How soft materials reveal their secrets under gentle pressure
Kalpit Bakal defended his PhD thesis at the Department of Mechanical Engineering on December 10th.
Soft materials surround us every day: in foods, cosmetics, medical implants, and living tissues. What makes them fascinating鈥攁nd challenging to study鈥攊s that they do not behave purely like solids or liquids. Instead, their mechanical response depends on time, scale, and environment. In this PhD research of Kalpit Bakal, Capillary Micromechanics was advanced into a sensitive and semi-automated technique capable of measuring both instantaneous and time-dependent mechanical properties of microscopic soft materials. The work demonstrates that viscoelasticity, poroelasticity, particle size, and microenvironment together govern how soft materials deform. These insights establish Capillary Micromechanics as a powerful tool for mechanophenotyping in biomaterials and cancer research.
Mechanical properties of soft materials are traditionally measured using rheometers鈥攄evices designed for bulk samples. However, when samples shrink to the micrometer scale, these tools become ineffective.
Existing microscale techniques often probe only surfaces or localized regions, missing how the entire object responds to deformation. This limitation is particularly problematic for materials such as hydrogels, biological tissues, and cell aggregates, where internal fluid flow and time-dependent effects play a crucial role.
Expanding Capillary Micromechanics beyond elasticity
To overcome these challenges, this research of Kalpit Bakal builds on Capillary Micromechanics, a technique in which soft microscopic particles are gently compressed inside a tapered glass capillary. By observing how the particle deforms, its mechanical properties can be extracted.
Earlier versions of the method focused mainly on simple elastic behavior. In this work, the technique was extended to capture time-dependent responses, allowing researchers to measure not only elasticity but also viscoelastic relaxation and flow-like behavior.
In addition, semi-automated testing procedures and automated image analysis were developed, significantly improving accuracy, reproducibility, and measurement speed.
Hydrogels reveal the role of water and size
Using polyacrylamide hydrogels鈥攑olymer networks swollen with water鈥攖he research explored how stiffness and relaxation dynamics change under different conditions.
The experiments showed that osmotic shocks stiffen the hydrogels, but surprisingly also slow down their relaxation. This effect is driven by macromolecular diffusion, highlighting the complex interplay between solid networks and fluid transport.
Crucially, particle size was found to strongly influence deformation dynamics: larger particles relax more slowly because water must travel longer distances through the network. This behavior is a clear signature of poroelasticity, where fluid flow through a porous solid governs mechanical response.
Probing tumor-like cell clusters
The technique was further applied to breast cancer cell spheroids, small three-dimensional clusters that mimic aspects of real tumors.
Capillary Micromechanics revealed that their mechanical properties depend strongly on growth conditions and extracellular matrix composition. Differences were observed not only in elasticity, but also in viscosity and strain-stiffening behavior鈥攎echanical features known to influence tumor progression and treatment response.
A new tool for soft matter and biomedical research
Together, these results demonstrate that soft-material mechanics cannot be described by stiffness alone. Instead, time, size, and microenvironment jointly determine how materials deform and relax.
By enabling reproducible, time-resolved measurements at microscopic scales, this work establishes Capillary Micromechanics as a versatile platform for studying biomaterials, soft matter systems, and cellular microtissues. With further automation and higher throughput, the technique holds strong promise for future applications in cancer research, tissue engineering, and materials design.
Title of PhD thesis: . Supervisors: Dr. Hans Wyss, and Prof. Jaap den Toonder.