OpenVstim is a low-cost open-source transcutaneous voltage stimulator capable of spinal cord electrical stimulation. Figure 8 shows the prototyped stimulator powered with a common 9V battery. The stimulator contains standard Electronic Muscle Stimulation (EMS) or Transcutaneous Electrical Nerve Stimulation (TENS) lead wires to connect stimulation electrodes. On-board linear potentiometer allows the user to control the stimulation intensity as well as turn on and off the stimulator. A minimum 3.6V DC is needed to power the entire stimulator. Stimulation parameters are listed in Table 2.
Table 2
Specifications of the Transcutaneous voltage stimulator (OpenVstim).
Channel | 1 (can be expanded to more channels) |
Waveform | Biphasic square wave |
Pulse Intensity | 0-128mA peak into 500Ω load |
Output Voltage | maximum 64V peak-to-peak |
Pulse duration | 50 µs (can be reprogrammed) |
Interpulse Interval | 1µs (can be reprogrammed) |
Stimulation Frequency | 20Hz (can be reprogrammed) |
Carrier Frequency | 10kHz (can be reprogrammed) |
Power | One 9V battery |
OpenVstim is a single channel voltage stimulator. However, the channel numbers can be easily increased by having more DC-DC boost converter and H-bridge driver modules. The stimulation parameters are all reprogrammable through the Arduino Uno microcontroller board. Since this is a voltage stimulator, the stimulation intensity depends on the electrode-tissue impedance. Peak intensity can be as high as 128mA for a typical 500Ω electrode-tissue impedance. Figure 9 illustrates the 10kHz biphasic burst stimulation. A 330kΩ resistive load is used to mimic the electrode-tissue impedance.
The biphasic stimulation burst generated by the OpenVstim stimulator contains few microseconds of inter-pulse interval (see Fig. 9). This is due to the slow activation of the Microcontroller’s GPIO logic pins. To standardize the stimulation, a faster activation of the logic pins is needed. This can be achieved by replacing the digitalWrite function with the following binary code:
PORTB = B00000XY0; // faster digitalWrite operation for Arduino Uno
The XY are the binary value for the GPIO pins 9 and 10. Replacing XY with 00 will send logic 0 to GPIO pins 9 and 10, and 11 send logic 1. When the code is implemented, OpenVstim generates faster stimulation burst with undetectable inter-pulse interval as shown in Fig. 10.
Components and modules used to build the OpenVstim stimulator are all listed in Table 3. Total cost of building the stimulator was only AU$66.65; however, can vary based on the market and supplies of the items. Reference prices and sources of all the components are also listed in the table. Special care is given to choose the most commonly available items so that anyone can build the stimulator in anywhere in the world. One should, however, check with their local market and supplier for the availability of the components and modules. In case of unavailability, a similar module can be used, but care must be given to connect appropriate circuit and power.
Table 3
Bill of Materials (BOM) of OpenVstim with itemized cost and source link.
Item no. | Item name | Model number | Description | Key feature | Units | Price (AU$) | Example source |
1 | M1 | Arduino Uno | Microcontroller unit (MCU) | 8 bits | 1 | 18.95 | www.ebay.com.au/ itm/402150942592 |
2 | M2 | XL6009 | DC-DC converter module | 50 V 1.5 A max output | 1 | 6.85 | www.ebay.com.au/ itm/273707870769 |
3 | M3 | DRV8871 | Full H bridge module | 45 V 3.5 A max output | 1 | 17.68 | www.ebay.com.au/ itm/363862724775 |
4 | SP1 | 50kΩ | Linear pot with on/off switch | 5 terminals | 1 | 5.35 | www.ebay.com.au/ itm/152893232674 |
5 | J1 | Battery clip | 9 V battery clip connector | T type | 1 | 2.92 | www.ebay.com.au/ itm/284538269404 |
6 | P1 | PCB board | Arduino Uno Shield board | For easy Prototyping | 1 | 6.99 | www.ebay.com.au/ itm/392331016225 |
7 | W1 | TENS cable | Electrotherapy Lead wires | To connect electrodes | 1 | 2.46 | www.ebay.com.au/ itm/254783965300 |
8 | B1 | Battery | 9V | 550 mAh | 1 | 5.45 | www.ebay.com.au/ itm/ 384397462542 |
Total cost: | AU$ 66.65 |
Table 4 shows the list of transcutaneous electrical stimulators used for non-invasive spinal cord stimulation therapy in patients with neurological injuries. Most TES system utilizes constant biphasic current pulses for neuromodulation. The design presented in the report, however, utilizes constant biphasic voltage pulses. An in-depth study on constant voltage vs constant current stimulation is warranted to take full advantage of TESCS therapy on functional recovery after neurological injuries.
Table 4
List of transcutaneous electrical stimulators used by the spinal cord stimulation research groups around the world.
Name | Company | Country | Approval | Mode | Channel | Power | Price |
Biostim-5 15 | Cosyma Inc. | Moscow, Russia | N/A | Current | 5 | Battery | N/A |
NeoStim-5 16 | Louisville, USA | N/A | Current | 5 | Battery | N/A |
ARCEX 17 | ONWARD Medical Inc | Eindhoven, Netherlands; Lausanne, Switzerland | FDA Breakthrough Device | Current | 4 | Battery | N/A |
Stimulette r2x 18 | Dr Schuhfried Medizintechnik GmbH | Vienna, Austria | N/A | Current | 2 | Line | N/A |
SCONE™ 19 | SpineX Inc. | Los Angeles, USA | FDA Breakthrough Device | Current | 2 | Battery | N/A |
SCiP 20 | Current | 2 | Battery | N/A |
DS8R 3 | Digitimer Ltd | UK, USA and Hong Kong | CE | Current | 1 | Line | £8,335 |
OpenXstim | Present report | Australia | Un-approved | Voltage | 1 | Battery | A$66.65 |
The presented stimulator (OpenXstim) provides an easy-accessible tool for non-invasive spinal cord stimulator to arguably for most communities around the world. However, the stimulator needed to be tested in TESCS settings to examine the efficacy of spinal cord neuromodulation. In future design, a voltage-to-current converter circuit will be added in the stimulator to generate constant biphasic current stimulation. This may require dynamic monitoring of load impedance to adjust the output compliance voltage of the stimulator. For the safety of unprofessional general users, however, constant voltage TES over constant current TES is advised.