Light based systems are frequently used in the field of medicine for diagnostic and surgical procedures, medical photography, phototherapy, etc. Phototherapy involves the use of light energy for the treatment of physical or mental illnesses. Illumination parameters such as wavelength and dose of therapy can be varied to have distinct effects on cells and tissues. This has necessitated the need to design a system which is programmable for various parameters related to therapy and can also be used by people at large for biomedical applications. The paper presents the design and implementation of a portable, hand-held and programmable Light Emitting Diode (LED) based phototherapy system using embedded technology. The system is designed around the ARM based TM4C123GH6PM microcontroller processor. The microcontroller has been programmed to allow for selection of various parameters such as frequency, duty cycle, and time of exposure. The matrix keypad interface and liquid crystal display (LCD) offer the ease of human interface. An LED driver circuit has been efficiently designed to modulate the output power of LED. Further, Super Bright LED (SLED) of wavelength ~633 nm has been successfully tested and results of optical characterization including spectral and spatial response have been presented in the results section. The designed system successfully achieves the programmable parameters required for dose optimisation required for enhancing the therapeutic effects of a phototherapy system.
Low Level Light Therapy (LLLT) or phototherapy is a noninvasive technique of exposing an injured tissue to light (continuous or pulsed monochromatic light) of wavelength ranging from 600 to 1000 nm in order to relieve pain, reduce inflammation, and enhance wound healing [
Till date, many research studies have been conducted to evaluate the effects of LLLT but none have been able to determine the parameters effective for wound healing applications. The major drawback is the nonavailability of a standard instrument which can be programmed for variable parameters such as frequency, duty cycle, and exposure time. Thus, one needs to develop a programmable and tunable instrument which can be used for laboratory experiments leading to a detailed investigation and analysis of the parameters required for the therapeutic treatment.
In this paper we have presented the design and analysis of programmable LED based system using embedded technology. In this design it is possible to vary the mode of operation such as continuous or pulsed. Also, it is possible to control the frequency of operation and duty ratio for optimal use in healing applications.
The design and development of the portable, programmable LED based phototherapy system are now discussed in detail.
The portable LED based phototherapy system comprises of three distinct units as illustrated in Figure 1. The units are: Pulse generator (processor module and user interface module), current driver circuit and the LED module. Processor is the core of pulse generator that instructs the system to operate in either continuous or pulsed mode. The keypad is used to accept user defined variables, i.e., frequency, duty cycle, and time of exposure. These illumination parameters are displayed on the LCD for user feedback and transmitted to microcontroller unit for pulse generation. The portable battery is used to power the designed system.
PHOTO (COLOR): System overview of LED based phototherapy system comprising of the pulse generator, driver circuit, and LED module.
The time duration for the operation of the device is also programmable. Finally, the output of the control system is routed to LED driver circuit which generates sufficient power to drive the LED module.
The pulse generator module is designed around the Texas Instruments TM4C123GH6PM microcontroller as its core controller and an interface module comprising of matrix keypad and LCD display which meets the design requirements of the system.
A microcontroller is a complete computer integrated onto a single silicon chip. This means that microcontroller has on-chip memories (RAM and ROM), a central processing unit (CPU), an arithmetic and logic unit (ALU), timers, watchdog timers, pulse width modulation modules, analog-to-digital converters, general purpose input/output ports, etc. The TM4C123GH6PM [
4x4 Matrix Keypad. Matrix keypad consists of an interconnected set of push buttons. The fundamental advantage of the 4x4 matrix keypad is that the sixteen keys require only eight general purpose input/ output (GPIO) pins to interface effectively, four GPIO pins for the rows and four GPIO pins for the columns. Keypad pins 1, 2, 3, and 4 are the column pins and keypad pins 5, 6, 7, and 8 are the row pins. The column pins are initialized as output pins and are successively set to logic high while the rest are set to logic low. Alternately, the row pins are initialized as input pins and are used to detect the press of a key. This process returns a coordinate value for the specific key that has been pressed, which is decoded to return the value which the key indicates (as opposed to the positional coordinate associated with the pressed key). This process relies on the fact that the time requirement to scan for the press of a key is at least an order of magnitude less than the time requirement to physically press a key. The row pins of the 4x4 matrix keypad are interfaced to general purpose input/output (GPIO) port E pins 0 to 3 (PE 0 – 3) of the TM4C123GH6PM microcontroller development board whereas the column pins of the keypad are interfaced to GPIO port C pins 4 to 7 (PC 4 – 7).
20x4 Liquid Crystal Display Module. The 20x4 liquid crystal display module incorporates a HD4478U liquid crystal display controller and driver along with a 20x4 dot-matrix liquid crystal display capable of displaying four rows of twenty characters each, a Random-Access Memory (RAM) unit and a Read Only Memory (ROM) unit. The RAM unit is divided into two discrete units, namely, Display Data RAM (DDRAM) and Character Graphics RAM (CGRAM). The DDRAM stores the microcontroller generated data to be displayed on the LCD screen. The CGRAM, on the other hand, stores any custom nonstandard characters a user may define. The ROM unit, called Character Graphics ROM (CGROM), stores the standard character data for the 5x7 dot-matrix characters and predefined commands used to control the operation of the LCD controller. The contents of this memory unit cannot be edited by the user interface.
The sixteen pins of the LCD module are connected as follows.
The LCD VSS (Pin 0), R/W (Pin 4), and LED- (Pin 15) pins are connected to ground. The VCC (Pin 1) is connected to the 5V power supply. The VEE (Contrast Adjustment) (Pin 2) is connected to the wiper of a 10 kΩ potentiometer whose other terminals are connected to VCC and ground. The Register Select (RS) (Pin 3) is connected to GPIO Port A pin 2 (PA 2). The Read/Write (R/W) (Pin 4) is connected to ground. The Enable (En) (Pin 5) is connected to GPIO Port A pin 3 (PA 3). Data pins (DB 0 – 7) (Pins 6 – 13) are interfaced to GPIO Port B pins (PB 0 – 7). The LED+ (Brightness Adjustment) (Pin 14) is connected to VCC through a 150 Ω resistor.
The supply from portable battery is connected across VBUS and ground pin of the TM4C123GH6PM microcontroller and an external reset push button is provided across the reset pin and ground. The square pulsed wave is generated at the GPIO Port D pin 6.
The program was written in C using the Advanced RISC Machines (ARM) instruction set developed on the Keil μVision version 4 Integrated Development Environment (IDE) which incorporates a text editor, linker, compiler, and debugger all in one development environment.
The TM4C123GH6PM microcontroller works at a bus frequency of 16 MHz using the on-chip oscillator. Alternatively, a crystal oscillator of 16 MHz may also be utilized which is much more accurate than the on-chip oscillator. By applying the phase locked loop (PLL), the TM4C123GH6PM can be configured to run at 80 MHz. The TM4C123GH6PM also boasts of a number of on-chip hardware timers which is a much more accurate way of generating delays as compared to firmware based delay generation techniques. We have programmed two such hardware timers: one to generate pulsed wave ON time and OFF time delays, and the other to generate the various in-system delays such as 1μs, 10μs, and 10 ms delays used in the LCD operation subroutines.
The flowchart represents the algorithm of software design of Pulse Generator unit of LED based phototherapy system is outlined in Figure 2. The primary function calls the various subroutines to perform specific tasks. The main function initializes the GPIO gating control (clock), calls the phase locked loop (PLL) function to operate the microcontroller at 80 MHz, initializes the output port for pulsed wave generation, and initializes the LCD module and keypad. Subroutines have been written for the individual functions required for the operation of the LCD as well as for scanning of individual keys and user defined variables by the 4x4 matrix keypad. It also calls the functions to accept user defined variables, i.e., frequency, duty cycle, and time. These variables are stored and used in the generation of the pulsed square wave.
PHOTO (COLOR): Flowchart representing the algorithm of software design of pulse generator unit of LED based phototherapy system.
Port A pins 2 and 3 are initialized as output control pins to the LCD, while Port B is initialized as LCD output register port. Port C pins 4 to 7 are initialized as output column pins to the 4x4 matrix keypad, and Port E pins 0 to 3 are initialized as input row pins to the 4x4 matrix keypad. Port D pin 6 is initialized as the output for the pulsed square wave.
LEDs are current controlled devices, such that even the slightest fluctuation in the terminal voltage across the LED causes a steep increase in the current flowing through it. Thereby, it is very easy to exceed the current limit of the LED, if the selected voltage source is not ideally chosen. The LED driver functions as a controlling element to provide the LED with stable potential difference across its terminals, or constant current through it.
An LED driver may be a simple and inexpensive current controlling resistor, an active constant current source, or a tightly controlled voltage regulator. The resistor is by far the simplest and most inexpensive approach for designing a driver circuit. However, it has a few drawbacks; specifically, it is quite inefficient as a current controlling element because it dissipates any excessive current as heat. Also, the resistor is not able to provide protection from voltage spikes from a nonideal power source. Alternately, a voltage regulator may be of two kinds, a linear voltage regulator or a switching regulator, such as an SMPS. A linear voltage regulator performs essentially the same function as a resistor but requires a minimum voltage drop of 1.5V. Switching regulators are much more efficient but may not be as accurate as linear regulators, requiring very precisely manufactured components with high tolerances which are expensive and difficult to source. A constant current source circuit, however, provides protection from voltage spikes and controls the current flowing through the LED, as opposed to very tightly controlling the voltage across the LED. With proper selection of power source, it can provide an efficiency of up to 95%.
As illustrated in Figure 3, the LED driver circuit is a modified current limiter circuit and comprises a feedback loop, consisting of a MOSFET Q
PHOTO (COLOR): PCB schematic of the LED based phototherapy system.
Power is delivered to the load, i.e., the LED, by a 5V portable power source while current regulation through the LED is achieved by means of three components, namely, MOSFET Q
If the voltage across RSET, i.e., voltage at the base of BJT Q
The component requirement and selection criteria for the design of current driver circuit are enumerated as follow.
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The real time PCB implementation of the LED based phototherapy system is represented in Figure 4(a). In order to perform various in-vivo and in-vitro studies of this phototherapy system with great ease, a compact and light weight prototype has been developed and is illustrated in Figure 4(b).
PHOTO (COLOR): PCB implementation of the designed systemPrototype of the developed LED based therapeutic system
The device offers the flexibility of adjusting the frequency, duty cycle, and time duration. Real time waveforms of pulsed output of LED driver circuit are illustrated in Figure 5 for various combination of frequencies and duty cycle (D) such as f = 5 Hz, D = 50%; f = 10 Hz, D = 75%; f = 50 Hz, D = 25%, and f = 100 Hz, D = 50%.
PHOTO (COLOR): Waveforms of pulsed wave with variable frequency and duty cycle.
In this section, we present the various measurements carried out to characterize the LED that is being used for the phototherapy application.
(a) Spectral Response. The spectral properties of the LED have been studied in detail using the LMS-6000 spectroradiometer. The plot of the relative spectral power distribution as a function of wavelength is given in Figure 6. It can be observed that the peak wavelength is at λp =633.9 nm. The observed Full Width at Half Maximum (FWHM) is 17.6 nm. The measured spectral flux of the LED is 28.708 lm at V=2.2V and I=200mA and efficiency is 66.63 lm/W.
PHOTO (COLOR): Spectral response of LED with a peak at 633.9 nm.
The International Commission on Illumination (CIE) chromaticity diagram, which is yet another way to study color, is given in Figure 7. The dominant wavelength, as determined from drawing a line through the color coordinates of the reference illuminant and the measured chromaticity coordinates of the LED is 623.6 nm.
PHOTO (COLOR): Chromaticity diagram.
(b) Spatial Response. The radiation pattern is a plot of spatial distribution of LED light intensity in the polar coordinate system [
PHOTO (COLOR): Radiation pattern of the LED.
Figure 9 illustrates the relative intensity plot in the 3D Cartesian plane using an optical arrangement of a CCD detector with data acquisition and image processing software and the LED [
PHOTO (COLOR): Intensity curve for LED.
PHOTO (COLOR): Relative radiant flux distribution of the LED.
A programmable LED based phototherapy device comprising of a pulse generator, a LED driver circuit, and a high powered LED has been developed and described in this work. A pulse generator specifically designed to drive the LED system with a design flexibility of varying time and duty cycles at low frequencies (1 Hz - 1 KHz) has been developed. The use of microcontroller to actualize the ever needed flexibility in design, eliminating the need of changing the software whenever a variable pulse is needed, has been achieved by this design. A pulse generator system with both keypad and LCD interface makes the device user friendly. The output frequency of the pulses of the microcontroller has been found to be extremely accurate. The driver circuit for the high power LED is low cost and only requires four discrete components. The various measurements have been carried out to characterize the LED which include spectral and spatial distribution of LED. In future work, the system will be upgraded to model the desired radiation pattern and optimize it for photo-therapy application. The described system provides easy programmability of variable illumination parameters required for phototherapy such as frequency, duty cycle, time of exposure, and power density, thus making this system apt for standardisation of treatment protocol for in vivo and in vitro studies. In the initial phase of our study, the system will be tested for the reduction of the infectious bacteria using microbial culture.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
The authors are thankful to Director DIPAS, DRDO for his keen interest and permission for experimental and publication purposes. They are also thankful to the Defence Research and Development Organization (DRDO), Government of India for financial support. They are also thankful to Prof. Enakshi K. Sharma (Dept of Electronic Science, University of Delhi) and her team for assistance in spectrum measurements of LED.
By Himani Kohli; Sangeeta Srivastava; Sanjeev Kumar Sharma; Satish Chouhan and Manan Oza