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Microvalves: Enabling a New Technology for Tissue Expansion Systems

Figure 1: AeroForm Tissue Expansion System (Image Credit: AirXpanders)

Each year, approximately 250,000 women in the U.S. are diagnosed with invasive breast cancer.1As a result, more and more patients with cancer in one or both breasts are making the difficult decision to undergo a unilateral or bilateral mastectomy to decrease their odds of developing cancer again.

Following a mastectomy, about 36 percent of women choose to move forward with breast reconstruction,2 which is the standard of care in many clinical settings. Typically, patients who select implant-based breast reconstruction require expansion of the sub-muscular space with saline expanders. This involves multiple time-consuming and often painful saline bolus injections over several months at the surgeon’s office.

In an effort to provide the patient with a more comfortable, gradual tissue expansion process that they can control, AirXpanders has developed and is investigating a breast tissue expansion system consisting of an implantable tissue expander, AeroForm, and a handheld radio-frequency dosage controller. The implant contains no batteries and all power is provided inductively by the hand-held dosage controller, which operates a first-of-its-kind microvalve to release gas from a reservoir contained within the expander.

The use of onboard gas as the expansion medium eliminates the need for saline bolus injections that are required for conventional tissue expanders. The dosage controller uses a 13.56 megahertz (MHz) frequency electromagnetic wave to both power and control the valve, allowing the surgeon and patient to control expansion non-invasively. The 13.56 MHz frequency, typically used for radio-frequency identification (RFID) systems, was chosen for its combination of reasonable antenna size, medium range and good skin penetration. The dosage controller communicates with the expander and allows the patient to administer 10 cc doses of carbon dioxide (CO₂) from a reservoir within the expander. The use of compressed CO₂ is one of the key technologies of the AeroForm expander and has been issued three U.S. patents. The key elements of the AeroForm tissue expansion system are shown in Figure 1.

CO₂ was selected due to its widespread use in common medical procedures. Operative procedures are performed with laparoscopic insufflators capable of flow rates of at least 4-10 liters per minute (L/min). CO₂ insufflation is preferred by most laparoscopic surgeons because CO₂ has a high diffusion coefficient and is a normal metabolic end product rapidly cleared from the body. Also, CO₂ is highly soluble in blood and tissues, and does not support combustion.

One of the key technologies that enable the device to be powered entirely by induction is the microvalve. The chosen valve technology, as shown schematically in Figure 2, is the magnetically activated, direct operation solenoid valve. The valve is designed to be in the normally closed position and is assembled and fabricated from micro-machined and micro-lapped stainless steel. This valve configuration offers a number of advantages, including low friction, low sensitivity to particulates, fast response time (< 10 milliseconds) and low internal volume. By using an appropriately designed coil and miniaturizing the valve seat, the valve requires very low power to operate, which is an important characteristic in an inductively- powered system. The power consumption, in the fully open state is 9 milliwatts at an applied potential of one volt and each valve opening consumes approximately 0.005 Joules, or 0.0012 calories of energy.

Figure 2: Schematic of Key Valve Elements, Valve is Shown Closed (Cross-Sectional View)
Figure 2: Schematic of Key Valve Elements, Valve is Shown Closed (Cross-Sectional View)

Achieving this level of efficiency required both a custom-designed solenoid and extreme miniaturization of the valve seat. The solenoid optimization parameters included a magnetic opening, hold and residual forces, energy efficiency and overall solenoid size. The final design met the goal of maintaining a safety factor of two on the force and energy parameters, while staying within the allocated size envelope for the solenoid. It was also necessary to miniaturize the valve seat to obtain the desired force margins. Figure 3 displays the valve seat with a dime for comparison. The valve seat is 0.15 millimeters (mm) in diameter and can almost fit within the ridges at the dime’s edge. To achieve optimum sealing, the valve seat is micro-lapped using a proprietary process developed by AirXpanders. Other valve components include a custom-designed flat spring that enables the desired, normally closed, leak-tight operation and lapped spacer shims to ensure precise valve opening. The entire assembly is contained within a stainless steel housing with a covering filter over the gas vent holes that prevents ingress of foreign material. A custom-designed electronics package controls valve actuation.

Figure 3: AirXpanders Microvalve Seat
Figure 3: AirXpanders Microvalve Seat

The flat spring is preloaded so that the valve remains closed unless the solenoid is energized using the dosage controller. The valve orifice is a micro-orifice with a diameter of 0.051 mm, which results in a maximum gas force of 1.71 gram-force (less than the weight of a dime). The total force exerted by the pre-loaded spring is 43 gram-force. During dosing, one volt is applied to the solenoid, resulting in a force of 95 grams minimum on the spring. At 95 grams, the solenoid provides greater than two times the spring sealing force, assuring consistent valve opening. The valve duty cycle is 250 milliseconds on with an off period to allow additional inductive charging. Figure 4 shows the valve closed and open. Note that the valve opening is accomplished by very small movement of the spring (0.1 mm).

Figure 4: Cross-Section View of AirXpanders Valve Assembly. Left: Detail of Closed Valve closed. Right: Detail of Open Valve
Figure 4: Cross-Section View of AirXpanders Valve Assembly. Left: Detail of Closed Valve closed. Right: Detail of Open Valve

Leak testing of the microvalve is conducted on each assembly at elevated pressure and shows leak rates of 4.5 x 10-3 standard cubic centimeters per minute at body temperature and pressure (sccmb) at 12.9 megapascals (MPa). Shelf life testing indicates a negligible leak rate in typical storage. Microvalve operation has been successfully demonstrated for inlet pressures up to 8.5 MPa with a measured flow rate of 1080 sccmb.

By developing an inductively-powered microvalve for its expansion system, AirXpanders was able to successfully engineer a wireless, patient-controlled device, providing women with a more convenient road to expansion. To date, AeroForm tissue expanders using this microvalve have been implanted in 98 patients in the U.S. and 61 patients in Australia across 17 clinical sites for a total of more than 250 implants (approximately 66 percent of patients undergo bilateral reconstruction when using AeroForm tissue expanders). The AeroForm system has been approved for commercial use in Australia and is currently in clinical trials in the U.S.

1Avon Foundation. Found at: Accessed March 20, 2015.
2Breast Found at: Accessed March 20, 2015.

Source:  Mdtmag News and AirXpanders

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