The Evolving Use of Magnets in Surgery: Biomedical Considerations and a Review of Their Current Applications

The novel use of magnetic force to optimize modern surgical techniques originated in the 1970s. Since then, magnets have been utilized as an adjunct or alternative to a wide array of existing surgical procedures, ranging from gastrointestinal to vascular surgery. As the use of magnets in surgery continues to grow, the body of knowledge on magnetic surgical devices from preclinical development to clinical implementation has expanded significantly; however, the current magnetic surgical devices can be organized based on their core function: serving as a guidance system, creating a new connection, recreating a physiologic function, or utilization of an internal–external paired magnet system. The purpose of this article is to discuss the biomedical considerations during magnetic device development and review the current surgical applications of magnetic devices.


Introduction
A magnet is a material that can produce a magnetic field based on the orientation of its atomic magnetic dipole moments. This magnetic field attracts unlike poles while repelling like poles. The resulting electromagnetic field and attractive force inherent in magnetic materials are useful properties in the development of novel surgical devices. Ever since the use of electromagnetic bougienage by Drs. Hendren and Hale for the lengthening of disconnected esophageal pouches in children with esophageal atresia, magnetic devices have been explored across the breadth of modern surgical procedures in almost every organ system (Table 1) [1].
As minimally invasive surgical (MIS) techniques, including endoscopy, laparoscopy, and robotic surgery, have allowed surgical procedures to be performed via smaller incisions (or no incisions) with improved patient outcomes (e.g., decreased hospital length of stay, faster return to activity), magnetic devices have been explored to augment existing MIS techniques and instruments [2]. With this aim, multidisciplinary and international collaboration have led to the rise in magnetic surgical device development from preclinical animal studies to the first in-human trials studying safety and feasibility [3]. This article serves to provide an overview of key biomedical considerations during device development and discuss the core functions of existing magnetic surgical devices. Intravascular paired ring magnets for side-to-side anastomosis between two blood vessels Obora et al., 1980 [4] 1981 Colorectal Presence of colostomy Internal-external paired magnetic ring system for colostomy closure Jansen et al., 1981 [5] 1989 Urology Urethral stricture Intraurethral miniaturized magnets for stricturoplasty Isakov et al., 1989 [6] 1992 Small intestine Small bowel fistula Intraluminal paired magnets to form compression anastomosis bypassing intestinal fistula Stepanov et al., 1992 [7] 1993 Hepatobiliary Biliary stricture Paired magnets to form compression anastomosis between proximal common bile duct and stomach or duodenum Saveliev et al., 1993 [8]

Biomedical Considerations for Magnetic Surgical Devices
The utility of a magnetic material in surgical device development is dependent on its strength, durability, and mitigation of potential toxicity in the human body. The strength of a material is based on the elements that comprise it and is determined by the product of a material's strength as well as the force required to demagnetize it (i.e., coercivity). This is known as the energy product (Gauss Oersted or Joules/m 3 ) which is graded on the N grading system-increasing strength correlating with a higher grade [9]. Currently, rare-earth elements, such as N52-grade neodymium-iron-boron (Nd-Fe-B), are commonly used for industrial magnetic devices as they have the highest recorded maximum energy product (474 kJ/m 3 ) [10]. This corresponds to a magnetic field of approximately 10,000 Gauss, which is 100 times stronger than a household refrigerator magnet at 100 Gauss [11]. One limitation of Nd-Fe-B magnets for industrial use is their relatively low Curie temperature, or the temperature at which a material loses its dipole alignment and subsequently its magnetism; however, the Curie temperature for Nd-Fe-B at 310 • C is well above environmental temperatures in medical use [12].
Another potential limitation is the relatively poor durability and potential toxicity of Nd-Fe-B magnets to human tissues. Nd and Fe are known to oxidize rapidly in air, leading not only to the corrosion and weakening of the magnet but also to the formation of reactive oxygen species (ROS) that have been shown to be cytotoxic to human cells [13,14]. Additionally, Nd is a brittle material that can break easily [15]. These limitations are mitigated by hermetically sealing the Nd-Fe-B magnetic core in a durable biocompatible coating. Considerations when selecting a coating include adhesion of the coating to the magnet, avoidance of magnet oxidation during the coating process, and the coating's durability, biocompatibility, and cost [16]. Examples of biocompatible medical-grade coating materials include titanium, parylene, and gold [17,18]. During device development, the stringent testing of physical and chemical properties is regulated by the United States Food and Drug Administration (FDA) to ensure the mitigation of these risks prior to clinical implementation. Key considerations during testing include the following: type of device, duration of contact with the human body, and nature of contact with the human body ( Table 2).
In addition to potential toxicity from the physical properties of Nd-Fe-B, one should consider the potential risks from exposure to a static magnetic field. Medical implants such as cardiac pacemakers or implantable cardiac defibrillators (ICD) may be at risk of interference from magnetic fields. Although the operation of implanted devices has not been shown to be adversely affected by static magnetic fields below 0.5 mT and has been proven to withstand magnetic resonance imaging (MRI), the proximity to medical implants-along with the size and strength of a magnetic surgical device-warrants consideration [19][20][21].

Implant device
Tissue/bone C, S, I, Sy, P, Sub, G, and Im In addition to the strength, durability, and potential toxicity of a magnetic device, a device's intended use and location in the human body, shape, thickness, and cost are all key factors to consider during device development. More specifically, the type of tissue that the device will be in contact with (e.g., intestinal tract, biliary tract, blood vessel wall, or bone) and the method of device placement/retrieval must be considered. For example, magnetic devices that are deployed intraluminally in the foregut may be more susceptible to degradation due to their increased exposure to gastric acid and digestive enzymes, relative to devices that are implanted in bone or deployed within blood vessels. Thus, these devices may warrant additional layers of hermetic sealing and more stringent durability testing. Additionally, devices that are placed within blood vessels must consider the size, shape, and coating to facilitate feasible placement into the blood vessel-as well as the prevention of device-associated thrombus formation [23]. Thus, the intended tissue type and device environment strongly influence device design. This highlights the importance of early collaboration between clinicians and bioengineers.
The risks of magnetic surgical devices during use must also be considered during device development. Although rare, potential off-target events associated with an in situ magnetic device can lead to catastrophic morbidity and even death. This is evident in reported cases of ingested rare-earth magnets that have led to bowel fistula, perforation, sepsis, and death [24]. In order to mitigate these off-target events, devices that utilize paired systems should consider the risk of injury to the intervening tissue and the use of unidirectional ferromagnetic backplates to minimize off-target effects [25]. Considering these risks, paired systems for magnetic compression anastomosis (e.g., intestinal, biliary, vascular) also require target tissues to be apposed, without any intervening tissue/fluid, to facilitate magnet mating, avoid magnet migration, and mitigate the risk of an off-target serious adverse event. Thus, while patient selection is ultimately key in device success, optimizing tissue-specific device specifications for its intended use is also paramount.

Core Functions of Magnetic Surgical Devices
The following sections serve to discuss the core functions of existing magnetic surgical devices on the market, in addition to those of novel devices currently under investigation. They also briefly discuss the clinical uses of the individual devices introduced, but are by no means comprehensive discussions of each device (Table 3).  GS (guidance system), NC (creating new connection), PHYS (recreating physiologic function), IE (internal-external paired system).

Guidance System
The ability of magnets to transmit a detectable electromagnetic field is of particular interest in the development of guidance systems for the placement of devices in the human body. This is evident in current devices that aid the placement of nasoenteric feeding tubes and peripherally inserted central venous catheters (PICCs) ( Table 1).
In order to provide nutrition to critically ill patients who may be incapable of eating by mouth, small-bore nasoenteric feeding tubes are frequently placed at the bedside. Conventional placement includes blind insertion, followed by radiograph confirmation prior to the use of the tube; however, this approach can place patients at risk of the tube being placed into the airways and causing pulmonary trauma (e.g., pneumothorax), as well as cause a delay in the use of the tube for hydration, nutrition, or vital medications while the radiograph is obtained and interpreted. In addition, if the radiograph is misinterpreted, tube feeding into the lungs can lead to catastrophic morbidity and even death. An alternative approach uses a stylet with an electromagnetic tip inside of the feeding tube during placement that transmits a signal to an external receiver unit placed on the patient's epigastric region [26]. This allows for the real-time visualization of the path of the tip of the tube to avoid placement into the lung, as well as for the confirmation of the correct placement in the stomach or small intestine without requiring radiograph confirmation. This has been shown to reduce the risk of pulmonary complications, such as feeding into the lungs or pneumothorax, while decreasing the delay in tube feeding initiation [27].
Additionally, this has reduced the burden on radiology resources and has the potential to lead to a cost avoidance of USD 346,000 over a 2-year period [28]. Similar to the placement of feeding tubes, the detection of an internal magnet for real-time guidance is also being used for PICC placement. PICCs are catheters that provide central venous access for medications, parenteral nutrition, repeated blood sampling, and invasive hemodynamic monitoring. These specialized catheters are commonly placed at the bedside under blind insertion with radiographic confirmation or in the radiology suite under live fluoroscopic guidance. While the former approach introduces an increased risk of malposition or cardiovascular injury that can result in arrhythmia or cardiac tamponade, the latter introduces increased radiation exposure, higher cost, and limited availability in resource-limited settings [31]. The magnetic approach uses an inner stylet with a magnetic tip that is detected by an external sensor placed on a patient's sternum, providing real-time tracking of the catheter's path, which has decreased rates of malposition and radiation exposure [29,30].
The potential of intravascular magnets in guiding the formation of arteriovenous fistulas (AVFs) in patients requiring long-term hemodialysis is being explored. AVFs continue to be created via conventional open surgery connecting an extremity artery to a vein; however, specialized centers are now exploring the ability to create AVFs in the radiology suite via percutaneous means. Specialized catheters with magnetic tips are inserted into the artery and vein of interest, which guide the catheters to the area of interest and align the vessels together, allowing a radiofrequency electrode to weld the vessels together to form a fistula [32].
Although not yet widely available, this introduces a novel use of paired magnets to guide a novel minimally invasive approach to AVF creation. Thus, existing magnetic surgical devices have improved conventional approaches by minimizing radiation exposure as well as the use of radiology resources, improving procedural accuracy, and creating a novel minimally invasive approach for a common vascular operation.

Magnetic Compression Anastomosis to Create New Connections
New connections can be created between two hollow organs (e.g., the esophagus, stomach, small intestine, colon, and bile duct) to bypass an obstructed segment of the gastrointestinal (GI) tract due to atresia, stricture, or mass-or to bypass the large absorptive surface of the small intestine (e.g., bariatric bypass surgery). Currently, open or laparoscopic approaches to the creation of an anastomosis are performed with either surgical staplers or hand-sutured methods; however, the use of compression to create an equally robust anastomosis resistant to anastomotic leak would create an approach that could further minimize the invasiveness of this procedure by decreasing or eliminating the need for incisions. This approach also has the potential to create an anastomosis in patients that require one to restore GI tract continuity, but that have medical or anatomic comorbidities that may prohibit conventional surgical methods [56]. For example, in patients with an obstructing gastric cancer and a history of multiple intra-abdominal operations or radiation therapy, subsequent dense adhesions may prohibit a safe palliative gastrointestinal bypass. Thus, an endoluminal magnetic approach would allow these high-risk surgical patients to receive treatment and avoid a permanent proximal gastrointestinal diversion.
The modern use of compressive force to create an anastomosis re-emerged in the 1980s with the creation of the biofragmentable bowel anastomosis ring (BAR) and compression anastomotic clips [110][111][112][113][114]. More recently, the use of paired magnets to create magnetic compression anastomoses (MCAs) has also been shown to form robust anastomoses [53]. Magnets are placed in the organs of desired connection via endoscopic-or laparoscopicassisted approaches. Magnets are guided to mate, and once the anastomosis is fully formed the magnets unite and are excreted or retrieved endoscopically [36,39,50].
Various paired magnet systems have been described in the literature (Table 3). Magnet configurations include spherical, circular discoid (flat or concave-convex), circularring-shaped, with varying inner diameter sizes, cylindrical, and bullet-shaped [50]. The magnetic cores of Nd-Fe-B or samarium-cobalt are commonly described, with coating methods varying between being single-layer (e.g., titanium oxide, polycarbonate) and multi-layer (e.g., inner nickel-copper-nickel coating, outer gold-parylene C coating for biocompatibility) [53]. Additional considerations during the development of MCA devices include how to best place the magnets into organs of interest (e.g., endoscopy, laparoscopy, and percutaneous) and the effects of magnet size/shape on magnet delivery as well as apposition, mating force, anastomosis formation, and the risk of postoperative complications (e.g., anastomotic leak, stricture). Prior to in-human use, extensive preclinical testing on device placement, biocompatibility, compressive force, and burst pressure performance is often performed [34,[36][37][38][39][40]43,115]. Burst pressure is used to test anastomotic resistance to leakage with maximal observed burst pressures in MCAs consistently being > 100 mmHg, which is above the physiologic intraluminal pressure [53]. Continued research aims to optimize magnet delivery via completely endoscopic means (i.e., incisionless surgery) (Figure 1) while improving anastomotic outcomes by reducing stricture rates ( Figure 2) [45,55,56].
include how to best place the magnets into organs of interest (e.g., endoscopy, laparoscopy, and percutaneous) and the effects of magnet size/shape on magnet delivery as well as apposition, mating force, anastomosis formation, and the risk of postoperative complications (e.g., anastomotic leak, stricture). Prior to in-human use, extensive preclinical testing on device placement, biocompatibility, compressive force, and burst pressure performance is often performed [34,[36][37][38][39][40]43,115]. Burst pressure is used to test anastomotic resistance to leakage with maximal observed burst pressures in MCAs consistently being > 100 mmHg, which is above the physiologic intraluminal pressure [53]. Continued research aims to optimize magnet delivery via completely endoscopic means (i.e., incisionless surgery) (Figure 1) while improving anastomotic outcomes by reducing stricture rates (Figure 2) [45,55,56].
The miniaturization of MCA device systems has also broadened their applicability to treat many pediatric conditions. A key example of this is in infants with esophageal atresia (EA)-a congenital malformation resulting in two separate non-communicating pouches that require a surgical anastomosis in order to achieve esophageal continuity and allow for nutrition via the mouth [116]; however, the current surgical options are sometimes associated with significant morbidity (e.g., musculoskeletal deformity, anastomotic leak or stricture, or vocal cord paresis)-the rates for which have remained relatively constant over the past 80 years [117]. Thus, the MCA devices currently used for EA repair utilize paired magnets that are placed into apposing pouches-one via the mouth and one via the stomach-and mated to create a completely endoscopic anastomosis. While the widespread adoption of this technique has been limited by early experience, observing the prohibitive rates of anastomotic strictures [54], newer devices with wider mating surfaces and unique mating geometries aim to solve this problem [53,55,56,118] (Figure 2).  The miniaturization of MCA device systems has also broadened their applicability to treat many pediatric conditions. A key example of this is in infants with esophageal atresia (EA)-a congenital malformation resulting in two separate non-communicating pouches that require a surgical anastomosis in order to achieve esophageal continuity and allow for nutrition via the mouth [116]; however, the current surgical options are sometimes associated with significant morbidity (e.g., musculoskeletal deformity, anastomotic leak or stricture, or vocal cord paresis)-the rates for which have remained relatively constant over the past 80 years [117]. Thus, the MCA devices currently used for EA repair utilize paired magnets that are placed into apposing pouches-one via the mouth and one via the stomach-and mated to create a completely endoscopic anastomosis. While the widespread adoption of this technique has been limited by early experience, observing the prohibitive rates of anastomotic strictures [54], newer devices with wider mating surfaces and unique mating geometries aim to solve this problem [53,55,56,118] (Figure 2).
Examples of in-human use of MCA include the following: • Cardiac and vascular surgeons are also utilizing paired magnet systems to create new connections between two arteries. Currently, the majority of vascular anastomoses are hand-sewn; however, in contrast to MCAs, magnetic vascular ports (MVPs) are permanent devices that are deployed within arteriotomies to form a vascular port opening on each vessel of interest. Ports are then mated to form anastomoses [69]. This method has been utilized most frequently for the minimally invasive direct coronary artery bypass (MIDCAB) procedure to connect the left internal mammary artery (LIMA) on the chest wall to the left anterior descending (LAD) artery on the heart [69]. In-human trials have demonstrated decreased anastomotic and total procedure times, with no device-related adverse events and favorable patency rates at 6-month follow-ups [68,70,[72][73][74]76,119]. Preclinical animal studies are also being used to explore the use of MCAs in the creation of vascular anastomoses, but these have not progressed to in-human trials at this point [120][121][122][123].

Recreating a Physiologic Function
The dysfunction of muscular sphincter complexes can lead to gastroesophageal reflux disease (GERD; lower esophageal sphincter) or incontinence (fecal; anal sphincter complex). Anti-reflux procedures for the treatment of medically refractory GERD, such as a Nissen fundoplication, can lead to abdominal bloating, inability to vomit, and persistent dysphagia [124]. Additionally, there are multiple surgical options for fecal incontinence, such as sacral nerve stimulation and artificial sphincter balloon implants, but the adoption of one has been limited by suboptimal outcomes [125]. Magnetic sphincter augmentation has the potential to recreate physiologic sphincter function. Current magnetic devices consist of small titanium-coated beads with Nd-Fe-B magnetic cores interlinked with titanium wires to form a flexible expandable ring (Table 3) [87]. This configuration allows for opening with increased dynamic pressure, but maintains a closed position at lower passive pressures, preventing reflux or incontinence. Devices are sized to the external diameter of the esophagus (e.g., GERD) or the anal canal (e.g., fecal incontinence) without compressing the underlying muscle [79,87]. Magnetic sphincter devices for GERD have demonstrated safety and the normalization of esophageal acid exposure (i.e., reflux), reduced or discontinued reflux medication use (i.e., proton pump inhibitors), and improved quality of life [78,80,81,[83][84][85]. Magnetic devices for fecal incontinence have also shown improvements in incontinence severity and quality of life, with comparable outcomes to sacral nerve stimulation, but their adoption has been limited due to reports of device-related infection and perineal pain [88,89,[126][127][128][129]. Magnetic sphincter augmentation is also being explored for urinary incontinence in the preclinical phase [90]. Thus, magnetically augmented sphincters are currently being used to recreate physiologic sphincter function, while also having the potential to be reversible (with device explantation).

Use of Internal-External Paired Magnet Systems
Systems that use an internal magnet guided by or acted upon by an external magnet have a variety of surgical applications, ranging from the surgical treatment of congenital disorders (e.g., scoliosis, pectus excavatum) to improving organ retraction during laparoscopic surgery ( Table 3). The surgical management of early onset scoliosis employs dynamic instrumentation to allow for continued longitudinal growth. Traditional growing rods require repeated surgeries to lengthen the rods as a child grows. Magnetic growing rod (MGR) systems use a similar system with single or dual implanted titanium spinal rods fixed to the spine cranially and caudally [130]; however, as a child grows, an external magnetic device can be used to rotate the rod's internal actuator, which lengthens the rod without the need for repeated surgeries [103]. In addition to a reduction in operations, the MGR system has also led to subsequent long-term cost savings, decreased infection rates, and similar rates of implant failure (e.g., rod or foundation failure) [104,[106][107][108][109]131].
The attractive magnetic force of an internal-external paired magnet system is also being employed to gradually remodel musculoskeletal deformities. Pectus excavatum is a congenital deformation of the cartilages that connect the ribs to the sternum that pushes the sternum inward and can cause the compression of the heart. Conventional repairs involve major surgical reconstruction. The modified Ravitch procedure requires the open removal of the abnormal cartilages, the fracturing of the sternum, and fixation into a satisfactory alignment. The less invasive Nuss procedure uses smaller incisions to place a titanium metal bar within the chest and behind the sternum to gradually remodel the chest wall over a 2-year period; however, both procedures are associated with a serious risk of injury to the heart, lungs, or major blood vessels, as well as significant postoperative pain. The Nuss procedure also requires a second procedure to remove the titanium bar after te remodeling of the chest wall is complete. The magnetic approach implants an internal Nd-Fe-B magnet with a ferromagnetic focusing plate encapsulated in a low-profile titanium shell to the anterior sternum [25,99]. The internal magnet is mated with a second magnet housed in an external polypropylene custom-fitted anterior chest wall brace [25]. This approach has been shown to decrease surgical risk while improving chest wall deformation and decreasing overall cost [100,101]; however, this technology has seen limited adoption due to major improvements in postoperative pain control with the advent of intercostal nerve cryoablation [132]. A similar use of an internal-external paired magnet system is also being studied in obstructive sleep apnea treatment-as an alternative to continuous positive airway pressure (CPAP)-by advancing the hyoid bone forward to maintain airway patency during sleep (Figure 3) [102,133]. Other surgical applications of an internal-external paired magnetic system include the use of internal detachable magnetic graspers manipulated by external magnets to aid in organ retraction during laparoscopic gallbladder surgery and bariatric surgery to decrease the number of incisions (Table 3) [91][92][93]95,96,134]. These multiple uses highlight the potential and versatility of magnetic surgical systems that can manipulate internal tissues via an external source. also being studied in obstructive sleep apnea treatment-as an alternative to continuous positive airway pressure (CPAP)-by advancing the hyoid bone forward to maintain airway patency during sleep ( Figure 3) [102,133]. Other surgical applications of an internalexternal paired magnetic system include the use of internal detachable magnetic graspers manipulated by external magnets to aid in organ retraction during laparoscopic gallbladder surgery and bariatric surgery to decrease the number of incisions (Table 3) [91][92][93]95,96,134]. These multiple uses highlight the potential and versatility of magnetic surgical systems that can manipulate internal tissues via an external source.

Conclusions
The use of magnetic devices in surgery is an actively growing field with various systems on the market or in preclinical development. Multidisciplinary collaboration during device development is recommended due to the combination of biomedical engineering and surgical considerations. Existing magnetic surgical devices can be used to guide the placement of catheters and tubes, create new connections in the GI tract as well as blood vessels, recreate physiologic sphincter function, and manipulate internal tissues via an external magnet. This versatility highlights the potential of magnetic devices to improve existing surgical techniques and minimally invasive approaches.
Author Contributions: Conceptualization, methodology, formal analysis, investigation, resources, data curation, writing (original draft preparation, review and editing), supervision, and project administration-W.G.L., L.L.E., S.M.J., and R.K.W. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors report no conflicts of interest.

Conclusions
The use of magnetic devices in surgery is an actively growing field with various systems on the market or in preclinical development. Multidisciplinary collaboration during device development is recommended due to the combination of biomedical engineering and surgical considerations. Existing magnetic surgical devices can be used to guide the placement of catheters and tubes, create new connections in the GI tract as well as blood vessels, recreate physiologic sphincter function, and manipulate internal tissues via an external magnet. This versatility highlights the potential of magnetic devices to improve existing surgical techniques and minimally invasive approaches.
Author Contributions: Conceptualization, methodology, formal analysis, investigation, resources, data curation, writing (original draft preparation, review and editing), supervision, and project administration-W.G.L., L.L.E., S.M.J. and R.K.W. All authors have read and agreed to the published version of the manuscript.