Understanding interfacial fracture behavior between microinterlocked soft layers using physics-based cohesive zone modeling

BioTrojans: viscoelastic microvalve-based attacks in flow-based microfluidic biochips and their countermeasures

Flow-based microfluidic biochips (FMBs) are widely used in biomedical research and diagnostics. However, their security against potential material-level cyber-physical attacks remains inadequately explored, posing a significant future challenge. One of the main components, polydimethylsiloxane (PDMS) microvalves, is pivotal to FMBs' functionality. However, their fabrication, which involves thermal curing, makes them susceptible to chemical tampering-induced material degradation attacks. Here, we demonstrate one such material-based attack termed "BioTrojans," which are chemically tampered and optically stealthy microvalves that can be ruptured through low-frequency actuations. To chemically tamper with the microvalves, we altered the associated PDMS curing ratio. Attack demonstrations showed that BioTrojan valves with 30:1 and 50:1 curing ratios ruptured quickly under 2 Hz frequency actuations, while authentic microvalves with a 10:1 ratio remained intact even after being actuated at the same frequency for 2 days (345,600 cycles). Dynamic mechanical analyzer (DMA) results and associated finite element analysis revealed that a BioTrojan valve stores three orders of magnitude more mechanical energy than the authentic one, making it highly susceptible to low-frequency-induced ruptures. To counter BioTrojan attacks, we propose a security-by-design approach using smooth peripheral fillets to reduce stress concentration by over 50% and a spectral authentication method using fluorescent microvalves capable of effectively detecting BioTrojans.

Material-level countermeasures for securing microfluidic biochips

Flow-based microfluidic biochips (FMBs) have been rapidly commercialized and deployed in recent years for biological computing, clinical diagnostics, and point-of-care-tests (POCTs). However, outsourcing FMBs makes them susceptible to material-level attacks by malicious actors for illegitimate monetary gain. The attacks involve deliberate material degradation of an FMB's polydimethylsiloxane (PDMS) components by either doping with reactive solvents or altering the PDMS curing ratio during fabrication. Such attacks are stealthy enough to evade detection and deteriorate the FMB's function. Furthermore, material-level attacks can become prevalent in attacks based on intellectual property (IP) theft, such as counterfeiting, overbuilding, etc., which involve unscrupulous third-party manufacturers. To address this problem, we present a dynamic material-level watermarking scheme for PDMS-based FMBs with microvalves using a perylene-labeled fluorescent dye. The dyed microvalves show a unique excimer intensity peak under 405 nm laser excitation. Moreover, when pneumatically actuated, the peak shows a predetermined downward shift in intensity as a function of mechanical strain. We validated this protection scheme experimentally using fluorescence microscopy, which showed a high correlation (R2 = 0.971) between the normalized excimer intensity change and the maximum principal strain of the actuated microvalves. To detect curing ratio-based attacks, we adapted machine learning (ML) models, which were trained on the force-displacement data obtained from a mechanical punch test method. Our ML models achieved more than 99% accuracy in detecting curing ratio anomalies. These countermeasures can be used to proactively safeguard FMBs against material-level attacks in the era of global pandemics and diagnostics based on POCTs.

Propagating Schallamach-type waves resemble interface cracks

Intermittent motion, called stick-slip, is a friction instability that commonly occurs during relative sliding of two elastic solids. In adhesive polymer contacts, where elasticity and interface adhesion are strongly coupled, stick-slip arises due to the propagation of slow detachment waves at the interface. Here we analyze two distinct detachment waves moving parallel (Schallamach wave) and antiparallel (separation wave) to applied remote sliding. Both waves cause slip in the same direction, travel at speeds much lesser than any elastic wave speed, and are therefore describable using the same perturbative elastodynamic framework with identical boundary conditions. A numerical scheme is used to obtain interface stresses and velocities for arbitrary Poisson ratio, along with closed-form solutions for incompressible solids. Our calculations reveal a close correspondence between moving detachment waves and bimaterial interface cracks, including the nature of the singularity and the functional forms of the stresses. Based on this correspondence, and coupled with a fracture analogy for dynamic friction, we develop a phase diagram showing domains of possible occurrence of stick-slip via detachment waves vis-á-vis steady interface sliding. Our results have interesting implications for sliding and stick-slip phenomena at soft interfaces.

Biomimicking interfacial fracture behavior of lizard tail autotomy with soft microinterlocking structures

Biological soft interfaces often exhibit complex microscale interlocking geometries to ensure sturdy and flexible connections. If needed, the interlocking can rapidly be released on demand leading to an abrupt decrease of interfacial adhesion. Here, inspired by lizard tail autotomy where such apparently tunable interfacial fracture behavior can be observed, we hypothesized an interlocking mechanism between the tail and body based on the muscle-actuated mushroom-shaped microinterlocks along the fracture planes. To mimic the fracture behavior of the lizard tail, we developed a soft bilayer patch that consisted of a dense array of soft hemispherical microstructures in the upper layer acting as mechanical interlocks with the counter body part. The bottom control layer contained a microchannel that allowed to deflect the upper layer when applying the negative pressure, thus mimicking muscle contraction. In the microinterlocked condition, the biomimetic tail demonstrated a 2.7-fold and a three-fold increase in adhesion strength and toughness, respectively, compared to the pneumatically released microinterlocks. Furthermore, as per the computational analysis, the subsurface microchannel in the control layer enabled augmented adhesion by rendering the interface more compliant as a dissipative matrix, decreasing contact opening and strain energy dissipation by 50%. The contrasting features between the microinterlocked and released cases demonstrated a highly tunable adhesion of our biomimetic soft patch. The potential applications of our study are expected in soft robotics and prosthetics.

Biomimetic fracture model of lizard tail autotomy

Lizard tail autotomy is an antipredator strategy consisting of sturdy attachment at regular times but quick detachment during need. We propose a biomimetic fracture model of lizard tail autotomy using multiscale hierarchical structures. The structures consist of uniformly distributed micropillars with nanoporous tops, which recapitulate the high-density mushroom-shaped microstructures found on the lizard tail's muscle fracture plane. The biomimetic experiments showed adhesion enhancement when combining nanoporous interfacial surfaces with flexible micropillars in tensile and peel modes. The fracture modeling identified micro- and nanostructure-based toughening mechanisms as the critical factor. Under wet conditions, capillarity-assisted energy dissipation pertaining to liquid-filled microgaps and nanopores further increased the adhesion performance. This research presents insights on lizard tail autotomy and provides new biomimetic ideas to solve adhesion problems.