Estimation of the surface mechanical properties of soft tissues mimicking phantoms using impact analyses: a comparative study

Background: Palpation is the most widely used approach to empirically assess the mechanical properties of superficial tissues. While elastography is used for volume measurements, it remains difficult to assess skin properties with non-invasive methods. This study aimed to compare the performances of an impact-based analysis method (IBAM) consisting in studying the dynamic response of a punch in contact with the tissue with other approaches available on the market. Materials and Methods: IBAM consists in analyzing the time dependent force signal induced when a hammer instrumented with a force sensor impacts a cylindrical punch placed in contact with soft tissue. Sensitivities to stiffness changes and to spatial variations were compared between IBAM and four other mechanical surface characterization techniques: IndentoPro (macroindentation), Cutometer (suction), MyotonPro (damped oscillation) and Shore Durometer (durometry) using soft tissue phantoms based on polyurethane gel. Results: For stiffness discrimination in homogeneous phantoms, IBAM was slightly better than IndentoPro and MyotonPro (by 20 % and 35 % respectively), and outperformed the Shore Durometer and Cutometer by a factor of 2 to 4. Furthermore, for stiffness and thickness variations in bilayer phantoms, the axial sensitivity of IBAM was between 2.5 and 4.5 times better than that of MyotonPro and IndentoPro. In addition, the Cutometer appeared to be severely limited by its measurement depth. Conclusion: IBAM seems to be a promising technique for characterizing the mechanical properties of soft tissue phantoms at relatively low depth after future ex vivo and in vivo validation studies with biological tissues (with both animal and in human experiments). This work could pave the way to the development of a decision support system in the field of dermatology and cosmetics.

💡 Research Summary

This paper introduces an Impact‑Based Analysis Method (IBAM) for non‑invasive, rapid assessment of the surface mechanical properties of soft tissues and compares its performance with four commercially available devices: IndentoPro (macro‑indentation), Cutometer (suction), MyotonPro (damped oscillation), and Shore Durometer (hardness). The authors built polyurethane gel phantoms that mimic human soft tissue, varying both Young’s modulus (30.3–105 kPa, corresponding to Shore OOO 20–50) and thickness (1–10 mm). For each phantom, five repeated measurements were taken with each device, and standard deviations were kept below 5 % to ensure reproducibility.

IBAM operates by striking a 4 mm aluminum punch, which is in contact with the phantom, with a 5 g hammer instrumented with a high‑frequency force sensor (sampling at 102.4 kHz). The recorded force‑time curve exhibits a primary impact peak (F₁ at time t₁) followed by a rebound peak (F₂ at time t₂). The authors define a dimensionless indicator R = t₂/t₁ and compute its average over five impacts (R̄) after outlier removal using a z‑score threshold of 2.5. The impact geometry, hammer mass, and punch diameter are fixed, standardizing the energy input and eliminating operator‑dependent variability.

Three experimental protocols were executed: (1) stiffness discrimination using homogeneous phantoms of constant 10 mm thickness, (2) depth (thickness) discrimination using homogeneous phantoms of varying thickness, and (3) axial sensitivity using bilayer phantoms where a soft upper layer of variable thickness sits on a rigid 10 mm base layer (E = 84.6 kPa).

In the stiffness discrimination test, all devices showed increasing indicator values with higher Young’s modulus, but IBAM’s R̄ changed most steeply. Compared with IndentoPro, IBAM improved stiffness discrimination by roughly 20 %; compared with MyotonPro, the improvement was about 35 %. Cutometer and Shore Durometer lagged behind by factors of 2–4.

For thickness discrimination, Cutometer’s suction depth saturated beyond 6 mm, rendering it unable to differentiate thicker samples, while the durometer’s hardness reading was essentially insensitive to thickness. IBAM, however, displayed a linear decrease of R̄ with increasing phantom thickness, reflecting longer rebound times as the impact energy travels through more material. Its axial sensitivity outperformed IndentoPro and MyotonPro by 2.5–4.5 times.

The bilayer experiments revealed that IBAM could detect subtle changes in the upper layer’s thickness and modulus. When the soft upper layer (E = 45.8 kPa) was thinned, R̄ rose sharply, indicating that a larger proportion of the impact energy reached the stiff base layer. MyotonPro and IndentoPro showed only modest variations, and Cutometer again failed due to limited suction depth.

Statistical analysis confirmed that the repeatability of IBAM is comparable to the commercial devices, yet its high‑frequency signal captures more detailed mechanical information, enabling finer discrimination of both stiffness and depth. The method is low‑cost, portable, and can be automated for real‑time feedback, making it attractive for clinical settings such as dermatology, plastic surgery, and cosmetic product testing.

The authors acknowledge that the current validation is limited to synthetic polyurethane phantoms. Future work must include ex‑vivo and in‑vivo studies on animal and human skin to verify the correlation between IBAM indicators and true biomechanical parameters. If successful, IBAM could become the core of a decision‑support system for personalized skin assessment, offering an objective alternative to subjective palpation and a more accessible option than expensive elastography equipment.