or crumpled staples that do not adequately close the wound, thereby creating more trauma, deterring healing of the wound, and increasing the risk of re-operation.

While ultra-smooth surface quality is not a top priority for all components in the medical device industry (and some components, such as osseointegrative surfaces in orthopedic implants requiring just the opposite), critical components that need high levels of resistance to corrosion, unwanted friction, and sterility require superfinished surfaces to perform their duties.

Multiple features can be machined in tandem with PECM, such as with the pockets on this surgical stapler anvil.

Challenges Associated with Superfinishing

However, producing optimal surface quality can be a significant engineering hurdle. Critical components are often comprised of tough, unique alloys that can be difficult to both machine and finish without incurring significant tool wear costs. These materials are often brittle, and are sensitive to both conventional, contact-based machining processes and contact-based finishing processes. Furthermore, the finishing processes for these materials may result in tool wear that can affect the tool’s precision over time, and/or leading to higher manufacturing costs.

Conventional finishing processes may also leave unwanted surface irregularities, as well. Some heat-based machining processes may involve “re-cast layers”, where the ejected material accidentally re-solidifies itself to the part. Contact-based processes may also leave “burrs”, which are small slivers of material that have been pushed out of the way, but not necessarily removed from the workpiece surface.

Let’s now discuss how PECM can alleviate manufacturing issues associated with part miniaturization and surface quality within robotic end-effector components.

PECM’s Miniaturization & Surface Quality Capabilities

There are a few ways PECM may be able to simultaneously miniaturize complex robotic end-effector components while providing high surface quality.

As there is no heat or contact used in the process, PECM is awarded several unique advantages. First, it is capable of machining features of a part that may be otherwise sensitive to thermal distortion and/or tool vibration found in conventional machining methods—Including downskin surfaces and thin walls; testing showed that PECM was capable of producing <0.075mm or <0.003″ thick walls with a 20:1 aspect ratio in hardened stainless steel.

Furthermore, PECM is more concerned with a material’s conductivity, as opposed to its hardness. This means that many tough materials used within end-effectors aren’t machined any slower by PECM (so long as they are conductive), which may include, but are not limited to, hardened stainless steel alloys and some titanium alloys.

PECM’s surface finishing capabilities are also an excellent fit for producing the surfaces required in critical RAS components. AS PECM uses no heat or contact, it therefore leaves no tool marks, burrs, or recast layers that could produce unwanted surface irregularities. PECM leaves a mirror-like surface quality on some materials used in end-effectors; even down to .005-.4um Ra. (.19-15.74uin).

Other conventional processes are certainly capable of producing similar tolerances and surface quality on these materials, however PECM is unique in that it both machines and finishes the part simultaneously, allowing faster cycle times and reducing the necessary steps required for producing optimal components. As PECM is also capable of producing and replicating complex features in high part volumes, PECM can machine complex features in end-effectors in tandem (with the proper cathode).

Gears

Complex gear profiles are the primary components that allow a robotic arm to move, and the material, tolerances, and surface quality of these gear profiles can have a significant effect on the robot’s lifespan, capabilities, and precision. We’ll review the importance of these qualities, their respective engineering challenges, and finally, discuss how PECM may alleviate these challenges.  

Strain Wave

Robotic equipment often utilizes strain wave gears, also known as harmonic drives, for their minimal backlash, compact features, and high-torque capabilities, usually within the elbows. With minimal friction and maximum reduction ratios, using compact strain wave gear profiles can improve equipment dexterity by simultaneously reducing the weight/size of critical parts and maximizing powertrain efficiency. In other words, strain wave gears in RAS ultimately help shorten the metaphorical gap between the surgeon’s intentions and the equipment’s movements.

However, strain wave gears are particularly challenging to machine. One of the most difficult components is the inner “flex-spline”, which must deform to the elliptical shape of the innermost wave generator. The thin walls and tight tolerances, combined with the requirement of a unique flexible (yet torsionally stiff) material for this flexspline is particularly challenging to machine with conventional, contact-based methods. These features are also extremely sensitive to thermal distortion or tool vibration inherent in these conventional processes, even including more advanced processes like electrical discharge machining (EDM).

Gear Material

When designing and implementing critical robotic surgery components, especially gears, a primary consideration is choosing the best material(s) for the application. The material of critical components can have profound effects on the dexterity, durability, and safety of the gear, and thereby the entire equipment. A good material can provide:

• Lightweighting (while maintaining functionality) improving dexterity
• Higher tensile strength and corrosion resistance, lengthening part lifetime

Within critical environments, such as within RAS equipment, the materials capable of fulfilling these objectives tend to be tougher, exotic alloys with lower levels of machinability, creating a significant engineering challenge for critical part manufacturers.

For example, many critical gears in robotic equipment (like strain wave gears) are usually comprised of unique alloy steels capable of high torque and flexibility (such as AISI 4030) containing nickel, chromium, and even some refractory metals including molybdenum. These advanced alloys, while dramatically increasing the gear’s durability and strength, can be difficult when machined, wearing down tooling quickly. As the tool is worn, its precision is decreased, which can be problematic when machining sensitive features and impacts the repeatability of the machining process.

Other gear types, such as planetary gears and rack-and-pinion gears, may also be utilized in the same gear trains as strain wave gears, or even within the end-effector joints. The criticality of these applications warrants gears with extremely tight tolerances and durable materials that resist corrosion, unwanted friction, and improve part lifetime, all of which can severely diminish the effectiveness of the robotic equipment and potentially cause a safety hazard by part breakage or unwanted movements.

Outside of medical environments, the robotic industry is also exploring the implementation of robots in harsh, unhospitable environments such as within nuclear reactors, extremely hot or cold locations across the globe, and even within space. This robotic equipment would thereby require materials capable of withstanding extreme temperature flux, and be capable of functioning for prolonged periods of time without lubrication.

In the distant future, robotic surgery equipment may even be utilized in extreme environments, such as within spacecraft, which would require unique strain wave gears comprised of exotic alloys. The high temperature flux in space would warrant gear materials capable of withstanding these environments, and would also require these gears to function for long periods of time without lubrication. Extensive research on this topic has produced unique ‘bulk metallic glass’ strain wave gears, which will be increasingly implemented in robotics within spacecraft, and, realistically, could be extended to robotic surgery devices potentially implemented in space in the not-so-distant future. Bulk metallic glass, however, is particularly challenging to machine with conventional methods in the geometries required for strain wave gears.

Gear Surface Quality

Surface quality has a considerable effect on the performance on gears in multiple ways, such as:

• Reducing friction within high-stress, high-contact gear trains
• Improving control ergonomics with smoother surfaces
• Improving corrosion resistance, impacting durability

If the critical gears within robotic instrumentation are machined with poor surface quality, a variety of problems can occur. For example, wear on gears can affect the performance or feel of the equipment over time, impacting the repeatability of the operation. Furthermore, unwanted friction can also create extra force on the gear train which may lead to larger motors.

However, producing high quality surfaces can be challenging for conventional methods. Many heat or contact-based finishing methods, such as electrical discharge machining or laser cutting, can produce surface defects in the form of machining marks, burrs, and recast layers.

How PECM Can Produce Complex Gear Profiles

While the material and surface quality required for critical gear profiles can be challenging to machine conventionally, PECM’s unique attributes can be advantageous for machining critical gears in robotic equipment, especially strain wave gears.

A primary advantage of the process for these components is its disregard for material hardness. PECM uses an electrochemical reaction to remove material and is unaffected by the material’s hardness. Therefore, PECM can machine most conductive materials at moderately equal rates, I.E. machining nickel superalloys or tough stainless steels at a similar speed to copper. In the context of complex gear profiles in RAS, PECM is capable of machining hardened stainless steels and bulk metallic glasses with high surface quality and repeatability.

PECM may also be capable of machining the unique features of some complex gear profiles, such as the thin walls used in the ‘flexspline’ of strain wave gears, or the tight-tolerance tooth profiles of a rack-and-pinion gear in a robotic end-effector. PECM may even be capable of machining gear tooth profiles that may have performance-related advantages for specific critical applications.

Furthermore, PECM is advantageous for machining gear profiles that require high surface quality. Since PECM involves no heat-affected zone or contact-based machining, there are no recast layers or burrs left after the process is complete. PECM has produced mirror-like surface quality down to .005-.4um Ra, or .19-1.57 uin Ra, in advanced materials relevant to these applications. These finishing qualities are inherent to the process itself, as PECM can machine and finish parts in a single operation.

Conclusion

As part volumes of critical RAS components are increasing as the global healthcare market seeks new, innovative ways to perform safer surgical procedures, so too will the engineering challenges associated with producing these critical components.  Whether it is machining advanced materials in strain wave gears, miniaturizing critical components within end-effectors, or providing unique surface quality on surgical tools, manufacturers should evaluate new, efficient machining technology, such as PECM, that can not only produce components with tight tolerances, but replicate those features to meet increased demand for RAS