Design and testing of low intensity laser biostimulator
© Valchinov and Pallikarakis; licensee BioMed Central Ltd. 2005
Received: 11 November 2004
Accepted: 13 January 2005
Published: 13 January 2005
The non-invasive nature of laser biostimulation has made lasers an attractive alternative in Medical Acupuncture at the last 25 years. However, there is still an uncertainty as to whether they work or their effect is just placebo. Although a plethora of scientific papers published about the topic showing positive clinical results, there is still a lack of objective scientific proofs about the biostimulation effect of lasers in Medical Acupuncture. The objective of this work was to design and build a low cost portable laser device for stimulation of acupuncture points, considered here as small localized biosources (SLB), without stimulating any sensory nerves via shock or heat and to find out a suitable method for objectively evaluating its stimulating effect. The design is aimed for studying SLB potentials provoked by laser stimulus, in search for objective proofs of the biostimulation effect of lasers used in Medical Acupuncture.
The proposed biostimulator features two operational modes: program mode and stimulation mode and two output polarization modes: linearly and circularly polarized laser emission. In program mode, different user-defined stimulation protocols can be created and memorized. The laser output can be either continuous or pulse modulated. Each stimulation session consists of a pre-defined number of successive continuous or square pulse modulated sequences of laser emission. The variable parameters of the laser output are: average output power, pulse width, pulse period, and continuous or pulsed sequence duration and repetition period. In stimulation mode the stimulus is automatically applied according to the pre-programmed protocol. The laser source is 30 mW AlGaInP laser diode with an emission wavelength of 685 nm, driven by a highly integrated driver. The optical system designed for beam collimation and polarization change uses single collimating lens with large numerical aperture, linear polarizer and a quarter-wave retardation plate. The proposed method for testing the device efficiency employs a biofeedback from the subject by recording the biopotentials evoked by the laser stimulus at related distant SLB sites. Therefore measuring of SLB biopotentials caused by the stimulus would indicate that a biopotential has been evoked at the irradiated site and has propagated to the measurement sites, rather than being caused by local changes of the electrical skin conductivity.
A prototype device was built according to the proposed design using relatively inexpensive and commercially available components. The laser output can be pulse modulated from 0.1 to 1000 Hz with a duty factor from 10 to 90 %. The average output power density can be adjusted in the range 24 – 480 mW/cm2, where the total irradiation is limited to 2 Joule per stimulation session. The device is controlled by an 8-bit RISC Flash microcontroller with internal RAM and EEPROM memory, which allows for a wide range of different stimulation protocols to be implemented and memorized. The integrated laser diode driver with its onboard light power control loop provides safe and consistent laser modulation. The prototype was tested on the right Tri-Heater (TH) acupuncture meridian according to the proposed method. Laser evoked potentials were recorded from most of the easily accessible SLB along the meridian under study. They appear like periodical spikes with a repetition rate from 0.05 to 10 Hz and amplitude range 0.1 – 1 mV.
The prototype's specifications were found to be better or comparable to those of other existing devices. It features low component count, small size and low power consumption. Because of the low power levels used the possibility of sensory nerve stimulation via the phenomenon of shock or heat is excluded. Thus senseless optical stimulation is achieved. The optical system presented offers simple and cost effective way for beam collimation and polarization change. The novel method proposed for testing the device efficiency allows for objectively recording of SLB potentials evoked by laser stimulus. Based on the biopotential records obtained with this method, a scientifically based conclusion can be drawn about the effectiveness of the commercially available devices for low-level laser therapy used in Medical Acupuncture. The prototype tests showed that with the biostimulator presented, SLB could be effectively stimulated at low power levels. However more studies are needed to derive a general conclusion about the SLB biostimulation mechanism of lasers and their most effective power and optical settings.
Nowadays lasers are widely used in therapy and diagnostics. They have been adapted to many medical procedures ranging from surgery, oncology, physiotherapy, dentistry, dermatology and biostimulation. The non-invasive nature of laser biostimulation have made lasers an attractive alternative in Medical Acupuncture at the last 25 years. Unfortunately, there is still an uncertainty as to whether they work or their effect is just placebo. Although a plethora of scientific papers published about the topic showing positive clinical results, there is still a lack of objective scientific proofs about the biostimulation effect of lasers in Medical Acupuncture.
The properties of acupuncture points, considered here as small localized biosources (SLB), have been extensively studied over the past 50 years. Research has shown SLB to be small area body regions, which exhibit unique, electrical, physiological and anatomical properties (e.g. high density of gap junctions, relatively low impedance etc.). They are considered to form groups, each group being arranged along a line, called meridian and related to an internal organ [1–4]. SLBs appear to be highly sensitive to mechanical, thermal, electrical or electromagnetic stimulation and are found to take place from the epidermis to a maximum depth of 2 cm [5–8]. It has been shown that with proper laser wavelength, intensity and collimation, low-level laser energy could be effectively delivered to SLB up to a 10 mm beneath the skin surface .
The objective of this work was to design and build a low cost portable laser device for effectively stimulation of SLB without exciting sensory nerves, and to find out a suitable method for objectively evaluating its efficiency. The attempt to define the optimal device parameters was based on the SLB properties, data about existing devices for low level laser therapy and on preliminary measurements performed in our laboratory. The latter suggest that the effect of SLB stimulation is also dependent on the polarization of the coherent emission in addition to its intensity, wavelength and modulation frequency. Therefore the device should provide a polarization adjustment, wide range of modulation frequencies, precise power settings and to have minimum size and cost.
Basic design and operating principle
The variable parameters of the laser output are: average output power, pulse width, pulse period, and sequence duration and repetition period. After each input parameter is selected, the total energy that would be delivered at the end of the stimulation session is automatically calculated and displayed. When defining the stimulation protocol, the software program reads the selected parameter value and automatically re-calculates the possible set of the other parameters, so that the user could not select inconsistent values or ones that would result in a total energy delivered that exceeds a certain safety limit. In stimulation mode the laser stimulus is applied according to the pre-programmed protocol.
The application can tolerate an elliptical beam shape and waveform aberrations, so circularization of the laser beam or correction of the waveform aberrations is not required.
Method for testing the device efficiency
Practical biostimulator circuit
RA0-RA2 – outputs used as LCD control signals
RA3-RA4 – outputs, used as DAC control signals
RB0-RB3 – either inputs or outputs, shared between the input control buttons B1-B4 and the LCD data bus DB4-DB7
RB4-RB6 – outputs, used as control signals for the laser diode driver
The LCD is implemented with the dot matrix alphanumeric character module U4 (Seiko Instruments L1682). It features low power consumption, high contrast, wide viewing angle, on-board controller and LSI driver (Samsung S6A0069). All functions required for the LCD drive are internally provided on the chip. Its internal operation is determined by signals sent from the microcontroller. These signals include:
Register select – RS
Read/Write – R/W
Data bus – DB4-DB7 (configured as inputs)
Read/Write Enable – E
When ports RB0-RB3 are configured as inputs, the LCD data inputs DB4-DB7 have no practical influence on the logic levels set by push buttons B1-B4. LCD operation is also not affected since DB4-DB7 content is read only on logic high at E (U4-pin 6), set by the microcontroller . When ports RB0-RB3 are configured as outputs, input buttons B1-B4 cannot alter their output logic levels because of resistor R2 connecting B1-B4 to common. Connector J1 is used for the microcontroller ICSP.
The laser diode driver is implemented with the highly integrated circuit U1 (Analog Devices AD9660), which combines a very fast output current switch with onboard analog light power control loops. It gets feedback current from the laser diode built-in photo detector (U1-pin 8), feeds it to a transimpedance amplifier (TZA) and then to two analog feedback loops where the bias and the active power levels of the laser are set . The two levels are proportional to the analog input voltage at the bias level input (U1-pin 14) and at the active level input (U1-pin 3). These inputs drive track and hold amplifiers with hold capacitors C4 and C5. The input voltage range on both inputs ranges from Vref to Vref + 1.6 V, requiring an offset of Vref to be created for common based signals. The bias level is chosen to be equal to Vref, where the active level is determined by the circuit realized with op-amp U5. It performs the level shift and scales the DAC output from Vref to Vref + 1.6 V. This solution is attractive because both DAC and op-amp can run off a single 5 V supply, and the op-amp does not have to swing rail-to-rail. The op-amp U5 output voltage level is given by:
Since the monitor current is proportional to the laser diode light power, the feedback loops effectively control the laser power to a level proportional to the analog inputs. The bias control loops is periodically calibrated via U1-pin 15, where the active control loop is continuously calibrated via U1-pin 1. Resistors R3 and R4 are used to avoid floating of inputs U1-pin 15 and U1-pin 2 when microcontroller ports are in a high impedance mode. The laser pulse modulation is done by switching between the bias and the active power levels according to the logic level at U1-pin 2, where logic high turns the modulation current on. The gain resistor R5 matches the feedback loop transfer function to the laser/photo diode D1. Capacitor C6 optimizes the TZA response, with larger values to slow TZA response. Lower values increase TZA bandwidth but may cause oscillations. When input U1-pin 16 is logic high, the onboard disable circuit turns off the output drivers and returns the light power control loops to a safe state. It is used during initial power up of U1 and when the laser is inactive. In case that input U1-pin 16 floats (after POR or other reset conditions) the driver is disabled. When U1 is re-enabled the control loops are recalibrated.
The DAC is implemented with the single 8-bit voltage output MAX517 (U3). It is controlled by the microcontroller via 2-wire serial interface (U3-pin 3, U3-pin 4), operates from a single power supply and swings rail-to-rail. POR ensures the DAC output is at zero volts when power is initially applied. It uses the power supply Vdd as reference (U3-pin 8) filtered by R12 and C9. The DAC's full-scale output voltage ranges from 0 to Vdd. Special attention was paid to the PCB layout design to minimize the crosstalk between analog inputs and digital outputs.
Beam dimensions (-3dB)
Average output power
Average output power density
Maximum irradiation per stimulation session
Collimating lens coupling efficiency (685 nm, f = 4.6 mm, NA = 0.53)
Polarizer transmittance (685 nm, optic axis parallel to the diode junction)
Quarter-wave retarder transmittance, (685 nm)
Total optical system transmittance (685 nm)
0–10 000 Hz
Pulse sequence duration
Pulse sequence repetition period
Number of pulse sequences
Maximum power consumption (5 V)
Device settings used during the preliminary measurements.
Average output power
Average output power density
Pulse sequence duration
Pulse sequence repetition period
Number of pulse sequences
The noise present in the signals is mainly composed of electromyographic signals and noise from the electrode-skin interface. The first two signal records (TH-8 and control) contain ECG artifacts and additional electromyographic noise since those two electrodes were positioned relatively distant from the reference electrode. The frequency bandwidth was limited to 200 Hz by sixth order low-pass Bessel filter and the signals were sampled with 1 kHz.
Recording to the EU regulation (Medical Device Directive 93/94) this device falls under class IIb in order to obtain the CE mark. The biostimulator prototype was categorized as Class IIIb laser product according to the International Standard for the Safety of Medical Laser Products IEC 601-2-22, as stated in Table 1.
The best solution for building a compact hand held biostimulator would be to design a custom made integrated circuit, but the cost would be much higher. We found a good alternative in using surface mount technology (SMT), commercially available integrated laser diode driver and a RISC Flash microcontroller. This solution resulted in a reduction in parts, size and power consumption. The proposed method for testing the device efficiency is very sensitive to precise electrode and stimulus positioning. Even a deviation of 3 mm from the exact SLB location may prevent the recording electrode from capturing signals from the source. The same deviation of the stimulus position also results of ineffective excitation of the targeted SLB and thus no SLB evoked potentials can be recorded. The method is also susceptible to the electrode-skin pressure, but not only due to its strong influence on the contact impedance. It was observed that the excessive electrode-skin pressure led to diminishing or even disappearing of the SLB signal, although the contact impedance was lower. This is most probably due to the pressure exerted on the SLB source that may affect the signal generation or transduction. Alternatively insufficient electrode-skin pressure led to excessive contact impedance and noise from the electrode-skin interface. The preliminary results suggest that a circularly polarized laser emission is most effective when used on the so-called Yang acupuncture meridians but not on Yin types. However more studies are needed to validate or disprove this observation.
The specifications of the prototype, built according to the proposed design, were found to be better or comparable to those of other existing devices. It features small size and low component count and power consumption. Because of the low power levels used the possibility of sensory nerve stimulation via the phenomenon of shock or heat is excluded. Thus senseless optical stimulation is achieved. The optical system presented offers simple and cost effective way for beam collimation and polarization change. The novel method proposed for testing the device efficiency allows for objectively recording of SLB potentials evoked by laser stimulus. Based on the biopotential records obtained with this method, a scientifically based conclusion can be drawn about the effectiveness of the commercially available devices for low level laser therapy used in Medical Acupuncture. The prototype tests showed that with the biostimulator presented, SLB could be effectively stimulated at low power levels. However more studies are needed to derive a general conclusion about the biostimulation mechanism of lasers in Medical Acupuncture and their most effective power and optical settings.
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