Conclusion
We propose an individually-controlled multi-tined expandable electrode for the on-demand RFA lesions. The interaction between pre-curved flexible stylet and relatively stiff cannula produces various shapes of our sub-electrodes. By tuning the expanded length and expanding speed of the stylets and cannulas, diverse trajectories can be realized on demand. Along with a rationally applied energy, three individually-controlled sub-electrodes can achieve three-dimensional on-demand conformal ablation efficiently. The ex vivo RFA experiments of porcine kidney and its comparison with a commercial electrode further demonstrate the conformability of our RFA lesions. Our electrode exhibits the potential to conformally ablate in vivo target tissue with various morphologic appearances on demand.
Experimental Section/Methods
Operation of the radiofrequency ablation system based on our RFA device
As illustrated in Figure 5, the RFA device and an electrode pad is connected to the RF generator for ablation. The servos in control box of our RFA device is actuated by Buslinker, which is powered by a battery pack. An Arduino board (Mega 2560 Pro) is used to receive messages from a personal computer, and send commands to the Buslinker and finally control the servos.
Trajectory fitting of the sub-electrode
Two hypotheses are assumed in this model:
1. The trajectory of sub-electrode is considered to be constructed by several segments of arcs, which is widely adopted in other works\cite{Swaney2013,Adebar2016,Gerboni2017}.
2. To simplify the model, the shape of stylet and cannula is assumed to change linearly during the morphing, and the fully expanded state (the stylet and cannula are both maximally expanded) of each morphing state is utilized for fitting.
Based on the hypotheses mentioned above, the structure parameters can be expressed as follows:
\(\theta=\frac{\theta_1l_c}{L_c},l_s=l_c\ \ \ \ \ \ \ \ \left(1\right)\)
\(\theta=\left[\theta_1\left(\frac{L_{ps}-L_s}{L_{ps}-l_c}\right)+\theta_2\left(\frac{L_s-L_c}{L_{ps}-l_c}\right)\right]\frac{l_c}{L_c},l_s\le L_{ps}\ \ \ \ \ \left(2\right)\)
\(\theta=\left[\theta_2\left(\frac{L_c+L_{ps}-L_s}{l_c}\right)+\theta_3\left(\frac{L_s-L_{ps}}{l_c}\right)\right]\frac{l_c}{L_c},l_s>L_{ps}\ \ \ \ \ \left(3\right)\)
\(r=r_1,l_s-l_c\le L_{ps}-L_c\ \ \ \ \ \ \ \ \ \ \ \ \ \left(4\right)\)
\(r=r_1\left(\frac{L_c+L_{ps}-L_s}{L_c}\right)+r_2\left(\frac{L_s-L_c}{L_c}\right),l_s-l_c>L_{ps}-L_c\ \ \ \ \ \left(5\right)\)
where \(\theta\) and \(r\) are the deflected angle of cannula and bending radius, respectively. \(\theta_1\) , \(\theta_2\) and \(\theta_3\) are the measured deflected angle of cannula at the end of each morphing state. \(r_1\) and \(r_2\) are the measured bending angle of stylet at the end of each morphing state. \(l_c\) and \(l_s\) are the expanded length of cannula and stylet, respectively. \(L_c\) and \(L_{ps}\) are the maximal expanded length of cannula (20 mm) and the length of pre-curved part of stylet (32 mm), respectively.
The \(l_c\) and \(l_s\) in equation (1) - (5) only need to satisfy the following criteria:
\(l_c\le l_s\ \ \ \ \ \ \ \ \ \ \ \ \ \ \left(6\right)\)
\(l_c+L_{ps}\ge l_s\ \ \ \ \ \ \ \ \ \ \ \left(7\right)\)
With equations (1)-(7), Figure 9 can be obtained accordingly.
Other Experimental Methods
Preparation of Tissue Phantom: As a widely used tissue phantom,\cite{Solazzo2005} agar (Shanghai Regal Biology Technology Co., Ltd, China) was adopted in this paper. 12 g agar as well as 8 g NaCl (Sinopharm Chemical Reagent Co., Ltd, China) was added in the 800g boiled water and stirred for 12 min to be fully dissolved. Then the solution was poured in a mold and cooled for 1 hour to be cured and finally the tissue phantom was made.
Preparation of Tissue Phantom with an Opaque Target: The evenly mixed solution of agar was first poured in a mold and cured for 8 min until there is a thin layer of film formed on the surface. Then a droplet of ink was dropped on the film and waited for 5 min until the ink was dried. Finally, the mixed solution was again poured in the mold and cooled for 1 hour for curation. (The tissue phantoms with an Opaque target were molded for the visualization of our target in Figure 9.)
Equipment: An RF generator (BanBianTian, China) worked at a power of 15 W in the experiments and the frequency of AC is nominally 550 ± 40 kHz. A magnetic mixer (HS 7, IKA, Germany) was used for the mixing and heating of the solution.
Image processing: The captured colored pictures are imported into MATLAB 2019a to transform to grayscale images. After a threshold is selected for each sets of images according to the brightness of the pictures, these grayscales images are then transformed to black and white images.
Finite Elements Analysis: COMSOL Multiphysics v5.6 (Stockholm, Sweden) was used for the radiofrequency ablation analysis. All the electromagnetic and thermal properties of the tissues are provided by COMSOL Multiphysics. For the simplicity of our model, blood perfusion and heat flux with the external environment are not considered, and only the conductive cone head and sub-electrodes are considered in this model. Arrhenius kinetics model was used to assess the damage of tissues. All the insets in Figure 10 are reprocessed for better visualization, the original pictures exported from COMSOL Multiphysics are illustrated in Figure 16.