Visualizing the location, morphology, and function of specific organs and tissues within the body is crucial for disease monitoring, management, and treatment^[1]. Currently, multiple mature imaging modalities are applied, including fluorescence imaging, ultrasound imaging, magnetic resonance imaging (MRI), computed tomography (CT), and positron emission tomography (PET)^[2]. To overcome the physical limitations of existing imaging technologies, contrast agents have been developed to enhance signal contrast and imaging resolution. They have found extensive application in cross-scale target imaging, encompassing blood vessels, body cavities, and other structures^[3].

Among various imaging technologies, ultrasound imaging is the most widely used due to its radiation-free nature, high penetration depth, cost-effectiveness, and ease of operation^[4-6]. However, its resolution, particularly in deep tissues, is relatively low on the order of millimeters^[7]. Furthermore, traditional B-mode ultrasound imaging encodes spatial information through the amplitude of reflected ultrasound waves, relying on grayscale information to identify imaging targets. Due to this inherent non-specificity, any high-echo tissue within the body can generate signal interference that obscures the imaging target. Consequently, the success of ultrasound-based diagnosis and treatment often heavily depend on the physician’s experience^[8].

To enhance ultrasound imaging, oral or injectable microbubble contrast agents are commonly used. These agents amplify the amplitude of ultrasound echo signals by strongly reflecting acoustic waves through gas bubbles^[9-11]. However, when circulating in the bloodstream, such contrast agents may pose risks or exhibit physiological toxicity^[9-10]. Furthermore, they also lack ultrasound specificity and do not fundamentally eliminate image artifacts caused by high-echo tissues^[11].

Acoustic metamaterials are a class of materials that flexibly manipulate acoustic waves through structural design, exhibiting novel acoustic properties^[12-14]. They are utilized in subwavelength imaging^[15-17], acoustic holography^[18-20], and acoustic absorption^[21-23]. By embedding submillimeter air cavity scatterers within hydrogel materials, an acoustic metastructured hydrogel (metagel) can be constructed, achieving applications in ultrasonic impedance tuning^[24-28]. Here, we developed an acoustic metagel film exhibiting a step-like acoustic spectral characteristic. Compared to non-specific imaging targets, it demonstrates a significant contrast in reflectivity across relatively high and low acoustic frequency bands. Utilizing a dual-frequency ultrasound probe, differential imaging in the frequency domain eliminates interference from high-echo tissues, enabling high-contrast ultrasound imaging. To optimize design parameters, we performed computational modeling of the ultrasonic reflection acoustic field. The metagel film maintains performance under deformation, achieving approximately 4 times reflectivity contrast enhancement across a 40% strain range. By designing dual-frequency excitation modes and integrating ultrasonic spectral analysis, we investigated reflectivity differences between the metagel film and zinc tablet during dual-frequency imaging, achieving a signal-to-noise ratio exceeding 50 dB and a 4 times amplification of the reflected ultrasonic signal amplitude. Furthermore, using adhesive hydrogel patches, we demonstrated the metagel film’s conformal attachment and imaging enhancement capabilities on complex curved surfaces, thereby providing an effective tool for interference-free ultrasound imaging.

The schematic structure of the metagel film is shown in Figure 1A, where a periodically arranged air-cell array is embedded within the hydrogel. At a specific frequency (termed the “cutoff frequency”), the reflectivity decreases sharply. Consequently, when ultrasound waves are incident on the metagel, lower-frequency acoustic waves are reflected while higher frequencies are transmitted [Figure 1B], which is a unique characteristic not found in human tissue or other conventional imaging targets. Therefore, by performing a differential operation between the relatively low-frequency image and the relatively high-frequency image, interference from any other tissue can be eliminated, thereby clearly preserving the metagel film itself [Figure 1C].

This unique property is attributable to the arrangement of the air cells. Each air cell is cylindrical with a uniform diameter d, occupies individual lattice sites with a lattice constant denoted as a and with a height denoted as h [Figure 1D and E]. According to the acoustic wave propagation equation:

(1) $$ \nabla^{2} P-\frac{\rho}{B} \frac{\partial^{2} P}{\partial t^{2}}=0 \\ $$

The velocity of sound in different media is determined by the medium’s properties: $$ v=\sqrt{\frac{B}{\rho}} \\ $$. In the Equation (1), P represents the acoustic pressure field, ρ and B denote the density and bulk modulus of the medium, respectively. Therefore, the primary parameters affecting the refractive index of acoustic waves between different materials are bulk modulus and density. Owing to the high water content (> 90%) of hydrogels, their bulk modulus and density are similar to those of water. In contrast, the bulk modulus of air cells is significantly lower than that of water, leading to strong scattering at the air-hydrogel interface. Moreover, the periodic arrangement of air cells forms a lattice structure, which induces Bragg scattering. This creates an acoustic bandgap analogous to that in crystalline structures, resulting in novel frequency-domain characteristics.

We conducted simulations to validate this principle (see Supplementary Figure 3 in Supplementary Materials for the simulation physics field settings). For a metagel with lattice constant a = 0.55, diameter d/a (denoted as filling ratio q) = 0.55, and h = 1 mm, the ultrasonic wave at 2 MHz undergoes focused reflection [Figure 2A], while the ultrasonic wave at 6 MHz is largely transmitted. The non-transmitted ultrasonic wave also scatters in surrounding directions [Figure 2B]. Its specific spectrum is shown in Figure 2C, with a cutoff frequency occurring at approximately 4.5 MHz. We also compared the reflected ultrasound spectra of other materials, including high-scattering bone and low-scattering soft tissue. Their reflection amplitude decreases slightly with increasing ultrasound frequency, which is attributed to the inherent attenuation of high-frequency ultrasound, resulting in a smooth reflection spectrum without a distinct step-like drop. Therefore, when high- and low- ultrasonic waves are simultaneously incident upon the metagel films and human organs, the metagel exhibits a distinct difference in reflection intensity, whereas human organs do not. Ultimately, by processing the amplitude of acoustic waves at different frequencies, the interfering acoustic field from high-echo tissues can be eliminated.

This peculiar mechanism can be explained by multiple scattering^[24]. When acoustic waves pass through a periodically arranged scattering structure (such as the air cells in a metagel), each scatterer causes scattering of the incident wave. These scattered waves then interact with each other again, creating a multiple scattering effect, which can be explained as follows:

The incident plane wave is expressed as:

(2) $$ P=e^{i k y} \\ $$

where k denotes the wave vector, and y represents the direction of ultrasonic incidence (simplified as normal incidence). Expressing Equation (2) as an expansion of a Hankel function of the first kind gives:

(3) $$ P_{j, l}=\sum_{n=-\infty}^{\infty} B_{n}^{(j, l)} H_{n}\left(k r_{j, l}\right) e^{i n \theta_{j, l}} \\ $$

The Equation (3) represents the scattered ultrasound from the l^th scatterer in the j^th incident cycle. $$ B_{n}^{(j, l)} \\ $$ is the scattering coefficient, H_n denotes the first-class Hankel function, (r_j,l, θ_j,l) are the polar coordinates centered on this scatterer. Due to the periodicity of the metagel structure, scatterers at the same position are simplified to have the same scattering coefficient.

The total scattering field is expressed as:

(4) $$ P^{\text {total }}=\sum_{j=-\infty}^{\infty} \sum_{p=1}^{N} P_{j, l} \\ $$

Further expand Equation (4) as a sum of plane waves of different diffraction orders:

(5) $$ P^{\text {total }}=\sum_{n=-\infty}^{\infty} P_{n} e^{\left(i k \cos \theta_{n} x+i k \sin \theta_{n}|y|\right)} \\ $$

where, $$ \cos \theta_{n}=n \frac{2 \pi}{k N a} \\ $$, N denotes the total number of scatterers.

For the n^th-order scattered wave, the transmission coefficient T_n and reflectance coefficient R_n are given by^[24]:

(6) $$ \begin{align} R_{n}=\frac{2}{k a \sin \theta_{n}} \sum_{q=1}^{N} e^{\left(-i \frac{2 n \pi}{N a} R_{0, q}\right)} \sum_{m=-\infty}^{\infty} B_{m}^{q}(-i)^{m} e^{i m \theta_{n}} \\ T_{n}=1-\frac{2}{k a \sin \theta_{n}} \sum_{q=1}^{N} e^{\left(-i \frac{2 n \pi}{N a} R_{0, q}\right)} \sum_{m=-\infty}^{\infty} B_{m}^{q}(-i)^{m} e^{i m \theta_{n}} \end{align}$$

For the principal reflection order, i.e., when n = 0^[24]:

(7) $$ R_{0}=\frac{2}{k a} \sum_{n=-\infty}^{\infty}(-1)^{n} B_{n} \\ $$

Therefore, as seen from Equation (7), the reflectivity is inversely proportional to a. This is related to the bandgap properties of Bragg scattering-type acoustic metamaterials, where ultrasonic waves at the bandgap center frequency possess wavelengths similar to the lattice constant. That is to say, a larger lattice constant results in a lower characteristic reflection frequency (corresponding to the cutoff frequency for the metagel). The following relationship holds^[29]:

(8) $$ f_{c u t o f f} \sim n \frac{v}{2 a} $$

We investigated the influence of different design parameters on the reflective properties of the metagel. As shown in Figure 2D, a larger lattice constant leads to a lower cutoff frequency, consistent with the conclusion derived from Equation (7). Simultaneously, the cutoff frequency increases with increasing fill factor, as depicted in Figure 2E. The height of the air cells within the metagel has a negligible impact on the cutoff frequency [Figure 2F].

Furthermore, we investigated the ultrasonic reflectivity at the cutoff frequency, which reflects the ratio of high-frequency (above cutoff frequency) to low-frequency (below cutoff frequency) ultrasonic reflectivity. As shown in Figure 2G-I, to ensure contrast between high- and low-frequency ultrasonic waves, a relatively high air-cell filling ratio and a small lattice constant are typically required. However, smaller dimensions pose manufacturing challenges and necessitate careful consideration in practical applications. The reflection spectra for each parameter are presented in Supplementary Figure 4. Therefore, the design of the metagel film can be scaled to accommodate ultrasonic transducers with different center frequencies [Supplementary Figure 5].

Due to the typically convoluted and non-planar nature of internal organs, conformal issues must be considered when using metagels as solid imaging films. Therefore, we investigated the ultrasonic reflection spectra under different strains, as shown in Figure 2J and K. Even at 40% bending strain, the metagel maintains strong reflection at low frequencies. The reflection spectra under various bending strains are depicted in Figure 2L. The cutoff frequency and reflectivity at this frequency remain nearly constant, while both high- and low-frequency reflectivities decrease. This reduction stems from deformation-induced performance loss but does not affect the contrast ratio, which remains high at approximately 4 times.

To further validate the imaging performance of the metagel film, we prepared metagel samples for testing. Using a single probe [Supplementary Figures 1 and 2] to simultaneously emit ultrasonic waves at two frequencies - 2 and 6 MHz - a technique termed dual-frequency scanning, we compared the reflection spectra of the metagel, pure hydrogel (without an air-cell structure, same thickness as the metagel), and zinc metal. As a high-echo material, zinc can be used in ultrasound imaging. Therefore, we selected zinc as an extremely high-echo interference source. As shown in Figure 3A and B, the metagel exhibits significant difference in peak reflection amplitude between 6 and 2 MHz frequencies, whereas the highly reflective zinc and the low-reflective pure gel remain nearly unchanged. Therefore, when subtracting the 6 MHz ultrasonic signal from the 2 MHz signal, the difference signals for zinc and pure gel approach zero, whereas the metagel signal is only partially subtracted, with most components retained. This principle forms the basis for eliminating high-echo interference through dual-frequency scanning. In addition, the metagel maintains stable reflective performance at various depths, up to 10 cm [Supplementary Figure 6]. During a 7-day continuous stability test, the metagel also maintained good performance [Supplementary Figure 7].

For the dualfrequency differential imaging proposed in this work, the identification of the metagel film relies primarily on frequency specificity. Therefore, in the axial direction, data containing the echo information of the entire object must be acquired to process a complete frequency spectrum, meaning that the axial resolution is comparable to the size of the device itself, approximately 1 mm. The lateral resolution is equal to the interval between line-scan ultrasonic transducer elements (0.15 mm). In contrast, traditional microbubble ultrasound contrast agents have a lateral resolution of approximately 2 mm and an axial resolution of 1 mm^[3].

Based on the above mechanism, we established a differential ultrasound imaging mode, as shown in Figure 4. A single ultrasound radiofrequency (RF) echo signal is converted into a digital signal, with the number of sampling points denoted as n. By performing spatial line scanning, m lines of RF data are acquired, yielding a 2D raw image matrix. The matrix element a_i,j is normalized and converted to grayscale intensity b_i,j. By subtracting the two sets of grayscale images b_i,j and c_i,j obtained from dual-frequency scanning (high and low frequencies, respectively), the image of the metagel film itself is preserved while images of other tissues are eliminated, thereby achieving high-contrast ultrasound imaging.

We demonstrate the practical application of this imaging modality. As shown in Figure 5A, a dual-frequency ultrasound probe simultaneously emits 2 and 6 MHz ultrasound waves to acquire echoes from a zinc tablet, pure gel, and metagel. These echoes are uniformly processed into ultrasound grayscale images, as depicted in Figure 5B-D. In the 2 MHz center-frequency ultrasound image, both the highly reflective zinc and metagel are visible. However, in the 6 MHz image, the metagel film exhibits low reflectivity due to its inherent spectral characteristics, resulting in an image that only reveals the zinc. Given inherent resolution differences in both 2 and 6 MHz ultrasound imaging, regions of interest (ROIs) were selected in the higher-resolution 6 MHz ultrasound images and delineated in the corresponding 2 MHz images, while the remaining areas are still processed according to the original 6 MHz image. In practice, this approach allows both accurate localization of the implant and preservation of the higher-resolution 6 MHz ultrasound images of the internal body structures. Differential imaging processing applied to these overlapping regions yielded the differential image shown in Figure 5D. The high-echo interference from the zinc tablet is nearly eliminated, leaving only the high-gray-value image of the metagel film, thereby achieving the objective of high-contrast ultrasound imaging. Specifically, the 2 MHz ultrasound imaging shown in Figure 5B corresponds to the previously reported metagel imaging method, i.e., using a metagel with a single-layer periodic air-column structure and directly performing ultrasound imaging at a center frequency of 2 MHz^[26].

We performed quantitative analysis of this imaging modality. Figure 5E demonstrates that the metagel film achieves a signal-to-noise ratio (i.e., the dB value above the noise signal) as high as 50 dB in dual-frequency differential imaging. Statistical analysis was conducted using Student’s t-test. For the metagel film, a significant difference was observed between the 2M and differential images (P = 0.0003). For zinc tablet, no significance (NS) was observed between the 2M and differential images (P = 0.3882). The signal-to-noise ratio further improves with repeated cyclic excitation of the dual-frequency probe [Figure 5F]. Compared to the grayscale image of the high-intensity reflector zinc, the metagel film exhibits 4 times increase in signal amplitude contrast [Figure 5G, Supplementary Table 2].

Given that hydrogels are inherently polymeric materials capable of flexible modification, they can be endowed with additional functionalities to serve diverse application scenarios. Here, we leverage the adhesive properties of hydrogels to achieve conformal attachment of metagels, further meeting the requirements for in vivo ultrasound imaging applications [Figure 6A]. We incorporated -NHS esters into the hydrogel to create a double-sided adhesive hydrogel patch^[30]. Amines introduced into the metagel film bonded to the adhesive layer. Similarly, proteins in the human body, rich in amino groups, also bonded effectively to the adhesive layer, as shown in Figure 6B. The adhesion performance of the adhesive layer at both interfaces is illustrated in Figure 6C. We adhered the metagel film to surfaces of three distinct shapes - C-, S-, and M-shaped [Figure 6D] - and imaged these objects, demonstrating the metagel film’s excellent conformal and high-contrast imaging capabilities. Considering the material’s ability to maintain its properties under deformation [Figure 2J-L], its flexible nature, and its adhesive properties, the conformal imaging performance of the metagel film is expected to be excellent. Using the following parameters: PRF = 4,000, scan line = 128, and pulse cycle = 6, the temporal resolution of differential imaging is calculated to be 37 frame per second (FPS), which is expected to effectively resist motion artifacts. Owing to the material’s ability to maintain its properties under deformation, the change in performance caused by the pressure dissipated from the external probe to the interior is negligible.

Due to the complexity of the internal environment, various tissues including fibrous tissue, connective tissue, and cartilage appear as high-echo structures in ultrasound images, thereby interfering with the physician’s assessment of the imaged area. Among these, bone exhibits strong acoustic reflection properties comparable to metal. Therefore, we further applied the metagel film to an ex vivo porcine rib tissue specimen, which simultaneously contains both high-echo (bone) and low-echo (fat and muscle, etc.) regions. Through dual-frequency differential imaging, we achieved high-contrast enhancement of the metagel film [Figure 6E]. This demonstrates that our metagel film not only conforms to complex human tissue geometries but also enables high-contrast imaging, effectively suppressing interference from strong echo signals - a unique capability unattainable with existing contrast agents. This fundamentally eliminates background interference through acoustic specificity. In addition, the swelling ratio of the metagel film is as low as 20%, which effectively minimizes performance degradation caused by swelling and deformation, and it can be stabilized by a 48-hour pre-swelling process [Supplementary Figure 8].

Our research proposes a high-contrast metagel film based on the acoustic modulation capabilities of flexible acoustic metamaterials. By leveraging the contrast between high- and low-frequency ultrasound echo signals, it eliminates echo interference from normal imaging targets, resulting in signal enhancement that surpasses that of metallic reflectors. The ultrasound imaging performance of the metagel film has been comprehensively validated through theoretical analysis, simulation calculations, and experimental testing. Composed entirely of hydrogel, the metagel film exhibits exceptional flexibility, biocompatibility, and complete biodegradability. Its integration with adhesive hydrogels further demonstrates the metagel film’s targeted imaging capabilities for specific tissues. The proposed metagel film not only overcomes the short duration of action associated with traditional microbubble contrast agents but also enables specific localization by eliminating high-echo backgrounds [Supplementary Table 3]. Our approach not only serves imaging device development and clinical research but also opens new avenues in soft materials, metamaterials, and applied acoustics.