Novel 3D printing-based probe for impedance spectroscopic examination of oral mucosa: design and preliminary testing with phantom models

Figures & data

Figure 1. (a) The final version of the bioimpedance spectroscopy (BIS) probe head with four high-purity silver electrodes. The electrode contact areas are 4 mm × 0.6 mm. (b) The good mechanical and electrical stability of the BIS probe is assured with a 3D printed plastic frame that ensures 1 mm insulation layer between adjacent electrodes. The wires terminated to 4 mm banana sockets are directly soldered to the silver electrode plates.

Table 1. Different steps towards successful probe construction.

Prototype (s)	Electrode materials	Insulating materials	Remarks
Prototype 1	1 mm thick non-passivated stainless steel (AISI316L)	Single material; Formlabs Standard Clear FLGPCL04	Heavily corroded during BIS measurement
Prototype 2	1 mm thick passivated stainless steel (AISI316L)	Single material; Formlabs Standard Clear FLGPCL04	Corroded and inconsistent impedance data found after few minutes
Prototype 3	0.6 mm thick passivated stainless steel (AISI316L)	Multimaterial; Vero PureWhite	Corroded, insulation also showed conductivity, no measurements done
Prototype 4	0.6 mm thick passivated stainless steel (AISI 304)	Single material; Formlabs Standard Clear FLGPCL04	Corroded after five to six minutes
Prototype 5	0.6 mm thick passivated stainless steel (AISI 304)	Teflon insulation (handmade probe)	Corroded after few measurements
Prototype 6	0.6 mm thick tantalum electrodes	Single material; Formlabs Standard Clear FLGPCL04	High impedance values
Prototype 7	0.6 mm thick high purity silver (fine silver 999, Peltolan Spectrolitnappi Ky, Pertunmaa, Finland)	Single material; Formlabs Standard Clear FLGPCL04	Fully functional, present data are based on this prototype

Seven different prototypes were realised.

Figure 2. A cross-sectional layout view of the custom-made bioimpedance probe with four rectangular high purity silver electrodes (A–D) inside the 3D printed insulation block (F).

Figure 3. Schematic overview of the experimental setup for bioimpedance spectroscopic measurements with 3D printed probe (modified from Emran et al. [9]).

Table 2. Mixtures of saline solutions and their nominal conductivities used as test solutions.

Nominal conductivity of test solution (mS/cm)
Solution 1: mixture of solution 1- & 1-part H2O	8 mS/cm
Solution 2: mixture of solution 2- & 1-part H2O	4 mS/cm
Solution 3: mixture of solution 3- & 1-part H2O	2 mS/cm
Solution 4: mixture of solution 4- & 1-part H2O	1 mS/cm
Solution 5: mixture of solution 5- & 1-part H2O	0.5 mS/cm
Solution 6: mixture of solution 6- & 1-part H2O	0.25 mS/cm

Figure 4. A custom-made concentric bioimpedance probe [9] used for comparison. Two terminal measurements were conducted by connecting outer ring and central pin, i.e., both current injection and voltage measurement was performed using the same electrode pair. The probe head is shown in the upper right corner of the picture.

Figure 5. Biological samples (5 mm thick slices) used to evaluate the performance of the novel 3D printed probe: (a) cucumber and (b) pork tongue with indicated measurement locations.

Figure 6. (a) Bode plots for all mixtures of NaCl solutions; (b) measured admittance as a function of nominal conductivity of NaCl test solutions shows excellent linearity (R²=0.999).

Figure 7. Impedance magnitude (left panel) and phase (middle panel) values at seven discrete frequencies (1 Hz to 1 MHz) in repeated BIS measurements (three scans) for cucumber sample. Inter-scan variation graphs (right panel) show relative standard deviation (RSD, %) of impedance magnitude and standard deviation (STD, °) of phase data. Data obtained with the 3D printed probe (upper part of figure) are shown in comparison to the concentric ring probe data (lower part of figure).

Figure 8. Impedance magnitude (left panel) and phase (middle panel) values at seven discrete frequencies (1 Hz to 1 MHz) in repeated BIS measurements (three scans) for porcine tongue sample. Inter-scan variation graphs (right panel) show relative standard deviation (RSD, %) of impedance magnitude and standard deviation (STD, °) of phase data. Data obtained with the 3D printed probe (upper part of figure) is shown in comparison to the concentric ring probe data (lower part of figure).

Emran S, Lappalainen R, Kullaa AM, et al. Concentric ring probe for bioimpedance spectroscopic measurements: design and ex vivo feasibility testing on pork oral tissues. Sensors (Basel, Switzerland). 2018;18(10):3378.

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