Category: Stimulators

Understanding Stimulation Hardware

Physiology studies often involve the recording of responses from subjects who are reacting to a particular sensory input being presented, otherwise known as stimulation. The presented stimuli can take several forms, and are tightly controlled in lab environments for the comfort and safety of the subject. Different types of human and animal stimulation consist of:

  • Electrical
  • Haptic (touch)
  • Thermal (temperature)
  • Olfactory (scent)
  • Auditory (sound)
  • Visual (light, such as a strobe)

The following is an introduction to some stimulation hardware commonly used in the research market:

Stimulator Modules such as the BIOPAC STM100C is one of the building blocks for many types of stimulation protocols and create a variety of pulse and waveform outputs for nerve conduction, evoked response, audio stimulus-response, and somatosensory response studies. The STM100C interfaces with the BIOPAC MP150 or MP160 data acquisition system and a wide range of devices such as headphones, LEDs, piezo transducer for audio alarms, or haptic transducers. The device also provides technician control of the stimulus level, and works in concert with the AcqKnowledge software Stimulation setup window.

Stimulus Isolation Adapters: These adapters are added to the STM100C module and multiply the output of the STM100C by 5 or 10x to provide the desired range for nerve conduction, muscle twitch, or other studies requiring a stronger stimulus than what the STM100C can generate on its own.

The STM200 is a standalone device that interfaces with the Analog Outputs on the front panel of the Universal Interface (UIM100C) or High Level Transducer Interface (HLT100C) modules. This is a very powerful stimulator, capable of outputting up to 1 amp. It can be used for human and animal applications, including isolated tissue bath preparations.

Linear Isolated Stimulator: The STMISOLA Linear Isolated Stimulator is the most advanced and powerful stimulator offered by BIOPAC. This type of unit can handle almost any application and far exceeds most typical hardware devices in terms of flexibility and control. Typically, stimulators can only generate simple unipolar or bipolar pulses. The STMISOLA, however, can output unipolar or bipolar arbitrary waves such as pulse (single or train), square, sine, triangle, exponentially decaying, modulated envelopes, and fully user-specified types. STMISOLA can be used for Transcranial Direct Current Stimulation (tDCS) and High Definition Transcranial Direct Current Stimulation (HD-tDCS. These are forms of electrical stimulation to effect neuro-modulation in the brain.

Stroboscope: The TSD122 Stroboscope resembles a handheld flashlight and is used for visual evoked response applications and for synchronizing triggers. The unit has a range from zero to a maximum of 12,000 flashes per minute.

Haptic Stimulator: The TSD190 Haptic Stimulator incorporates an internal electromagnetically actuated plunger which can be used to mechanically stimulate a 1.5 mm diameter area of skin surface; both plunger force and travel can be infinitely adjusted between zero and a specified maximum value. This type of stimulator connects directly to the STM100C Stimulator Module and is used for tactile or somatosensory studies.

Thermal Stimulator: The STMTHERM* Thermal Stimulator is used for applying 5-second hot or cold temperature pulses to a  4 cm x 4 cm skin area. AcqKnowledge software controls the actual output from the MP160 data acquisition unit and the stimulator hardware multiplies the signal to achieve the desired voltage or current levels.

Auditory Stimulator: The OUT101 Tubephone is used to deliver aural stimulus in the form of clicks or tones for auditory evoked responses studies such as auditory brainstem response (ABR) and resembles a tube, which is worn in the ear. It connects directly to the STM100C Stimulator Module.

We hope the above thumbnail sketch serves as an introduction to various stimulator hardware and common stimulation protocols. Please visit the above linked content for more in-depth information.

We also invite you to view our free webinar, Stimulation Techniques for Researchers, available on-demand.

*STMTHERM Thermal Stimulator will be available soon! Stay tuned.

BIOPAC offers a wide array of wired and wireless equipment that can be used in your research. To find more information on solutions for recording and analyzing signals such as ECG, heart rate, respiration and more using any platforms mentioned in this blog post, you can visit the individual application pages on the BIOPAC website.

Holiday Happiness: Gratitude or Gifts?

Studies investigated the anxiety of gift-giving, the effects of social support in stressful situations, and what impact meditation has on heart rate, ECG, and blood pressure.

December has arrived, and for many the holidays bring a rush to check items off of a long shopping list for parents, siblings, friends, in-laws, and many more. Often we struggle to find that “perfect” gift, but are the physical goods we exchange the true source of the warmth and happiness that characterizes the season?

An article by Nomura investigated anxiety induced by gift-giving. To test this, EDA data was recorded with a BIOPAC data acquisition system and STP100 while participants responded to stimuli. The participants visualized a specific relative while being presented with potential gift options, and were asked how likely they were to purchase the potential gift for that relative. The data found that people who self-reported thinking of in-laws were the most stressed, while those who reported thinking of children or same-gender friends were least stressed.

A psychosomatic study by Inagaki and Eisenberger found that mental and physical well-being benefit from forms of social support. They tested this by administering mild shocks with the BIOPAC STMISOLA to male patients who held the hand of a female partner lying in an fMRI. The results suggest that this support (hand-holding) provided possible stress-reduction for both the male and female participants.

Another article on nonverbal communication and well being by Shaltout, et al. tested the effects of meditation and the subsequent ECG, blood pressure, and heart rate effects. The BIOPAC ambulatory BioHarness and NIBP Blood Pressure System gathered data while participants underwent several mediation episodes, during which they were instructed to focus on loving kindness towards a subject. The results found that participants had decreased heart rate.

Research suggests direct, loving interactions with those who are close to us have many stress-reducing benefits. So when you are shopping for gifts, remember that spending time is more important than spending money. To read more studies about love and stress reduction, visit the BIOPAC publication page.

BIOPAC offers a variety of equipment that can be used in studies similar to the ones presented. To find more information on solutions for recording and analyzing signals such as EEG, EMG, respiration, and EDA using the MP160/MP150, MP36, and other platforms, you can visit the individual application pages on the BIOPAC website. 

BIOPAC Systems, Inc. provides life science researchers and educators with data acquisition and analysis systems that inspire people and enable greater discovery about life. Visit us at www.biopac.com.

Electrical Stimulators for Physiological Research: Compliance

All stimulators incorporate a fundamental characteristic known as “compliance.” Compliance comes in two flavors, namely voltage compliance and current compliance. Voltage stimulators emulate a voltage source, up to a certain current compliance. Current stimulators emulate a current source,

By Alan Macy, BIOPAC Systems, Inc. 

All stimulators incorporate a fundamental characteristic known as “compliance.” Compliance comes in two flavors, namely voltage compliance and current compliance. Voltage stimulators emulate a voltage source, up to a certain current compliance. Current stimulators emulate a current source, up to a certain voltage compliance. The stimulator compliance limit is important information. Here are examples:

Current Compliance

A voltage stimulator is being used to establish a field stimulation in a tissue bath. The voltage pulse is 100 volts, with a width of 1 ms. The bath establishes a 1000 ohm impedance between stimulation electrodes. Therefore, 100 volts/1000 ohms is 100 ma. In this situation, the voltage stimulator must be able to provide a current compliance up to 100 ma for at least 1 ms. If the stimulator’s current compliance is limited to 50 ma, then the output voltage will be limited to 50 ma * 1000 ohms or 50 volts, even if the stimulator is being instructed to output 100 volts.

Voltage stimulators commonly operate with the help of high voltage DC-DC converters and storage capacitors to provide the needed compliance current. If the storage capacitors can’t store enough charge or are not sufficiently supplied, then voltage stimulators may exhibit a quality known as voltage droop. Voltage droop is the quality when the stimulator output voltage can’t be maintained over a specific time interval, due to lack of sufficient current compliance. If the above stimulator can only be current compliant to 100 ma for 0.5 ms, then the output voltage pulse will be 100 volts for 0.5 ms and then will decay in direct proportion to the current compliance limit.

Voltage Compliance 

A current stimulator is being used for neuromuscular stimulation between two small electrodes. The current pulse is 10 ma, with a width of 1 ms and there is a 10,000 ohm impedance between stimulation electrodes. Therefore, 10 ma * 10,000 ohms is 100 volts. In this situation, the current stimulator must be able to provide a voltage compliance up to 100 volts for at least 1 ms. If the stimulator’s voltage compliance is limited to 50 volts, then the output voltage will be limited to 50 volts / 10,000 ohms or 5 ma, even if the stimulator is being instructed to output 10 ma.

Current stimulators commonly operate with the help of high voltage DC-DC converters and storage capacitors to provide the needed compliance voltage. If the storage capacitors can’t store enough charge or are not sufficiently supplied, then current stimulators may exhibit a quality known as current droop. Current droop is the quality when the stimulator output current can’t be maintained over a specific time interval, due to lack of sufficient voltage compliance. If the above stimulator can only be voltage compliant to 100 volts for 0.5 ms, then the current pulse will be 10 ma for 0.5 ms and then will decay in direct proportion to the voltage compliance limit.

Example

In the case of transcranial Direct Current Stimulation (tDCS), a constant current of up to 2 ma (Is) may be applied through the cranium. Assuming a combined series electrode / skin junction resistance of 20 Kohms, a cranium resistance of 100 ohms and other loop series resistances considered negligible, then the total stimulation loop impedance (Zs) is 20.1 Kohms. Accordingly, in this situation, the voltage compliance (Vs) of the tDCS current stimulator must be at least:

Is * Zs = Vs
0.002 amp * 20,100 ohms = 40.2 volts

Given the variable nature of the combined series resistances in the stimulation loop, it’s generally helpful to double the expected compliance voltage requirement of the stimulator. Accordingly, in this case, a stimulator compliance voltage of 80 volts or higher is recommended. Because the series resistance in the loop is largely a function of the electrode / skin interface junctions, the choice of electrode type and contact area has a great effect on the voltage compliance requirements of the current stimulator. Other factors contributing to the voltage compliance requirement are the equilibrium potentials and over-potentials associated with the specified electrode / electrolyte / skin junctions. Furthermore, if high resistance electrode leads are used, the potential across the leads may become material.

 

BIOPAC Systems, Inc. provides life science researchers and educators with data acquisition and analysis systems that inspire people and enable greater discovery about life. BIOPAC lets you measure physiology anywhere, with innovative, compatible wireless, wearable solutions that can be used by anyone for meaningful discovery. We make high-quality scientific tools for physiology measurement and interpretation with superior compatibility and world-class customer service and support. Visit us at www.biopac.com.

Electrical Stimulators for Physiological Research

Highly-controlled electrical stimulation capability can provide a unique tool for the life science researcher to discern sensory function and associated physiological responses in living subjects. There are two types of electrical stimulators, voltage and current.

By Alan Macy, BIOPAC Systems, Inc.

Any physiological measurement is concerned with the subject of stimulus and response, because all physiological processes are subject to change with applied stimuli. The processes of life encapsulate the idea that environmental change precipitates sensory activity that results in stimulation. Because sensory activity is mediated by the flow of ions in the nervous system, highly controlled electrical stimulation capability can provide a unique tool for the life science researcher to discern sensory function and associated physiological responses in living subjects.

Computer-based systems can be used to define very exacting electrical stimuli and those signals can be directed to a subject via electrodes. The level of stimuli can be extremely wide and the shape of the stimuli can be finely controlled. Signals can be introduced directly into nerves or muscle fibers to evoke a specific response.

There are two types of electrical stimulators, voltage and current. A voltage stimulator behaves like a voltage source, in that it outputs a variable voltage with low output impedance. A current stimulator behaves like a current source, in that it outputs a variable current with high output impedance.

The primary physical expression that characterizes the behavior of electrical stimulators is:

Vs = Is * Zs

where:

Vs – stimulation voltage from stimulator
Is – stimulation current from stimulator
Zs – impedance of stimulation loop

Zs is the collective impedance of the complete stimulation loop. From a simplified view, Zs consists of the series combination of two electrode / skin junction impedances and the stimulated tissue volume. Zs has resistive and capacitive components, so the capacitive components charge in accordance to the expression:

Ics = Cs * (dVcs/dt)

where:

Ics – stimulation current through Cs
Cs – capacitive component associated with Zs
dVcs/dT – change in stimulus voltage, across Cs, as a function of time

Accordingly, electrical stimulation is associated with the charging of the stimulation loop capacitance. If fast electrical stimulation is required, then the stimulator should have high current level drive capability. For a given Cs, the speed of charging (dVcs/dt) will be higher (faster) as Ics is increased.

There are two major sub-impedances in the stimulation loop that define the portion of the stimulator output voltage (Vs) that is directly across the tissue volume (Vt):

Ze – electrode to site impedance (total of both sites)

Zt – stimulated tissue volume impedance

Vs = Is * Zs

Vs = Is * (Ze + Zt)

Vt = Vs * (Zt / (Ze + Zt))

Because of this relationship, the value of Vt might not be reliably specified, given a certain Vs.For any given tissue volume, electrical stimulation can be specified in terms of voltage across the tissue volume (Vt) or current through the tissue volume (It). In practice, because two terminal stimulation is simple to apply, current stimulation is often used because the current through the tissue volume (It) is the same as the sourced stimulation current (Is). However, if the stimulation current (It) is specified then Vt may be indeterminate. Sometimes, both (It) and (Vt) are required to be known, such as with bioimpedance measurements. In this case, voltage monitoring electrodes can be placed on the tissue volume so both (It) and (Vt) can be identified.

Compliance, an essential characteristic of electrical stimulators, is addressed in Electrical Stimulators for Physiological Research: Compliance.

BIOPAC Systems, Inc. provides life science researchers and educators with data acquisition and analysis systems that inspire people and enable greater discovery about life. BIOPAC lets you measure physiology anywhere, with innovative, compatible wireless, wearable solutions that can be used by anyone for meaningful discovery. We make high-quality scientific tools for physiology measurement and interpretation with superior compatibility and world-class customer service and support. Visit us at www.biopac.com.

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