Technical Note | ElastoSens™ Bio

How does the ElastoSens™ Bio work?

by Dr. Dimitria Camasão
Senior Application Specialist, Rheolution Inc.

Dr. Antoine Frayssinet
Senior Application Specialist, Rheolution Inc.

THE ELASTOSENS™ BIO SPECIFICATIONS AND TECHNOLOGY

The ElastoSens™ Bio is a compact analytical instrument that measures the viscoelastic properties of soft materials. The dimensions of one instrument are 26.8 cm (10.5 in), 23 cm (9 in), and 50.8 cm (20 in) for height, width, and length, respectively, and it weighs 17 kg.
ElastoSens™ Bio uses vibrations to obtain contactless viscoelastic measurements. Viscoelasticity Testing of Bilayered Materials (VeTBiM) is the patented technology behind the ElastoSens™ Bio [1]. VeTBiM consists in measuring the free response of a bi-layered system to a vibrational impulse. The bi-layered system consists of (i) a sample over (ii) a flexible membrane located at the bottom of the patented sample holder (Figure 1). The sample holder is composed of a solid polycarbonate body with a PDMS elastomer-based membrane at its bottom.

Figure 1. Patented sample holder of the ElastoSens™ Bio.

During testing, this sample holder is firmly attached to a mechanical unit inside the instrument (Figure 2A). The mechanical unit applies at each measurement point a vertical pulse vibration into the bilayered structure (sample holder + sample) in the range of 0.1 to 0.2 mm in amplitude in order to gently induce its free linear resonance (Figure 3A). The spectrum of resonance of the bilayered structure is measured using the laser probe pointing to the center bottom of the sample holder and processed to extract its resonance frequency and amplitude. Sample height is measured using an ultrasonic probe located right at the center top of the sample holder (Figure 3B).

Knowing the geometrical properties of the bilayered structure (height is measured and diameter is known) and the mechanical properties of the flexible membrane, the resonance properties of the bilayered structure only depend on the unknown viscoelastic properties of the sample. An inverse problem is automatically applied by the instrument to obtain the viscoelastic properties of the sample including linear shear storage modulus (G’), linear shear loss modulus (G’’), shear complex modulus (G*) and the loss tangent tan(d). The viscoelastic data are stored and displayed in real time (Figure 3C).

Figure 2. Measuring chamber of the ElastoSens™ Bio. (A) Mechanical unit where the sample holder is placed. (B) Sensor of temperature.

Figure 3. Diagram of a testing in the ElastoSens™ Bio. (A) Single bilayered system (sample holder + sample) under stimulation in the measuring chamber of the ElastoSens™ Bio, (B) Resonance of the bilayered system, free of any constraint and monitored by a class 1 laser and an ultrasonic probe (for the height), (C) Data processing and display in the software of the ElastoSens™ Bio. [2]

In addition, the rate of G’ evolution (derivative of G’ as a function of time), the height of the sample (precision of ± 0.2 mm) and the temperature (precision of ± 0.1 °C) are also displayed at each measurement point. The sample height captures in real time any volume changes of the sample that may be caused by swelling, shrinking or degradation. Due to the non destructive nature of the technology, the sample can be removed from the instrument and the sample can be reused for other purposes or retested multiple times (Figure 4).

Figure 4. Sample holder can be removed after the test from the measuring chamber of the ElastoSens™ Bio. Sample can be kept in the sample holder for testing at later time points or reused for other purposes.

ENVIRONMENTAL FEATURES

The ElastoSens™ Bio can measure the viscoelastic properties of a sample during the transition from liquid-to-solid, solid-to-liquid or directly at the solid state of the material. These measurements can be combined with environmental stimuli such as temperature, light or gases to study the behavior of materials in response to such stimuli.

Temperature control

In the ElastoSens™ Bio, a sensor of temperature is located inside the chamber and the thermoelectric peltier heating-cooling system is calibrated to go from 4 °C to 70 °C with a precision of ± 0.1 °C. The calibration is done using an external calibrated temperature probe inserted into a sample of 7 mL of water. The peltier heating-cooling system heats or cools down the air of the inner measuring chamber through an engineered ventilation system. Heating speed is approximately 10 °C/min and cooling speed is approximately 5 °C/min from 70 °C to room temperature, and lower as approaching 4 °C.

Bio mode and sterility

The bio mode of the ElastoSens™ Bio is an alternative to the peltier system for tests done at 37 °C. In this mode, both ventilation ports of the measurement chamber are closed (Figure 5) blocking the air circulation and deactivating the peltier system. Internal resistors embedded inside the measuring chamber are able to maintain the temperature at 37 °C. This mode is recommended when working with cells or bacteria to conserve a sterile environment limiting contamination from the air. This mode can also be chosen to avoid or reduce drying of the sample especially if long tests are conducted.

When a sterile environment is needed, the sample holder can be sterilized with ethanol 70% for up to 4 hours. The chamber can be cleaned with a clean cloth and an antiseptic liquid sprayer. The ElastoSens™ Bio can be placed inside a biological hood and the measuring chamber kept open for a few hours. The sample is kept in the sample holder which can be placed in a 6 well plate (or another sterile recipient) during incubation between tests.

Figure 5. The air circulation can also be blocked in the Bio Mode, by closing the vent at each side of the chamber

Photostimulation

Nine LEDs are inserted on the interior side of the measuring chamber lid. Three wavelengths are available (365 nm, 385 nm and 405 nm) with a total of three LEDs per wavelength. For a given wavelength, the disposition of the LEDs is alternated to ensure an homogeneous irradiation to the sample from the top (Figure 6). Each wavelength can be turned on either individually or combined with other wavelengths. Their intensity can also be adjusted from 0 to approximately 24 mW/cm² (depending on the wavelength and sample volume).

Figure 6. LEDs located at the lid of the ElastoSens™ Bio featuring three wavelengths (365 nm, 385 nm and 405 nm).

Atmospheric control

The composition of gases inside the measuring chamber can be changed on the ElastoSens™ Bio. The lid of the measuring chamber includes three channels that allow the user to access the inside of the measuring chamber while the lid is closed during testing. The access channels are made of hard nylon plastic and have an internal diameter of 2.5 mm (0.1”). The channels can be connected to tubes from the exterior using 5/32” connectors. All three channels can be used as entrance or exit, and also other combinations can be possible, such as using one only channel for gas entrance and one other channel for gas exit (Figure 7A). As examples of applications, the user can reduce undesirable oxidation or hydrolysis reactions by replacing the air surrounding the sample with an inert gas such as nitrogen or argon. Alternatively, the injection of CO₂ or O₂ can be useful when working in hypoxia conditions with cells or bacteria. Humidity inside the measuring chamber could also be controlled using these channels. To note, the control of gases injected into the chamber has to be done by other instruments independently of the ElastoSens™ Bio. Not just gases, but flexible tubes or sensors can also fit in the channels such as flexible probes to measure other physical properties such as temperature and pH (Figure 7B).

Figure 7. Lid of the ElastoSens™ Bio’s measuring chamber with the access channels. (A) Flexible tubes inserted in the access channels allowing the user to inject and aspire gases into and outside of the measuring chamber during testing. (B) Schematic example of a temperature probe installed in the access channels.

Testing configuration

Testing duration and environment, as well as measurements duration and intervals can be fully customizable in the ElastoSens™ Bio through its user-friendly software. The ElastoSens™ Bio can measure samples with shear storage modulus (G’) from a few Pa to 1 MPa, with a volume varying between 300 µL and 7 mL depending on its final stiffness.

Configuration

Testing configuration starts by selecting the mode of operation of the ElastoSens™ Bio known as soft and stiff modes (Figure 8). The soft mode is used for samples that have a final G’ (G’ in the final gel state) lower than 500 Pa. The stiff mode is used for samples that have a final G’ higher than 500 Pa. After creating a file for the sample, the user can start creating the measurement sequences.

Figure 8. Testing configuration window in the ElastoSens™ Bio App.

Measurement sequences

One sequence is composed of time, temperature and photostimulation parameters (Figure 9). In the time configuration, the user can select for how long (‘test duration’) and how often (‘step’) the measurements are going to be taken, the later if the user wants ‘Multiple’ measurements. A single test can last days with measurements taken with a minimum step time of 2 seconds for the stiff mode and 8 seconds for the soft mode. There is also the option of ‘None’ in which case no measurement will be done and the instrument can be used just as an environmental chamber, and ‘Single’ for only one measurement. The temperature configuration can be set directly in the equipment (‘Manual’) or programmed in the sequence configuration (‘Constant’ or ‘Profile’). A profile of temperature can be designed to follow a predefined thermal history by determining the start and end temperatures with or without temperature steps. Photostimulation can be activated in each sequence. LEDs of different wavelengths can be used individually or combined and their light intensity can be adjusted from 0 to 100% (the latter corresponding to approximately 24 mW/cm², depending on the wavelength and sample volume). The user can create more than one sequence in the same test. For example, the user may create one sequence with the LED 405 nm on for 10 minutes, and another sequence without light to see if G’ still increases after the light is switched off. Or another test can be composed of a sequence in which the temperature will increase and another sequence in which the temperature will decrease, to study the behavior of the sample under a cycle of temperature.

Figure 9. Measurement sequence configuration in the ElastoSens™ Bio App.

Testing of the empty sample holder

Once the sequence(s) is created, there is the step of testing the sample holder empty. This step is important because the viscoelastic properties of the membrane are included in the calculations to obtain the viscoelastic properties of the sample. The viscoelastic properties of the membrane can have slight differences according to its temperature. For a higher precision in the measurements, it is then recommended to pre-incubate the sample holder at the temperature of the testing before performing this step.

Run test

Finally, the test is ready to be started. The first measurement will start in less than a second after the play button is clicked. All the parameters mentioned above (G’, G’’, G*, rate, height, temperature) can be monitored in real time and exported from the tablet.

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