Foodborne pathogens cause hundreds of millions of unique cases of illness and hundreds of thousands of deaths annually. In the U.S. alone, contamination leads to about 50 million cases each year, according to the Food and Drug Administration (FDA), and more than $15 billion in annual losses, according to U.S. Department of Agriculture data. Tackling this threat requires rapid, scalable diagnostics suited for both field deployment and centralized laboratories. Detecting a specific bacterial species ~0.5 µm in diameter at a concentration of just one colony-forming unit (CFU) per sample — the sensing capability required by the FDA for such species — presents substantial technical challenges. Achieving this extreme sensitivity demands sophisticated, often costly detection systems. Accurate identification is essential to prevent false positives and requires clear differentiation of the target bacterium from other microorganisms. Additionally, sample preparation must ensure uniform distribution of the target cells, prevent loss during handling, and avoid damage or contamination that could compromise analysis. Traditional culture-based enumeration methods can be reliable but slow, often requiring up to 48 h to produce results. Rapid detection alternatives, such as polymerase chain reaction (PCR) — which currently dominates the rapid foodborne pathogen detection market — and enzyme-linked immunosorbent assay (ELISA), deliver results more quickly. But these methods require specialized equipment and technical expertise. Quantifying bacteria at such low concentrations (1 CFU per sample) demands highly precise dilution and plating techniques, to ensure that measured counts reflect the true concentration in the original matrix. Meeting FDA criteria further requires rigorous method validation to confirm consistent detection at the specified sensitivity and specificity levels. This performance can be achieved only through a combination of advanced microbiological techniques, meticulous sample handling, and stringent analytical validation. Important limitations of PCR are also evident in comparison to other methods — including those that use soft-matter (liquid crystal-) based detection methods. For example, PCR amplifies targeted DNA sequences in vitro using thermostable polymerases, and the high sensitivity of this amplification process makes it prone to contamination from amplicons, leading to false positives. Furthermore, PCR is unable to distinguish between DNA from live versus dead cells and is susceptible to inhibition by biological or chemical contaminants that disrupt DNA polymerase activity. The liquid crystal detection approach, on the other hand, mitigates and even avoids several of these PCR-specific issues. It relies on pathogen-specific antibody binding followed by optical analysis. For the liquid crystal-based detection method, the target pathogen must first be enriched from the food matrix using an accredited matrix- and pathogen-specific protocol. A measured portion of the matrix is incubated in a selective culture broth designed to isolate and promote the pathogen’s growth while suppressing background flora. Enrichment is essential because naturally occurring pathogen concentrations in food are typically far too low for direct rapid detection. The goal is to grow the pathogen to a level where small-volume test samples contain sufficient cells for reliable analysis. More broadly, this soft-matter photonics-based biosensing platform exploits the birefringence, or optical anisotropy, of homeotropic liquid crystals for the detection of pathogens. Scientists from the company Crystal Diagnostics, with collaborators from Northeast Ohio Medical University and Kent State University’s Liquid Crystal Institute, developed such a method1. In addition to their ubiquity in display technologies, liquid crystals’ molecular alignment responds to thermal, electric, magnetic, mechanical, and/or chemical stimuli, producing measurable optical signals. This enables thermochromic indicators for medical, cold chain, and process monitoring; label-free detection of surfactants, toxins, and explosives; and passive wearable sensors that visualize temperature, pressure, or humidity changes. The exploitation of the optical anisotropy of the liquid crystals occurs through birefringent light caused by the pathogen complex disturbances in Crystal Diagnostics’ AccuPath product line. Then, in application, a compact polarization imaging system captures liquid crystal alignment perturbations in real time, delivering results within minutes from the scalable, deployable food safety solution (Figure 1). Figure 1. Crystal Diagnostics’ AccuPath unit is used in preparation for testing. The system is deployed in the Northeast Ohio Medical University laboratory. Courtesy of Crystal Diagnostics. Lyotropic chromonic liquid crystals Crystal Diagnostics’ optical biosensing strategy uses a variant of liquid crystals that are “lyotropic chromonic” (see Sidebar above). These liquid crystals are known to undergo reversible self-assembly in aqueous environments. The molecules stack face-to-face to form polydisperse molecular columnar aggregates, with alignment driven by concentration and solvent conditions. Unlike conventional surfactants, or surface-active agents, lyotropic chromonic liquid crystals (LCLCs) exhibit low toxicity due to their organic components. This quality, combined with their exceptional compatibility with biological systems, makes LCLCs a strong candidate for use in biosensing. Further, their concentration-dependent phase behavior enables tunable transitions from isotropic to nematic and smectic phases. And at higher concentrations or elevated temperatures, LCLCs adopt a uniform, densely packed alignment, which enhances their potential as signal amplification media in optical detection platforms. In the case of Crystal Diagnostics’ technology, the self-aligned LCLCs are disturbed by functionalized pathogen-specific antibodies that are bound to the pathogens to form a responsive matrix within microfluidic slide cells. The LCLCs spontaneously align without external fields. Antigen-antibody binding events disrupt the mesophase architecture, inducing localized anisotropy changes. These perturbations alter light transmission under polarized illumination, offering a label-free optical readout that is scalable, and that therefore supports use in high-throughput centralized laboratories as well as smaller on-site evaluation laboratories. Moreover, using a commercially available CMOS sensor, a user can capture these birefringence-based optical signatures, and the results can be processed via imaging algorithms to detect target pathogens. The method is also label-free and, as mentioned, offers compatibility with compact optical hardware. Influencing alignment LCLCs exhibit concentration- and solvent-dependent phase transitions that govern their molecular alignment. At elevated concentrations, discotic mesogens self-organize into nematic, smectic, or columnar phases, favoring homeotropic alignment, where the molecular director is oriented perpendicular to the substrate. A minimization of free energy drives this transition. Anchoring forces and intermolecular interactions within the liquid crystal matrix cause this minimization of free energy. The transition that the LCLCs undergo takes place over a short alignment period. Also, it is modulated by concentration, flow dynamics, and surface anchoring layers — for example, polyimide films — within the gap of the sample test cell. In concentration, rod-like aggregates form with long-range order and uniform alignment. This molecular arrangement induces optical anisotropy, producing polarization-dependent effects that are critical for photonic biosensing applications. Further, ordered mesogen alignment induces optical anisotropy, where refractive index and absorption vary with wavelength, propagation direction, and polarization. This principle is integral to technologies such as liquid crystal displays (LCDs), as well as the Crystal Diagnostics’ AccuPath system. Homeotropic alignment In many liquid crystal implementations, dynamic alignment driven by electric-field coupling offers real-time molecular reorientation. This is a familiar principle, enabling active modulation in displays and tunable optics, among other devices. Despite its utility for certain commercial liquid crystal devices, this coupling method incurs higher complexity and energy consumption. In contrast, surface-bound self-alignment provides a fixed optical configuration that is well suited to passive systems, including biosensors using LCLCs. In Crystal Diagnostics’ system, for example, surface-bound alignment is established through boundary interactions during sample cell manufacture and the liquid crystal’s affinity to self-align over time. Birefringence in homeotropically aligned LCLCs arises from their inherent optical anisotropy. In this alignment, the long axes of the LCLC molecules are oriented normally, or perpendicular to the substrate, creating a well-defined optical axis. As linearly polarized light travels through the medium, it encounters distinct refractive indices depending on its polarization relative to the director: Parallel polarization experiences the extraordinary index, while perpendicular polarization encounters the ordinary. This anisotropy results in a phase shift between polarization components, producing interference effects and observable depolarization. Polarizing optical microscopy reveals these phenomena via interference colors, especially when monochromatic illumination is used. Additionally, Crystal Diagnostics’ AccuPath system obtains a sensitive optical signature — similar in principle to mechanisms used in LCD technology and other optical modulation systems. The birefringence changes, induced by pathogen-bead complexes within an otherwise uniformly aligned LCLC volume, deliver this signature. Slide design The sample slide cell is manufactured by bonding two soda-lime glass substrates, each polished and coated with a polyimide alignment layer using a UV-cured adhesive embedded with glass spacers. This configuration ensures a reproducible width within the cell for the liquid crystal and pathogen bead complex solution, improving homeotropic alignment of LCLCs (Figure 2). Unlike actively aligned systems in LCD technologies, for example, LCLCs passively orient over a short time period post-injection. Alignment efficiency can be enhanced by tuning the alignment layer, cell gap, fill dynamics, illumination wavelength, and/or LCLC formulation. Figure 2. A simplified AccuPath sample slide construction and function diagram. Courtesy of Crystal Diagnostics. Upon mixing with analytes, the LCLC is introduced into the laminated sample cell, where molecular assemblies stack and align perpendicular to the alignment layer on the glass substrates. The concentration-dependent anchoring effect fixes one end of each column to the opposing coated surfaces. Bead/pathogen aggregates disturb this alignment, causing local light depolarization, which is then imaged using a CMOS sensor and imager. The imaging is performed using a monochromatic LED array, with light diffused and polarized before entering the sample. As it traverses the LCLC volume, optical disturbances from bead/pathogen complexes induce birefringence. A second crossed linear polarizer is used to analyze this birefringence, and it is then detected by a monochrome CMOS camera. Polarization and transmitted light intensity changes provide high-contrast optical signatures of pathogen-induced birefringent events, enabling sensitive and low-noise detection of the live pathogen-bound biological complexes (Figure 3). Figure 3. A simplified imager design diagram. Courtesy of Crystal Diagnostics. Results/proof of concept After the pathogen samples are enriched and mixed with aqueous LCLCs, the liquid crystals self-align in a specially constructed sample slide cell. The technique of immunomagnetic separation isolates target bacteria from the sample media to form specific bead-pathogen complexes that disturb the homeotropic alignment of the LCLC strands. These disruptions ultimately produce birefringent patterns when polarized light passes through the sample to be imaged for analysis (Figure 4). Figure 4. A process diagram — from sample preparation to image — using Crystal Diagnostics’ solution for foodborne pathogen detection. Courtesy of Crystal Diagnostics. Unlike conventional methods such as PCR, which can produce false positives, the technique enables pathogen(s) detection in active growth phases during enrichment. The detection process — from enriched samples to test sample preparation by binding, washing, and mixing with liquid crystals for alignment — is automated for biosafety and efficiency. It requires minimal user input and enables high-throughput user analysis (Figure 5). Figure 5. The AccuPath user interface delivers clear positive or negative test results from images taken of each sample. Courtesy of Crystal Diagnostics. The detection sensitivity depends on the size of bead-pathogen aggregates, which must sufficiently deform the LCLC columnar alignment to exceed the background birefringence of the aligned LCLC. Optimization of alignment conditions — such as polyimide layer chemistry, cell spacing, and sample infusion parameters, among others — helps to reduce phase transition time and enhances signal contrast. Future refinements, targeting improved performance, may include simplified capture and amplification protocols that heighten aggregate-induced deformation while suppressing nonspecific receptor pairing. They may also include the resolution of smaller bead complexes via optimization of the LCLC alignment and sample cell design. Reference 1. G. Lundeen (March 2025). Method for rapid foodborne pathogen detection with liquid crystal materials proceedings. Proc SPIE, Vol. 13387, OPTO 2025, Emerging Liquid Crystal Technologies XX, Article No. 133870A, San Francisco, www.doi.org/10.1117/12.3043787. Liquid Crystal and Soft Matter Photonics Fundamentals Discotic mesogens are liquid crystal compounds distinguished by rigid, disc-shaped cores that self-assemble into columnar structures. They display characteristic liquid crystalline behavior and can exist in various phases, including nematic, smectic, and columnar. Their architecture combines rigid and flexible segments, enabling them to exhibit both fluid-like and solid-like properties. The nematic phase is a liquid crystal state in which molecules align along with a common orientation without exhibiting positional order. In the smectic phase, liquid crystals exhibit both orientational and positional order. This unique arrangement underpins their importance in numerous technologies, most notably in display applications. Lyotropic chromonic liquid crystals (LCLCs) consist of molecules with rigid polyaromatic cores and peripheral ionic groups that aggregate in aqueous environments. LCLCs exhibit phase transitions as a function of both temperature and the concentration of molecules in a solvent, typically water. Most LCLCs are nontoxic to biological cells, making them effective as signal-amplifying media in real-time biosensors. The term “chromonic” — a shorthand for phrases such as “lyotropic mesophase formed by soluble aromatic mesogens” — was also intended to evoke associations with dyes. In chemistry and chemical physics, a mesophase, or mesomorphic phase, refers to a state of matter intermediate between solid and liquid. In liquid crystals, homeotropic alignment refers to the molecular arrangement in which rod-like liquid crystal molecules orient perpendicular to the substrate surface. It is one of several distinct alignment modes observed in liquid crystalline systems. Linearly polarized light waves oscillate in a single plane, and the magnitude of the electric field vector varies sinusoidally as a function of time. Immunomagnetic separation (IMS) is a powerful laboratory technique used to isolate specific cells, proteins, or nucleic acids from complex mixtures using magnetic particles coated with antibodies.