Elastomer Microlasers for Biosensing

• Elastomer microlasers are microscopic laser resonators made from low-modulus elastomers that support whispering gallery mode lasing for sensitive biosensing.
• They are fabricated via microfluidic techniques to produce uniform, monodisperse beads with tunable sizes and high throughput.
• These devices integrate soft polymer mechanics with optical transduction to measure forces down to the piconewton scale in biological environments.

Elastomer microlasers are microscopic laser resonators composed of highly deformable polymer materials, engineered to serve as sensitive biosensors for mechanical forces in cellular and tissue environments. Their defining characteristic is the integration of a low-modulus elastomer matrix, doped with an optical gain medium, into a monodisperse spherical microresonator geometry that supports whispering gallery mode (WGM) lasing. These systems facilitate direct, quantitative optical readout of mechanical deformation in physiological settings, leveraging the unique intersection of soft condensed matter physics, microfluidics, polymer chemistry, and photonics (Bayrak et al., 27 Dec 2025).

1. Materials and Polymer Design

The elastomer system utilizes a commercially available two-component silicone gel (Nusil LS1-3252), with a 1 : 1 weight ratio mixture of components A and B. Curing proceeds via a platinum-catalyzed hydrosilylation reaction yielding a transparent crosslinked elastomer distinguished by refractive index $n \approx 1.52$. The manufacturer’s durometer ("00–25") corresponds to a bulk elastic modulus $Y$ in the range of $10$–$100$ kPa. Atomic force microscopy (AFM) nanoindentation on single microbeads (diameter 7–17 μm) yields $Y = 5$–$15$ kPa, with an average near 10 kPa. This modulus is approximately an order of magnitude higher than oil-droplet WGM sensors (effective stiffness $\ll 1$ kPa) but vastly lower than conventional polymer or glass (GPa-scale stiffness), striking a balance between deformability and mechanical stability for biological force detection.

A fluorescent organic dye, specifically a coumarin derivative (C545T), is dissolved in component B prior to curing, ensuring homogenous distribution post-polymerization. This forms a uniform gain medium, with the resultant beads demonstrating both biocompatibility and optical transparency essential for in situ biosensing.

2. Microfluidic Fabrication of Elastomeric Microresonators

Controlled synthesis leverages a co-flow focusing microfluidic device comprising two coaxially aligned glass capillaries (dispersed-phase inlet, 1 μm tip; continuous-phase outlet, 100 μm) affixed inside a 3D-printed PLA chamber. The dispersed phase (elastomer plus dye) and continuous phase (glycerine with a surfactant such as Tween 20 or DSPE-PEG-biotin) are flow-regulated via independent N₂ gas lines. Typical operating pressures ($P_{\mathrm{disp}}=500$ mbar, $P_{\mathrm{cont}}=200$ mbar) yield diameter $D\approx22$ μm, with bead size tunable from 4 to over 15 μm by adjusting the pressure ratio. Doubling both pressures reduces bead diameter to $\approx10$ μm, and monodispersity below 5% is achieved.

Produced droplets are collected and thermally cured at 65 °C for 95 minutes to complete crosslinking. The dye remains evenly distributed throughout the cured beads. The approach enables high-throughput production rates (~200 beads/minute), with sub-5% polydispersity, facilitating scalable fabrication for multiplexed sensing applications.

3. Whispering Gallery Mode Resonator Physics

The elastomer microbeads function as spherical dielectric WGM resonators, supporting modes defined by the resonance condition

$m\,\lambda = 2\pi n R, \qquad m \in \mathbb{N}$

where $R$ is bead radius and $n$ is refractive index. The effective mode volume ($V_{\mathrm{mode}}$) for these spheres is

$$V_{\mathrm{mode}} = \frac{\int \varepsilon(\mathbf r)\,\lvert E(\mathbf r)\rvert^2\,dV}{\max_{\mathbf r}\left[\varepsilon(\mathbf r)\,\lvert E(\mathbf r)\rvert^2\right]}$$

scaling as $V_{\mathrm{mode}} \sim \alpha(\lambda/n)^3$ with $\alpha$ of order unity.

Quality factors are defined by

$$Q = \frac{\lambda}{\Delta\lambda} = \omega\,\tau_{\mathrm{ph}}$$

where $\Delta\lambda \approx 50$ pm at $\lambda \approx 520$ nm, resulting in $Q \gtrsim 10^4$. The lasing spectrum typically manifests as multiplets of alternating TE/TM modes with a free spectral range $\Delta\lambda_{\mathrm{FSR}} \sim 1$ nm, with optical resonance peaks exceeding the amplified spontaneous emission background intensity by three orders of magnitude.

4. Lasing Behavior and Optical Characterization

The lasing threshold is characterized by a simplified pump intensity expression

$$I_{\mathrm{th}} \propto \frac{h\nu\,V_{\mathrm{mode}}}{\sigma_{\mathrm{em}}\,Q\,\tau_{\mathrm{sp}}}$$

where $h\nu$ is photon energy, $\sigma_{\mathrm{em}}$ is emission cross-section, and $\tau_{\mathrm{sp}}$ the spontaneous emission lifetime. Experimentally, single-pulse lasing threshold energies ($E_{\mathrm{th}}$) for beads of 15–20 μm diameter range from 2 nJ (for larger beads) to 11 nJ (smaller beads). The corresponding fluences are tens to several hundred $\mu$J/cm².

In spectral measurements, above-threshold WGM lasing output presents sharp lines whose positions and linewidths are sensitive to the bead’s radius and refractive perturbations. The system supports a direct mapping between external mechanical forces and the optical domain via WGM shift and linewidth modulation.

5. Mechanical Deformation and Force Responsivity

The beads’ low Young’s modulus ($Y = 5$–$15$ kPa) facilitates pronounced mechanical deformation in response to biologically relevant forces. Under uniaxial compression,

$F = Y\,A\,\frac{\Delta L}{L}$

with $F$ the force, $A$ the area of contact, $L$ the original length, and $\Delta L$ the deformation. For spherical beads compressed by, e.g., a glass bead, this manifests as a measurable oblate deformation.

The WGM modes provide a direct optical transduction of radius changes: $$\frac{\Delta\lambda}{\lambda} \approx \frac{\Delta R}{R}$$ and experimental data yield a sensitivity $\frac{\Delta\lambda}{\Delta F} \approx 20$ pm/nN. Mode linewidths broaden from 50 pm (unloaded) to approximately 200 pm at $F\approx7$ nN, increasing at a comparable rate.

Dynamic AFM-lasing experiments show reversible and nearly linear tracking of mode centers with force, with only slight viscoelastic hysteresis. The detectable force range extends up to $\approx$50 nN before modal overlap, and force resolution is limited by the mode linewidth and optical slope to $\lesssim$3 pN in principle (practical limits are $\lesssim$50 pN). This extends the measurable force range by one to two orders of magnitude compared to oil-droplet WGM and hydrogel bead sensors.

6. Biocompatibility, Cellular Integration, and Biosensing Applications

The elastomer microlasers retain structural integrity and WGM lasing capability in phosphate-buffered saline and cell culture media. Biocompatibility is established via 3T3 fibroblast assays, with no detectable change in cell morphology or viability after 24 hours in contact with the beads. Uncoated beads resist cellular adhesion, whereas lipofectamine-functionalized beads adhere to and are internalized by cells, enabling intracellular or pericellular force sensing.

The Young’s modulus closely matches that of soft tissues (1–100 kPa), corresponding to physiological forces from several piconewtons up to tens of nanonewtons. This enables elastomer microlasers to probe contractile forces in cardiac tissue, detect cell–cell and cell–matrix interactions within spheroids or organoids, and operate in optically challenging deep-tissue environments where traditional optical force sensors are limited.

7. Summary of Key Performance Metrics

The following table compares essential materials and optical properties of elastomer microlasers relative to alternative WGM or force-sensing probes:

Parameter	Elastomer Microlasers	Oil-Droplet WGM	Glass WGM / Polystyrene
Young’s Modulus ($Y$)	5–15 kPa	$\ll$1 kPa	GPa range
Size Control (radius, $R$)	4–15 μm, polydisp. <5%	Variable	Fixed
$Q$-factor	$\gtrsim$10⁴	10³–10⁴	10⁶–10⁹
Lasing Threshold	2–11 nJ	Similar or higher	Higher (stiffer, less gain)
Biocompatibility	High, stable in aqueous media	Oil–water limitations	Limited (hard, nonbiomimetic)

These attributes uniquely position elastomer-based WGM microlasers as tunable, stable, and biocompatible force sensors for multiplexed biosensing in complex cellular and tissue contexts (Bayrak et al., 27 Dec 2025).