Cell mechanical testing service

Cell Mechanical Testing Service – Comprehensive Biomechanical Characterisation for Tissue Engineering, Drug Development and Single‑Cell Research

As an ISO/IEC 17025 accredited contract research laboratory, we offer specialised cell mechanical testing services to Bulgarian and international research institutes, pharmaceutical companies, and biotechnology firms. The mechanical properties of cells – including stiffness, viscoelasticity, adhesion force, and traction – are critical indicators of cellular health, differentiation, disease state (such as cancer progression), and response to therapeutic agents. Our test platform integrates advanced atomic force microscopy (AFM), optical tweezers, micropipette aspiration, microfluidic deformation devices, and traction force microscopy to provide a comprehensive mechanical characterisation of suspended and adherent cells, 3D spheroids, and engineered tissues. All methods are aligned with ISO 13099 (Colloidal systems – Methods for zeta potential determination – part 1: Electroacoustic and electrokinetic phenomena), ASTM E2491 (Standard guide for evaluating the kinetic and equilibrium behaviour of antibodies and other binding proteins), and BDS guidelines for biological testing. Our reports are recognised by the Bulgarian Food Safety Agency (BFSA), the National Centre for Infectious and Parasitic Diseases (NCIPD), and leading European and Bulgarian research funding and regulatory bodies.

Cell Types and Biological Samples We Test

Our biomechanics laboratory handles a wide range of cell types and biological constructs. Typical test samples include:

• Adherent cell lines – cancer cell lines (e.g., HeLa, MCF‑7, MDA‑MB‑231, A549), fibroblasts, endothelial cells, epithelial cells, and mesenchymal stem cells (MSCs)
• Suspension cells – immune cells (lymphocytes, macrophages, neutrophils), haematopoietic stem cells, and circulating tumour cells (CTCs)
• Primary cells – patient‑derived cells (e.g., fibroblasts, chondrocytes, osteoblasts) for personalised medicine studies
• 3D cell aggregates – spheroids, organoids, embryoid bodies, and microtissues for regenerative medicine and cancer research
• Engineered tissues – decellularised scaffolds, hydrogels with embedded cells, and bioprinted constructs
• Extracellular matrix (ECM) components – collagen, fibrin, Matrigel™, and synthetic hydrogels with defined mechanical properties
• Bacterial and yeast cells – for microbiological biomechanical studies and antimicrobial testing

Single‑Cell Mechanical Properties – Stiffness, Viscoelasticity and Adhesion

• Atomic Force Microscopy (AFM) indentation – ISO 13099 / ASTM E2491 – We use a high‑resolution AFM with a calibrated cantilever (spring constant 0.01‑10 N/m) and a spherical or pyramidal tip. Single cells are indented at a controlled speed (0.1‑10 µm/s) and the force‑distance curve is recorded. The apparent Young’s modulus is calculated using the Hertz‑Sneddon contact mechanics model (for spherical indenter) or the Oliver‑Pharr method (for sharp indenters). This method can assess cell stiffness at the nanoscale (localised regions of the cytoskeleton) and at the whole‑cell level. For Bulgarian oncology research, we routinely measure the stiffness of healthy vs. malignant cells; cancer cells are typically softer (lower modulus) than normal cells.
• Micropipette aspiration – ISO 13099 / ASTM E2491 – We apply a controlled negative pressure (0.01‑5 kPa) to a single cell via a glass micropipette (inner diameter 3‑8 µm). The cell’s deformation into the pipette is video‑recorded and analysed using a standard viscoelastic model (standard linear solid or Kelvin‑Voigt) to derive the instantaneous elastic modulus, the long‑term elastic modulus, and the apparent viscosity. This is particularly useful for suspension cells and cells that are difficult to adhere for AFM.
• Optical tweezers – single‑cell stretching – Using a dual‑beam optical trap, we stretch a single cell (captured between two beads) and measure the force‑extension curve. The trapped cell is subjected to sinusoidal or step‑strain oscillations, allowing the calculation of the storage modulus (G′) and loss modulus (G″) as a function of frequency (0.1‑10 Hz), providing a complete viscoelastic profile.
• Microfluidic deformation – high‑throughput analysis – For high‑throughput screening (up to 1 000 cells per minute), we use a microfluidic device with a constriction channel (5‑10 µm). Cells are driven through the channel at controlled flow rates, and the deformation index (DI = 1 – circularity) is measured from high‑speed images. This method is valuable for drug screening and for detecting cell populations that respond to biomechanical modulation.
• Traction force microscopy (TFM) – For adherent cells, we measure the contractile forces exerted on the substrate. Cells are cultured on a deformable substrate (polyacrylamide hydrogel with fluorescent beads). The bead displacement field is recorded and the traction stresses are reconstructed using a finite‑element or Fourier‑based algorithm. This provides information on cell‑substrate adhesion and the cellular contractile machinery, which is essential for studying cell migration, wound healing, and cancer invasion.

Bulk Mechanical Properties – Spheroids, Organoids and Engineered Tissues

• Unconfined compression of 3D constructs – ASTM D695 / ISO 844 – We subject cylindrical spheroids or tissue constructs (diameter 0.5‑2 mm) to unconfined compression (e.g., using a texture analyser or a custom micro‑compression device). The force‑displacement curve is converted to stress‑strain data, and the compressive modulus (E) and the strain‑hardening region are determined. This mimics the mechanical environment of tumours, cartilage, and developing tissues.
• Indentation of spheroids – using a spherical indenter – For smaller spheroids (100‑300 µm), we perform indentation with a spherical AFM probe (diameter 20‑50 µm) to map the local stiffness across the spheroid. The spatial variation of stiffness (from the outer rim to the core) is plotted, which is important for understanding nutrient gradients, hypoxia, and necrotic core formation.
• Tensile testing of microtissues – ASTM D638 (adapted) – For fibre‑like or strip‑shaped engineered tissues, we perform a uniaxial tensile test using a micro‑tensile tester with a low‑load cell (0.1‑10 N). The elastic modulus, tensile strength, and failure strain are measured, which are crucial parameters for vascular grafts and cardiac patches.
• Stress relaxation and creep – dynamic mechanical analysis (DMA) – ISO 6721 – We apply a constant deformation (step strain) or constant stress to the tissue construct and monitor the force or displacement relaxation over time. The relaxation time constant (τ) and the equilibrium modulus are derived, characterising the tissue’s viscous behaviour.
• Burst pressure of tissue‑engineered vessels – ISO 7199 (adapted) – For vascular constructs, we pressurise the vessel segment with saline or culture medium and record the pressure at which leakage or rupture occurs. This is an essential quality control test for vascular grafts.

Substrate and ECM Mechanical Characterisation – Matrix Stiffness and Topography

• Substrate stiffness – AFM or nano‑indentation of hydrogels – We measure the elastic modulus of the substrate (e.g., polyacrylamide, alginate, collagen gel) that cells are cultured on. This is critical because cells sense and respond to substrate stiffness (durotaxis). We provide calibration of the substrate modulus for your specific cell type.
• Surface topography – SEM and profilometry – ISO 4287 / ASTM E1813 – We characterise the surface roughness (Ra, Rz) and the pore size of the substrate using scanning electron microscopy (SEM) or a contact profilometer. This parameter influences cell adhesion, migration, and differentiation.
• Viscosity of extracellular matrix (ECM) solutions – ISO 3219 / ASTM D2196 – For liquid ECM components (e.g., collagen type I solutions, Matrigel™), we measure the viscosity and shear‑thinning behaviour using a rotational rheometer, which is important for injectable tissue engineering applications.
• Adhesion strength – cell detachment assay – We use a parallel‑plate flow chamber or a spinning disk device to apply a defined shear stress (0.1‑10 Pa) to adherent cells and measure the fraction of cells that detach as a function of time and shear stress. The critical shear stress (τcrit) is calculated as the value at which 50 % of cells detach, providing a quantitative measure of cell‑substrate adhesion.

Biomechanical Responses to Stimuli – Mechanical and Biochemical Modulation

• Drug‑induced stiffness changes – We measure the change in cell stiffness (AFM or microfluidic) before and after exposure to candidate drugs (e.g., chemotherapeutic agents, cytoskeletal modulators). A significant change in stiffness can be used as a biomarker for drug efficacy and as a method for screening potential drug candidates.
• Temperature‑dependent mechanical properties – ISO 3386 (adapted) – We measure the viscoelastic properties (storage and loss moduli) of cells at physiological (37 °C) and sub‑physiological (room temperature, 4 °C) temperatures to assess the temperature sensitivity of the cytoskeletal network.
• Osmotic swelling and volume regulation – Using a micro‑perfusion system or a micro‑pressure probe, we change the osmolarity of the medium and measure the volume and pressure changes in single cells. This is important for studying cell volume regulation (regulatory volume decrease/increase) and for understanding water transport through channels (aquaporins).
• Mechanical fatigue of tissue constructs – cyclic compression (ISO 14817 adapted) – We apply a cyclic compressive load (0.5‑2 Hz, 10‑100 cycles) to engineered cartilage or bone constructs to simulate physiological loading and measure the change in modulus and deformation after cycling, assessing the construct’s fatigue resistance.
• Cell migration and invasion – mechanical constraints – We measure the migration speed and persistence (from time‑lapse microscopy) of cells through micro‑channels of defined width (3‑20 µm) and assess how changes in cell deformability affect invasion and metastasis potential.

Data Analysis and Modelling – Extracting Mechanical Parameters

We provide advanced data analysis and modelling to convert raw force‑displacement or deformation data into meaningful mechanical parameters.

• Contact mechanics models – Hertz, JKR (Johnson‑Kendall‑Roberts), Sneddon, and Oliver‑Pharr models are applied to AFM indentation curves to extract the Young’s modulus, adhesion energy, and hardness.
• Viscoelastic models – We fit stress‑relaxation and creep curves to the standard linear solid (SLS) model, the power‑law model (the structural damping model), or the Maxwell‑Wiechert model to derive the instantaneous modulus, the relaxed modulus, and the relaxation time spectrum.
• Finite‑element modelling (FEM) – For complex geometries (such as spheroids, organoids, and cell‑laden hydrogels), we develop FEM models (using Abaqus or COMSOL) to simulate the experimental deformation and validate the derived material parameters.
• Machine learning for high‑throughput data – For microfluidic deformation data, we apply clustering and classification algorithms (e.g., random forest, support vector machines) to identify cell populations with distinct mechanical signatures, enabling rapid phenotypic screening.

Quality Control, Reference Materials and Standardisation

To ensure the reliability and reproducibility of our testing, we adhere to rigorous internal and external quality control measures.

• Calibration of AFM cantilevers – using the thermal noise method and the reference cantilever method, with traceability to SI units.
• Reference cell lines – we maintain a panel of reference cell lines with known mechanical properties (e.g., from the literature or from previous in‑house characterisation) to validate the performance of the instruments and protocols.
• Standardised test substrates – we use defined hydrogel formulations with known stiffness (e.g., polyacrylamide gels crosslinked to produce Young’s moduli of 1, 10, and 100 kPa) to verify the consistency of indentation measurements.
• In‑house reference standards – we have developed and validated internal reference standards for calibration, including bead standards, microparticles, and cell spheroids, which allow us to monitor instrument performance over time.
• Blind testing and inter‑operator variability – we perform regular blinded replicates and inter‑operator comparisons to quantify and reduce operator‑induced variability.

Report Acceptance & Compliance with Bulgarian and EU Research Standards

All cell mechanical tests are performed under our ISO/IEC 17025 accreditation and in compliance with Good Laboratory Practice (GLP) principles, where applicable. Our final test reports include a complete description of the sample, the test method, the instrument calibration status, the raw and processed data (with statistical analysis: mean, standard deviation, coefficient of variation, confidence intervals), the derived mechanical parameters (Young’s modulus, viscoelastic constants, adhesion force, traction stresses), graphical representations (force‑distance curves, stress‑strain plots, stiffness maps), and a clear interpretation and conclusion on the mechanical phenotype of the sample. These reports are accepted by the Bulgarian Food Safety Agency (BFSA), NCIPD, and by Bulgarian and international research and regulatory bodies for project reports, peer‑reviewed publications, patent applications, and regulatory filings. Bilingual (Bulgarian/English) versions are available for international collaboration and submission.