SAINT PAUL, Minn. – Daniel Friedrichs, Senior Principal Engineer for Surgical Energy at Minnetronix Medical, was recently featured in the 10th Anniversary edition of IEEE Power Electronics Magazine for his article titled, Surgical Energy: A Unique Power Electronics Application in Healthcare.
The article covers the history of surgical energy as a therapy, three different timely applications (radiofrequency ablation, electroporation, and pulsed field ablation) and how power electronics design and development is one of the key ingredients in making this technology work.
Friedrichs leads the development of surgical energy systems for Minnetronix Medical, a development and manufacturing partner to the medical device industry. He has specialized in medical power electronics for 15 years and holds over 35 patents related to medical applications of power converters.
The full article can be found below:
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Introduction
Surgical energy systems, a specialized niche in power electronics, directly connect power converters to patients to deliver electrical energy for medical therapies such as radiofrequency ablation, electroporation, and pulsed field ablation. These systems are extensively used in hospitals and clinics for treating a wide range of health conditions yet are relatively unknown to the broader technical community. This article introduces the field of surgical energy, describes the power electronics design challenges unique to these applications, and highlights surgical energy’s significant positive impact on human health.
While other medical devices exist that provide electrical current to tissue (for example: pacemakers and neurostimulators), surgical energy refers to a very literal connection between power electronics and patients in which the output of a power converter delivers significant voltage (hundreds to thousands of volts) to modify tissue (thermally or electrically) surgically (Figure 1). As such, the power electronics technologies involved are familiar to readers of this magazine, but this medical application may not be.
Far from a rarity, there are over 300 different models of these types of medical devices on the market today [1]. The breadth of health conditions they treat spans from head to toe on the human body; Figure 2 illustrates over 60 examples of diseases for which surgical energy is either currently being used or investigated.
Background
While human knowledge of electrical shock hazards predates the harnessing of electricity, it was not until 1891 that scientists observed that these shock hazards have an important frequency dependence. D’Arsonval demonstrated in 1893 that high-frequency (hundreds-of-kHz) electrical current could pass through tissue without risk of electrical “shock” (muscle contraction); a discovery that rapidly precipitated the establishment of the field of surgical energy [2]. The first clinical uses were for electrosurgery, a method of using high-frequency electrical current to cauterize bleeding tissue [3].
Today, the use of surgical energy has expanded far beyond electrosurgery to encompass a wide variety of uses that include both thermal and non-thermal mechanisms, traditional and minimally invasive procedures, and therapies that either work alone or in combination with a drug or other adjuvant. Three examples (radiofrequency ablation, electroporation, and pulsed field ablation) have been selected (due to their timeliness, impact, and interest to the power electronics community) for review in this article.
Radio Frequency Ablation: Clinical Application
For many types of tumors (including those of the liver, kidney, and breast), surgical resection (removal) is therapeutic. However, many tumors are not candidates for surgical resection, and even those that can be surgically resected may involve an invasive operation. Surgical energy offers an alternative: rather than resecting a tumor, it may be ablated (killed) via a minimally invasive procedure, with the harmless remnant tissue left in place [4].
Radio frequency (RF) ablation is the most common example of this therapy, and the procedural steps are briefly described here. A needle electrode is guided to the center of the tumor (often using X-ray or ultrasound imaging for positioning); Figure 3(a). Once placed, the electrode is connected to a high-frequency (radio frequency) source of current. As current flows from the electrode into the tumor tissue, it encounters electrical resistance and generates I2R heat; Figure 3(b). As heat spreads outwards through the tumor, thermal necrosis causes cell death; Figure. 3(c). Once the volume of ablated tissue encompasses the volume of the tumor, the current is stopped, the needle is removed, and the therapy is complete; Figure 3(d). Residual (dead) tissue is either absorbed or encapsulated by the body.
While not applicable to every patient or disease state, in many cases, radiofrequency ablation offers a potentially curative and minimally invasive treatment. The healthcare impact of substituting invasive surgery with a minimally invasive approach is large and obvious.
Radio Frequency Ablation: Power Electronics
The power electronics used to generate the high-frequency currents used in RF ablation typically follow the architecture shown in Figure 4.
After an AC/DC stage, an adjustable DC/DC converter provides a variable voltage to an inverter, which feeds a filter (typically a resonant filter, such as an LLC or LCLC). For electrical safety, it is critical that only high-frequency AC reaches the patient, so a transformer provides galvanic isolation (and often a step up in voltage). A typical output specification is 0-200 W, 0-200 V, and is at a fixed frequency in the range of 200 kHz-4 MHz.
Electrodes used for RF ablation typically have integral thermocouples. Electrode temperature is the main input to the control system, which adjusts the amplitude of the DC/DC stage to maintain tissue temperature just below 100°C (any higher may boil water in the tissue, generating steam and interrupting the electrical circuit). Output voltage and current sensors further limit the output to comply with a procedure-specific “power curve” which defines the maximum allowed output current, power, and voltage as a function of load impedance [5]. As every application or therapy has slightly different requirements, hundreds of variations on this basic architecture exist as unique products in the surgical energy market.
From Thermal Energy to Electric Fields
Surgical energy therapies which rely on thermal energy (such as radio frequency ablation) are in widespread use today and can be found in any hospital or clinic. While innovation continues to occur with this technology (and new products are regularly introduced), much attention is focused on a surgical energy therapy that utilizes a different mechanism of action: electric fields.
Cell walls contain pores actuated by small voltages due to the differing ionic concentrations between the intra- and extra-cellular environments. This trans-membrane potential can be overcome by the application of an external electric field, allowing the opening and closing of cellular pores at will. This technique, termed electroporation, has become a mainstay of laboratory science but recently has also been employed in the following examples of in vivo applications.
Cargo Delivery Electroporation
Control of cellular pores can allow the introduction of genes, drugs, vaccines, or other such cargo into a cell whose cell wall would otherwise not admit it. Once the field is removed, the pores re-close, trapping the cargo within the cell where it can have some therapeutic effect [6]. Figure 5 illustrates the cellular-level mechanics of this process.
Cargo Delivery Electroporation
Control of cellular pores can allow the introduction of genes, drugs, vaccines, or other such cargo into a cell whose cell wall would otherwise not admit it. Once the field is removed, the pores re-close, trapping the cargo within the cell where it can have some therapeutic effect [6]. Figure 5 illustrates the cellular-level mechanics of this process.
One clinical use of this technology is a gene therapy currently being developed to treat wet macular degeneration, a disease of the eye that causes progressive blindness. In this therapy, a DNA strand is modified to encode a behavior that arrests the progression of the disease. This DNA strand is injected into a specific collection of cells within the eye; Figure 6(a). Then, a set of electrodes is brought into contact with the injected tissue; Figure 6(b). An electroporation power supply is used to deliver a sequence of electric pulses that transfect the DNA strand into the cells; Figure 6(c). Once the therapy is completed, the DNA strand re-programs those cells to behave as bio-factories that produce a compound which perpetually treats the disease without the need for recurring therapy; Figure 6(d).
Many other applications of electroporation are forthcoming and include electro-chemotherapy (directly delivering chemotherapeutic agents to cancerous cells, reducing systemic drug burden and associated side-effects), vaccine delivery (enabling “bare” DNA vaccines, which are quick to develop, quick to scale up, and amenable to worldwide transport without cold storage requirements), and several other gene therapies [7].
Pulsed Field Ablation
The number of pores opened in a cell undergoing electroporation is proportional to the applied field strength: a higher voltage results in more pores opening. A threshold exists where too many pores have been opened and even after removal of the external electric field, the cell is unable to reclose all pores and dies. This process is illustrated in Figure 7.
While not desirable in cargo-delivery electroporation, this effect can be intentionally invoked to ablate diseased or disordered tissue [8]. In contrast to radiofrequency ablation, this effect is non-thermal, relying entirely on electric fields to kill cells. This electric-field mediated tissue ablation (termed “pulsed field ablation,” due to the use of repeated short-duration pulses) has many potential benefits in certain diseases, but one particularly notable feature is that it can be tissue-type selective.
As an example of the clinical utility of this feature, consider the disease of atrial fibrillation. Atrial fibrillation is a heart condition where one chamber of the heart beats erratically (or “fibrillates”) due to an errant conduction pathway (a “short circuit”) developing within the heart’s conduction system, as shown in Figure 8. In this fibrillating state, the heart is not pumping efficiently and retains blood for much longer than a heartbeat. This stagnant blood may form a clot, which can cause a stroke if it escapes the heart. This disease is potentially devastating, and not uncommon (affecting approximately 1 in 10 adults over age 65) [9].
Today, one surgical treatment for atrial fibrillation is the use of a radiofrequency ablation system to thermally ablate the tissue responsible for the errant conduction pathways [5]. While effective, this therapy risks collateral damage to important structures like the esophagus or the phrenic nerve. The physician has limited ability to avoid this collateral damage, as all tissues (being approximately 70% water) have similar heat capacities and thus, little tissue selectivity exists when using a thermal ablation mechanism – only the physician’s skill guides the thermal energy to the correct target.
Pulsed field ablation offers a significant measure of added safety against collateral tissue injury due to the different susceptibilities different tissue types have to electroporation. Figure 9 illustrates the magnitude of these differences [10]. By adjusting the output amplitude to the desired level, it is possible to deliver an ablation therapy that selects only the target tissue and has no effect on non-target tissues.
Many other applications of pulsed field ablation are being investigated, as well, and include the treatment of prostate cancer or hyperplasia (ablating cancer cells without impacting connective tissue or nerves), as well as cancers of the gastrointestinal tract, head and neck, liver, lung, and pancreas [11]. In addition to the tissue-selectivity benefit, there are many other potential benefits to pulsed field ablation, including faster procedures, greater consistency, and quicker patient recovery.
Electric Field Power Electronics
The power electronics used for both electroporation and pulsed field ablation look similar; it is the output amplitude that distinguishes them. While many architectures exist in this nascent field [12], a typical approach is shown in Figure 10. After an AC/DC stage, a high-voltage DC/DC converter charges a bank of high-voltage capacitors. Output parameters (which include output amplitude, pulse width, number of pulses, intra-pulse delays, etc.) are either set by the user (via a user interface) or pre-programmed for a given therapy. Once ready to deliver, an IGBT (or other switch) connects the capacitor bank to the patient in the prescribed sequence.
It should be obvious that significant electric shock hazards may exist in this therapy, given the bank of high-voltage capacitors that are connected to the patient. Timing of pulses is critically important to safety and may drive uncommon design decisions (such as the use of FPGAs versus microcontrollers for control of output transistors [13]). In applications where current may traverse the heart, synchronization to the heartbeat is commonly required to avoid inducing arrythmia. Output amplitudes and rates of change are significant enough to require considerable engineering effort to select components capable of withstanding high stress levels.
Conclusions
Surgical energy is a critically important application of power electronics that, today, finds use in most surgical procedures performed [14], and is used in less acute settings (such as dentists’ offices and dermatologists’ offices), as well, yet is relatively unknown to the public. Rapid advancements in the field are creating therapies that are even less invasive and more effective and apply to diseases and conditions across the entire human body.
Future advancement in the field of surgical energy will likely involve the connection of disparate technologies to create a standard-of-care which is data-driven and analytical, in contrast to today’s empirical and observational approach. Today, a patient may receive an imaging test to diagnose a condition, but that data often stops there. In the future, such data may supply a finite element analysis model which could allow planning and prediction of the effects of a surgical energy treatment. Instead of applying the same therapy to all patients, such a model could output a patient-specific energy prescription. With tools like biocompatible 3D printing, patient-specific fixtures could be created to ensure electrodes are optimally positioned and avoid sensitive nearby structures. Surgical robotics may allow electrode placement with greater precision than otherwise humanly possible. Intra-procedural imaging tools may allow real-time monitoring of efficacy, and intra-procedural data may be collected and returned to the model to confirm that the delivered therapy was equivalent to the plan.
The future of surgical energy is power electronics applied to sundry human health conditions to realize minimally invasive, highly directed therapies with few side effects and fast recovery. As new surgical energy modalities evolve, the need for high-performance power electronics will accompany them. Surgical energy is an opportunity to literally connect power electronics to meaningful advances in human health!
Acknowledgement
This article is an adaptation of a presentation given during the IEEE APEC 2024 Plenary Session in Long Beach, CA, USA.
About the Author
Daniel Friedrichs (dfriedrichs@minnetronixmedical.com) leads the development of surgical energy systems for Minnetronix Medical, a design, development, and manufacturing partner to the medical device industry. He has specialized in medical power electronics for 15 years and holds over 35 patents related to medical applications of power converters. Dr. Friedrichs has a Ph.D. in Electrical Engineering from the University of Colorado Boulder, CO, USA. He is a licensed professional engineer and a senior member of IEEE.
References
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About Minnetronix Medical
Since 1996, Minnetronix Medical has accelerated medical device breakthroughs as a design, development, and manufacturing partner to leading device companies around the world. Today, through lifecycle efficiency, opportunity realization, and increased utility, the company creates value in key technology segments that include optical systems, RF energy, fluid and gas management, and stimulation & active wearables. From design and manufacturing services to whole product solutions, Minnetronix has the expansive industry insight and intentional technical acumen to deliver better medical devices to market, faster.