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A wearable cardiac ultrasound imager


Gallium–indium eutectic liquid steel, toluene, ethyl alcohol, acetone and isopropyl alcohol have been bought from Sigma-Aldrich. SEBS (G1645) was obtained from Kraton. Silicone (Ecoflex 00-30) was purchased from Easy-On because the encapsulation materials of the machine. Silicone (Silbione) was obtained from Elkem Silicones because the ultrasound couplant. Aquasonic ultrasound transmission gel was purchased from Parker Laboratories. 1-3 composite (PZT-5H) was bought from Del Piezo Specialties. Silver epoxy (Von Roll 3022 E-Solder) was obtained from EIS. Anisotropic conductive movie cable was bought from Elform.

Design and fabrication of the wearable imager

We designed the transducer array in an orthogonal geometry, just like a Mills cross array (Supplementary Fig. 34), to realize biplane normal views concurrently. For the transducers, we selected the 1-3 composite for transmitting and receiving ultrasound waves as a result of it possesses superior electromechanical coupling18. As well as, the acoustic impedance of 1-3 composites is near that of the pores and skin, maximizing the acoustic vitality propagating into human tissues19. The backing layer dampens the ringing impact, broadens the bandwidth and thus improves the spatial decision18,51.

We used an automated alignment technique to fabricate the orthogonal array. The present technique of bonding the backing layer to the 1-3 composite was to first cube many small items of backing layer and 1-3 composite, after which bond every pair collectively one after the other. A template was wanted to align the small items. This technique was of very low effectivity. On this examine, we bond a big piece of backing layer with a big piece of 1-3 composite after which cube them collectively into small items with designed configurations. The diced array is then mechanically aligned on adhesive tape with excessive uniformity and ideal alignment.

Electrodes primarily based on eutectic gallium–indium liquid steel are fabricated to realize higher stretchability and better fabrication decision than present electrodes primarily based on serpentine-shaped copper skinny movie. Eutectic gallium–indium alloys are sometimes patterned via approaches comparable to stencil lithography52, masked deposition53, inkjet printing54, microcontact printing55 or microfluidic channelling56. Though these approaches are dependable, they’re both restricted in patterning decision or require refined photolithography or printing {hardware}. The subtle {hardware} makes fabrication difficult and time-consuming, which presents a problem within the improvement of compact, skin-conformal wearable electronics.

On this examine, we exploited a brand new know-how for patterning. We first screen-printed a skinny layer of liquid steel on a substrate. A key consideration earlier than display printing was learn how to get the liquid steel to moist the substrate. To resolve this drawback, we dispersed massive liquid steel particles into small microparticles utilizing a tip sonicator (Supplementary Fig. 2). When microparticles contacted air, their outermost layer generated an oxide coating, which lowered the floor rigidity and prevented these microparticles from aggregating. As well as, we used 1.5 wt.% SEBS as a polymer matrix to disperse the liquid steel particles as a result of SEBS might moist properly on the liquid steel floor. We additionally used SEBS because the substrate. Due to this fact, the SEBS within the matrix and the substrate might merge and treatment collectively after display printing, permitting the liquid steel layer to stick to the substrate effectively and uniformly. Then we used laser ablation to selectively take away the liquid steel from the substrate to kind patterned electrodes.

The massive variety of piezoelectric transducer parts within the array requires many such electrodes to handle every component individually. We designed a four-layered high electrode and a standard floor electrode. There are SEBS layers between totally different layers of liquid steel electrodes as insulation. To show all electrode layers to connect with transducer parts, we used laser ablation to drill vertical interconnect accesses21. Moreover, we created a stretchable shielding layer utilizing liquid steel and grounded it via a vertical interconnect entry, which successfully protected the machine from exterior electromagnetic noises (Supplementary Fig. 8).

Earlier than we hooked up the electrodes to the transducer array, we spin-coated toluene–ethanol resolution (quantity ratio 8:2) on the highest of the multilayered electrode to melt the liquid-metal-based elastomer, also referred to as ‘solvent-welding’. The softened SEBS offered a adequate contact floor, which might assist kind a comparatively sturdy van der Waals drive between the electrodes and the steel on the transducer floor. After bonding the electrodes to the transducer array, we left the machine at room temperature to let the solvent evaporate. The ultimate bonding energy of greater than 200 kPa is stronger than many industrial adhesives22.

To encapsulate the machine, we irrigated the machine in a petri dish with uncured silicone elastomer (Ecoflex 00-30, Easy-On) to fill the hole between the highest and backside electrodes and the kerf among the many transducer parts. We then cured the silicone elastomer in an oven for 10 min at 80 °C. Because the filling materials, it suppresses spurious shear waves from adjoining parts, successfully isolating crosstalk between the weather18,19. With that being mentioned, we predict the principle purpose for the suppressed spurious shear waves is due to the epoxy within the 1-3 composite, which limits the lateral vibration of the piezoelectric supplies. The Ecoflex because the filling materials could have contributed however not performed a chief position as a result of the kerf is just not too huge, solely 100 to 200 µm. We lifted off the glass slide on the highest electrode and immediately lined the highest electrode with a shielding layer. Then we lifted off the glass slide on the underside electrode to launch your complete machine. Lastly, screen-printing an roughly 50-μm layer of silicone adhesive on the machine floor accomplished your complete fabrication.

Characterization of the liquid steel electrode

Current wearable ultrasound arrays can obtain wonderful stretchability by serpentine-shaped steel skinny movies as electrodes19,26. The serpentine geometry, nevertheless, severely limits the filling ratio of useful elements, precluding the event of techniques that require a excessive integration density or a small pitch. On this examine, we selected to make use of liquid steel because the electrode owing to its massive intrinsic stretchability, which makes the high-density electrode potential. The patterned liquid steel electrode had a minimal width of about 30 μm with a groove of about 24 μm (Supplementary Fig. 3), an order of magnitude finer than different stretchable electrodes18,26,57. The liquid steel electrode is right for connecting arrays with a small pitch58.

This liquid steel electrode exhibited excessive conductivity, distinctive stretchability and negligible resistance change below tensile pressure (Fig. 1b and Supplementary Fig. 4). The preliminary resistance at 0% pressure was 1.74 Ω (similar to a conductivity of round 11,800 S m−1), comparable with reported research59,60. The resistance step by step elevated with pressure till the electrode reached the roughly 750% failure pressure (Fig. 1b and Supplementary Fig. 4). The relative resistance is a parameter broadly used to characterize the change within the resistance of a conductor (that’s, the liquid steel electrode on this case) below totally different strains relative to the preliminary resistance58,59,60. The relative resistance is unitless. When the pressure was 0%, the preliminary resistance R0 was 1.74 Ω. When the electrode was below 750% pressure, the electrode was damaged and the resistance R on the breaking level was measured to be 44.87 Ω. Due to this fact, the relative resistance (R/R0) on the breaking level was 25.79.

To research the electrode fatigue, we subjected them to 100% cyclic tensile pressure (Fig. 1c). The preliminary 500 cycles noticed a gradual improve within the electrode resistance as a result of the liquid steel, when stretched, might expose extra surfaces. These new surfaces have been oxidized after contacting with air, resulting in the resistance improve (Supplementary Fig. 4). After the preliminary 500 cycles, the liquid steel electrode exhibited steady resistance as a result of, after a interval of biking, there weren’t many new surfaces uncovered.

This examine is the primary to make use of liquid metal-based electrodes to attach ultrasound transducer parts. The bonding energy between them immediately decides the robustness and endurance of the machine. That is particularly crucial for the wearable patch, which can be subjected to repeated deformations throughout use. Due to this fact, we characterised the bonding energy of the electrode to the transducer component utilizing a lap shear take a look at. The liquid steel electrode was first bonded with the transducer component. The opposite sides of the electrode and the component have been each fastened with stiff supporting layers. The supporting layer serves to be clamped by the tensile grips of the testing machine. Samples can be broken if they’re clamped by the grips immediately. Then a uniaxial stretching was utilized to the pattern at a pressure charge of 0.5 s−1. The take a look at was stopped when the electrode was delaminated from the transducer component. A SEBS movie was bonded with a transducer component and we carried out the lap shear take a look at utilizing the identical technique. The height values of the curve have been used to symbolize the lap shear energy (Fig. 1d). The bonding energy between the pure SEBS movie and the transducer component was roughly 250 kPa, and that between the electrode and the transducer component was about 236 kPa, which have been each stronger than many industrial adhesives (Supplementary Desk 2). The outcomes point out the sturdy bonding between the electrode and the component, stopping the electrodes from delamination below varied deformations. This sturdy bonding doesn’t have any limitations on the ultrasound pressures that may be transduced.

Characterization of the transducer parts

The electromechanical coupling coefficient of the transducer parts was calculated to be 0.67, on par with that of business probes (0.58–0.69)61. This superior efficiency was largely owing to the approach for bonding transducer parts and electrodes at room temperature on this examine, which protected the piezoelectric materials from heat-induced harm and depolarization. The part angle was >60°, considerably bigger than most earlier research18,62, indicating that a lot of the dipoles within the component aligned properly after bonding63. The massive part angle additionally demonstrated the distinctive electromechanical coupling efficiency of the machine. Dielectric loss is crucial for evaluating the bonding course of as a result of it represents the quantity of vitality consumed by the transducer component on the bonding interface20. The common dielectric lack of the array was 0.026, on par with that of the reported inflexible ultrasound probes (0.02–0.04)64,65,66, indicating negligible vitality consumed by this bonding strategy (Supplementary Fig. 1b). The response echo was characterised in time and frequency domains (Supplementary Fig. 1c), from which the roughly 35 dB signal-to-noise ratio and roughly 55% bandwidth have been derived. The crosstalk values between a pair of adjoining parts and a pair of second nearest neighbours have been characterised (Supplementary Fig. 1d). The common crosstalk was beneath the usual −30 dB within the subject, indicating low mutual interference between parts.

Characterization of the wearable imager

We characterised the wearable imager utilizing a industrial multipurpose phantom with many reflectors of various varieties, layouts and acoustic impedances at varied places (CIRS ATS 539, CIRS Inc.) (Supplementary Fig. 11). The collected information are offered in Prolonged Information Desk 1. For a lot of the checks, the machine was first hooked up to the phantom floor and rotated to make sure the most effective imaging aircraft. Uncooked picture information have been saved to ensure minimal data loss attributable to the double-to-int8 conversion. Then the uncooked picture information have been processed utilizing the ‘scanConversion’ operate offered within the k-Wave toolbox to revive the sector-shaped imaging window (restored information). We utilized 5 occasions upsampling in each vertical and lateral instructions. The upsampled information have been lastly transformed to the dB unit utilizing:

$${I}_{{rm{new}}}=20times {log }_{10}({I}_{{rm{previous}}})$$


The penetration depth was examined with a bunch of traces of upper acoustic impedance than the encompassing background distributed at totally different depths within the phantom. The penetration depth is outlined because the depth of the deepest line that’s differentiable from the background (6 dB greater in pixel worth). As a result of the deepest line out there on this examine was at a depth of 16 cm and was nonetheless recognizable from the background, the penetration depth was decided as >16 cm.

The accuracy is outlined because the precision of the measured distance. The accuracy was examined with the vertical and lateral teams of line phantoms. The bodily distance between the 2 nearest pixels within the vertical and lateral instructions was calculated as:

$$Delta y=frac{{rm{imaging}},{rm{d}}{rm{e}}{rm{p}}{rm{t}}{rm{h}}}{{N}_{{rm{pixel}},{rm{v}}{rm{e}}{rm{r}}{rm{t}}{rm{i}}{rm{c}}{rm{a}}{rm{l}}}-1}$$


$$Delta x=frac{{rm{imaging}},{rm{w}}{rm{i}}{rm{d}}{rm{t}}{rm{h}}}{{N}_{{rm{pixel}},{rm{l}}{rm{a}}{rm{t}}{rm{e}}{rm{r}}{rm{a}}{rm{l}}}-1}$$


We acquired the measured distance between two traces (proven as two shiny spots within the picture) by counting the variety of pixels between the 2 spots and multiplying them by Δy or Δx, relying on the measurement path. The measured distances at totally different depths have been in contrast with the bottom fact described within the information sheet. Then the accuracy might be calculated by:



The lateral accuracy was offered because the imply accuracy of the 4 neighbouring pairs of lateral traces at a depth of fifty mm within the phantom.

The spatial resolutions have been examined utilizing the lateral and vertical teams of wires. For the resolutions at totally different depths, the total width at half most of the purpose unfold operate within the vertical or lateral instructions for every wire was calculated. The vertical and lateral resolutions might then be derived by multiplying the variety of pixels throughout the full width at half most by Δy or Δx, relying on the measurement path. The elevational resolutions have been examined by rotating the imager to kind a forty five° angle between the imager aperture and the traces. Then the intense spot within the B-mode pictures would reveal scatters out of the imaging aircraft. The identical course of as calculating the lateral resolutions was utilized to acquire the elevational resolutions. The spatial resolutions at totally different imaging areas have been additionally characterised with the lateral group of wires. 9 wires have been positioned at ±4 cm, ±3 cm, ±2 cm, ±1 cm and 0 cm from the centre. The lateral and axial resolutions of the B-mode pictures from these wires have been calculated with the identical technique.

Notice that the lateral decision worsens with the depth, primarily due to the obtain beamforming (Supplementary Fig. 15). There are two beamformed indicators, A and B. The lateral decision of the A degree (x1) is clearly higher than that of the B level (x2). The truth that lateral decision turns into worse with depth is inevitable in all ultrasound imaging, so long as obtain beamforming is used.

As for various transmit beamforming strategies, the wide-beam compounding is the most effective as a result of it could actually obtain an artificial focusing impact in your complete insonation space. The higher the focusing impact, the upper the lateral decision, which is why the lateral decision of the wide-beam compounding is healthier than the opposite two transmit strategies on the similar depth. Moreover, the multiple-angle scan used within the wide-beam compounding can improve the decision at high-angle areas. The multiple-angle scan combines transmissions at totally different angles to realize a worldwide excessive signal-to-noise ratio, leading to improved resolutions.

The elevational decision can solely be characterised when the imaging goal is immediately beneath the transducer. For these targets which are distant from the centre, they’re tough to be imaged, which makes their elevational resolutions difficult to calculate. When characterizing the elevational decision, the machine ought to rotate 45°. On this case, a lot of the mirrored ultrasound waves from these wires can not return to the machine owing to the big incidence angles. Due to this fact, these wires can’t be captured within the B-mode pictures. One potential resolution is to lower the rotating angle of the machine, which can assist seize extra wires distributed laterally within the B-mode picture. Nonetheless, a small rotating angle will trigger the elevational picture to merge with the lateral picture, which will increase the error of calculating the elevational decision. Contemplating these causes, we solely characterised the elevational decision of the imaging targets immediately beneath the transducer array.

The distinction decision, the minimal distinction that may be differentiated by the imaging system, was examined with greyscale objects. The collected B-mode pictures are proven in Fig. 2. As a result of the targets with +3 and −3 dB, the bottom distinction out there on this examine, might nonetheless be acknowledged within the pictures, the distinction decision of the wearable imager is set as <3 dB.

The dynamic vary in an ultrasound system refers back to the distinction vary that may be displayed on the monitor. The distinction between an object and the background is indicated by the common gray worth of all pixels within the object within the show. The gray worth is linearly proportional to the distinction. The bigger the distinction, the bigger the gray worth. As a result of the show window was utilizing the info sort ‘uint8’ to distinguish the greyscale, the dynamic vary was outlined because the distinction vary with a gray worth starting from 0 to 255.

The item with −15 dB distinction has the bottom common gray worth, whereas the article with +15 dB distinction has the very best (Supplementary Fig. 16). In our case, there are six objects with totally different contrasts to the background within the phantom. The very best gray worth obtained from the article of +15 dB distinction was 159.8, whereas the bottom gray worth from the article of −15 dB distinction was 38.7. We used a linear match to extrapolate the contrasts when the corresponding common gray values have been equal to 255 and 0, which corresponded to contrasts of 39.2 dB and −24.0 dB, respectively. Then the dynamic vary was decided as:



The lifeless zone is outlined because the depth of the primary line phantom that isn’t overwhelmed by the preliminary pulses. The lifeless zone was examined by imaging a selected set of wire phantoms with totally different depths proper beneath the machine (Supplementary Fig. 11, place 4) immediately and measuring the road phantoms that have been seen within the B-mode picture.

The bandwidth of the imager is outlined because the ratio between the total width at half most within the frequency spectrum and the centre frequency. It was measured by a pulse-echo take a look at. A bit of glass was positioned 4 cm away from the machine and the reflection waveform was collected with a single transducer. The collected reflection waveform was transformed to the frequency spectrum by a quick Fourier rework. The complete width at half most was learn from the frequency spectrum. We obtained the bandwidth utilizing:



Distinction sensitivity represents the potential of the machine to distinguish objects with totally different brightness contrasts20. The distinction sensitivity was examined with the greyscale objects. The distinction sensitivity is outlined because the contrast-to-noise ratio (CNR) of the objects having sure contrasts to the background within the B-mode picture:

$${rm{CNR}}=frac{left|{mu }_{{rm{in}}}-{mu }_{{rm{out}}}proper|}{sqrt{{sigma }_{{rm{in}}}^{2}+{sigma }_{{rm{out}}}^{2}}}$$


wherein μin and σin are the imply and the usual deviation of pixel depth throughout the object, and μout and σout are the imply and the usual deviation of pixel depth of the background.

The insertion loss is outlined because the vitality loss through the transmission and receiving. It was examined in water with a quartz crystal, a operate generator with an output impedance of fifty Ω and an oscilloscope (Rigol DS1104). First, the transducer acquired an excitation within the type of a tone burst of a 3-MHz sine wave from the operate generator. Then the identical transducer acquired the echo from the quartz crystal. Given the 1.9-dB vitality lack of the transmission into the quartz crystal and the two.2 × 10−4 dB (mm MHz)−1 attenuation of water, the insertion loss may very well be calculated as:

$${rm{Insertion}},{rm{l}}{rm{o}}{rm{s}}{rm{s}}=left|20times {log }_{10}left(frac{{V}_{{rm{r}}}}{{V}_{{rm{t}}}}proper)+1.9+2.2times {10}^{-4}occasions 2dtimes {f}_{{rm{r}}}^{2}proper|$$


Simulation of the acoustic subject

The simulation computes the basis imply sq. of the acoustic strain at every level within the outlined simulation subject. The basis imply sq. is outlined within the equation beneath and provides a mean acoustic strain over a sure time length, which is pre-defined in a packaged operate of the software program. Within the equation, xi is the simulated acoustic strain on the ith time step.

$${x}_{{rm{RMS}}}=sqrt{frac{1}{n}({x}_{1}^{2}+{x}_{2}^{2}+cdots +{x}_{n}^{2})}$$


Determine 2c is the simulated root imply sq. of the transmitted acoustic strain subject by the orthogonal transducers. The simulation was carried out utilizing the MATLAB UltraSound Toolbox67. Each-dimensional phased array within the orthogonal transducers offers a sector-shaped acoustic strain subject. The simulation merges two such sector-shaped acoustic strain fields. The imaging process was carried out with the identical parameters because the simulations.

Within the simulation, we outlined the transducer parameters first: the centre frequency of the transducers as 3 MHz, the width of the transducers as 0.3 mm, the size of the transducers as 2.3 mm, the pitch of the array as 0.4 mm, the variety of parts as 32 and the bandwidth of the transducers as 55%. Then we outlined wide-beam compounding (Supplementary Fig. 13) because the transmission technique: 97 transmission angles, from −37.5° to +37.5°, with a step measurement of 0.78°. Then the acoustic strain subject was the general impact of the 97 transmissions. Lastly, we outlined the computation space: −8 mm to +8 mm within the lateral path, −6 mm to +6 mm within the elevational path and 0 mm to 140 mm within the axial path.



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