Minimally invasive experimental methods that can measure local rate dependent mechanical properties are essential in understanding the behaviour of soft and biological materials in a wide range of applications. Needle based measurement techniques such as Cavitation Rheology (Zimberlin et al., 2007) and Volume Controlled Cavity Expansion (VCCE, Raayai-Ardakani et al. (2019a)), allow for minimally invasive local mechanical testing, but have been limited to measuring the elastic material properties. Here, we propose several enhancements to the VCCE technique to adapt it for characterisation of viscoelastic response at low to medium stretch rates ($\text{10−2}$ - 1 s${}^{-1}$). Through a carefully designed loading protocol, the proposed technique performs several cycles of expansion–relaxation at controlled stretch rates in a cavity expansion setting and then employs a large deformation viscoelastic model to capture the measured material response. Application of the technique to soft PDMS rubber reveals significant rate dependent material response with high precision and repeatability, while isolating equilibrated states that are used to directly infer the quasistatic elastic modulus. The technique is further established by demonstrating its ability to capture changes in the rate dependent material response of a tuneable PDMS system. The measured viscoelastic properties of soft PDMS samples are used to explain earlier reports of rate insensitive material response by needle based methods: it is demonstrated that the conventional use of constant volumetric rate cavity expansion can induce high stretch rates that lead to viscoelastic stiffening and an illusion of rate insensitive material response. We thus conclude with a cautionary note on possible overestimation of the quasistatic elastic modulus in previous studies and suggest that the stretch rate controlled expansion protocol, proposed in this work, is essential for accurate estimation of both quasistatic and dynamic material parameters.

可以测量局部速率相关的机械性能的微创实验方法对于理解软材料和生物材料在广泛应用中的行为至关重要。基于针的测量技术（例如空化流变学（Zimberlin等人，2007）和体积受控的腔膨胀（VCCE，Raayai-Ardakani等人（2019a）））允许进行微创的局部机械测试，但仅限于测量弹性材料性能。在这里，我们建议对VCCE技术进行一些增强，以使其适用于中低拉伸速率下的粘弹性响应的表征（$\text{10−2}$ -1秒${}^{-\mathrm{1个}}$）。通过精心设计的加载方案，所提出的技术在型腔扩展设置中以受控的拉伸速率执行了数个扩展-松弛周期，然后采用大变形粘弹性模型来捕获测量的材料响应。该技术在软质PDMS橡胶上的应用显示出显着的速率相关材料响应，具有很高的精度和可重复性，同时隔离了用于直接推断准静态弹性模量的平衡状态。通过展示其捕获可调谐PDMS系统的速率相关材料响应变化的能力来进一步建立该技术。测得的软PDMS样品的粘弹性能用于解释基于针法的速率不敏感材料响应的早期报道：已经证明，恒定体积速率的腔体膨胀的常规使用可以引起高拉伸速率，这导致粘弹性硬化和对速率不敏感的材料响应的幻觉。因此，我们在以前的研究中对准静态弹性模量可能高估的警告性结论作为结论，并建议这项工作中提出的拉伸速率控制的膨胀方案对于准确估算准静态和动态材料参数至关重要。