The review, by Shubham Gupta and Varatharajan Prasannavenkadesan of Queen Mary University of London, synthesises studies published between January 2000 and July 2026, drawn from PubMed, Scopus and Web of Science and selected for mechanical relevance. Rather than reviewing suture materials or clinical outcomes in isolation, the authors organise the literature around three tribological interfaces: needle against tissue, suture against tissue, and suture against suture. Their central claim is that low resistance is desirable during penetration and pull-through, controlled sliding is needed while the knot is advanced, and substantial strand-to-strand friction becomes essential once the knot is tightened and must resist slippage.

The needle stage begins before the tissue surface is even broken. Initial contact produces local indentation and viscoelastic deformation, so the reaction force climbs until a critical puncture point is reached. Experimental work on sheep skin and tendon has shown a characteristic force build-up, with skin producing two peaks corresponding to entry through the upper surface and emergence from the underside. Increasing tissue pre-tension raises penetration resistance, while the effect of insertion speed depends strongly on tissue type: varying speed from 1 to 10 millimetres per second had no significant effect in skin, yet faster insertion reduced the force and time needed to puncture liver. Once the needle has passed through, the measured load reflects several mechanisms at once, including cutting at the tip, friction along the embedded shaft and tissue compression generated when the needle deviates from its natural curvature. The review is careful to distinguish total insertion force from friction force proper, noting that the two are often conflated in the literature.

Geometry and trajectory matter enormously. Larger needles and triangular cutting tips produce greater puncture forces than smaller, round-bodied needles, and perpendicular entry gives the lowest resistance among tested angles. Skin and muscle resist insertion more strongly than fat, and vascular structures introduce marked force fluctuations. Perhaps most strikingly, experiments with passive curved needles showed that following the needle’s arc during insertion, rather than advancing linearly, reduced average axial force by 45.87 percent, transverse force by 55.76 percent and bending moment by 57.36 percent in tissue-mimicking gel. Robotic trajectory planning that follows the needle arc and minimises unnecessary tissue displacement, combined with state estimation of deformation and friction forces during passage, offers a route to controlling these interactions in real time.

Once the needle has created a passage, the thread slides against a newly damaged, deformable surface, and the resulting drag includes adhesion, viscoelastic deformation around the suture and, for rough multifilament threads, local ploughing and abrasion. The experimental evidence overturns a common intuition that smoother always means lower friction. In capstan tests, nylon monofilament with a surface roughness of about 4.25 micrometres produced a higher apparent coefficient of friction than coated Vicryl despite being smoother, because smoother monofilaments increase the real contact area with flat substrates and thus adhesive friction. Yet in pull-through testing through a skin substitute, nylon produced a mean drag force of 0.25 newtons against 0.42 newtons for polyglycolic acid multifilament of comparable size, a reduction of roughly 40 percent. Multifilament architecture becomes particularly punishing when individual strands can engage, compress or scrape the walls of the tissue channel.

Diameter, hydration and operating conditions further modulate drag. When polyglycolic acid multifilament diameter increased from 0.19 to 0.55 millimetres, pull-through force rose from roughly 0.35 to 1.15 newtons, although friction did not increase proportionally with the estimated normal force. Increasing normal load from 0.1 to 0.5 newtons reduced the coefficient of friction by 36 percent in one suture-artificial skin pair, while raising sliding velocity from 5 to 25 millimetres per second increased it by 26 percent. Hydration effects are contact-dependent and sometimes counterintuitive: blood and interstitial fluid in porcine tissues reduced adhesion relative to dry artificial skin, whereas fetal bovine serum increased the apparent coefficient for Vicryl from 0.40 to 0.77, raising friction by 88.9 percent for that material while simultaneously damping stick-slip fluctuations. Damage patterns follow the same logic, with polyglycolic acid producing a larger damaged area at the entry boundary than at the exit and retaining more tissue debris than nylon.

Surface engineering offers ways to tune these interfaces, but no single strategy wins everywhere. A plasma-polymerised aminosilane coating applied to curved suture needles lowered penetration force by up to 35 percent compared with conventional silicone-coated controls and remained stable during repeated insertions. A polydopamine-PTFE-activated-carbon coating reduced maximum insertion and extraction forces by 62 and 64 percent in a PVC phantom and by 49 and 30 percent in bovine kidney, with atomic-force microscopy indicating a 65.7 percent reduction in surface friction. For threads, chitosan coating reduced the dynamic coefficient of friction of braided silk while modestly improving tenacity and knot strength. By contrast, dopamine and graphene oxide coatings on polyglycolic acid sutures changed friction very little, with coefficients of 1.37 and 1.42 against 1.38 untreated, even as they improved coating uniformity, wettability and mechanical strength. Antimicrobial treatments can also shift handling: antibacterial processing of braided silk changed suture-suture friction by up to 32.8 percent and increased tissue-drag resistance and bending stiffness by roughly 50 percent.

The knot stage inverts the design goal entirely. A surgical knot does not become secure merely because strands touch; tightening generates normal pressure where filaments cross, allowing suture-suture friction to resist sliding. In polypropylene monofilaments, a dynamic friction coefficient of 0.20 was measured, and knot strength followed a power-law dependence on pre-tension with an exponent of about 1.56. Recent work on general filament mechanics shows that contact pressure within tight knots is strongly localised along curved contact regions, that tension drops sharply at self-contact or nip points where one filament pinches another, and that increased sliding strength with tying tension arises mainly from the simultaneous growth of normal contact force and contact length rather than from plastic deformation. Clinical data reinforce the material dependence: No. 5 FiberWire had the highest failure load at 625 newtons but also the greatest slippage at 14 millimetres, while No. 2 Ethibond slipped only 0.1 millimetres despite lower tensile capacity, and monofilament knots generally require at least five throws with blocking half-hitches.

The review also maps how these interactions are measured and modelled. Capstan and linear-sliding tests provide controlled comparisons of suture surfaces, while the penetration friction apparatus measures needle puncture and suture pull-through sequentially as the skin substitute deforms around the moving thread. Force platforms reveal that novices generate maximum forces of about 4.7 newtons in laparoscopic suturing against 2.6 newtons for experienced surgeons, and that technique matters: closing a standardised 5-newton wound required mean maximum pulling forces ranging from 3.46 newtons for a pulley suture to 8.11 newtons for a horizontal mattress. Finite-element models, from multiscale tendon simulations to spring-based representations of individual sutures on fixed grids, expose contact stresses that are difficult to measure directly. Robotic force feedback has already shown measurable benefits, reducing median maximum force from 3.4 to 2.8 newtons without degrading suturing scores, and lowering mean applied force from 1.53 to 1.04 newtons in robotic interrupted suturing.

The authors’ conclusions point toward a discipline-wide shift. No clinically validated optimal friction range currently exists, because reported values depend on material, surface structure, hydration, tightening force and knot configuration and are not expressed in comparable quantities. The priorities they identify are concrete: standardised wet-tissue testing protocols that specify hydration, pre-tension, insertion angle, pulling velocity and repeated passages; clearer experimental separation of cutting, deformation and friction components; repeated-passage evaluation of coating durability and possible material transfer to tissue; tissue-specific needle and suture selection informed by widely varying damage thresholds, which range from about 1.25 newtons in fallopian tube to 11.43 newtons in fascia; and adaptive robotic control that estimates friction, cutting and deformation separately and adjusts trajectory, speed and grip during each stitch. Friction, in this view, is not an enemy to be eliminated but a resource to be scheduled, spent sparingly during passage and banked heavily at the knot.

Subject of Research: Tribology and interfacial mechanics of surgical suturing, including needle-tissue penetration, suture drag, coatings and knot security

Article Title: Friction and interfacial mechanics of surgical suturing: needle–tissue penetration, suture drag and friction-control strategies

Article References: Gupta, S., & Prasannavenkadesan, V. (2026). Friction and interfacial mechanics of surgical suturing: needle–tissue penetration, suture drag and friction-control strategies. Frontiers in Mechanical Engineering, 12, Article 1963772. https://doi.org/10.3389/fmech.2026.1963772

Image Credits: AI Generated

DOI: 10.3389/fmech.2026.1963772

Keywords: surgical suturing, biotribology, needle-tissue interaction, suture drag, knot security, friction control, surface coatings, medical robotics, finite element modelling, wound closure, suture materials, force feedback

The research, published in Frontiers in Medical Engineering by Eve Sobirey of Hamburg University of Technology and colleagues at the University Medical Center Hamburg-Eppendorf, set out to quantify how different surface coatings change the friction behavior of PolyJet-printed vascular models. The goal was deliberately modest and practical: not to replicate the exact tribology of living brain vessels, for which no quantitative reference values exist, but to bring the unrealistically high catheter advancement forces of printed models down to a range that feels plausible to experienced hands. The work was carried out in the context of the HANNES simulator, a Hamburg-developed training platform that replicates the arterial path from the groin to the head and runs real interventional instruments under pulsatile flow at physiological pressure.

The team’s earlier work had established that PolyJet printing outperforms alternatives such as stereolithography when it comes to keeping tiny vessel lumens open and geometrically faithful. Stereolithography models suffered from unreliable patency in small diameters, while PolyJet models of the middle meningeal artery reproduced complex vascular geometries with open channels. But when interventional physicians handled the PolyJet models, they judged them significantly too rough, and the resulting friction undermined the realism of catheter navigation. Previous attempts at friction-reducing coatings elsewhere had either been applied to larger vessels or evaluated only in silicone models, without objective force measurements or validation by experienced users.

For the new study, the researchers systematically tested eight coating systems drawn from three functional families. Elastic coatings such as Plasti Dip, a solvent-based synthetic rubber, and RT 601, an addition-curing silicone elastomer, work by smoothing the printed layer structure and softening the contact between catheter and wall. Friction-reducing polymer coatings, including Hard Coat Finish and the polyurethane-forming Aquasure, create continuous smooth layers that minimize surface asperities. Silicone oil forms a lubricating boundary film, Teflon spray deposits a solid low-surface-energy sliding layer of PTFE, and the hydrophilic system X-tec HP3066 retains water at the surface to form a hydrated boundary layer that prevents direct solid-on-solid contact. The team also tested three blood-mimicking fluids: tap water, a water-glycerin mixture, and water with a small amount of baby shampoo, whose surfactants lower surface tension and improve wettability.

Measuring friction inside a two-millimeter channel under clinically meaningful conditions required a purpose-built test bench. A cylindrical test specimen, abstracted from the anatomy of the foramen spinosum of the middle meningeal artery, was connected to the HANNES fluid system and continuously flushed with blood-mimicking fluid at 37 degrees Celsius. A guidewire clamped in a universal testing machine was cycled back and forth through a stationary catheter at a constant one millimeter per second, with a force sensor of plus or minus 0.01 newton accuracy recording the forces generated by the interaction of wire, catheter, fluid, and vessel wall. Twenty cycles served to compare coatings, and a separate sixty-cycle series assessed short-term stability under repeated loading.

The results were striking. The uncoated reference specimen showed a median catheter advancement force of 0.18 newtons in water, with force peaks reaching 0.37 newtons. Every one of the eight coatings reduced advancement forces, but by very different amounts. In water, Plasti Dip brought the median down to 0.04 newtons and Teflon spray to 0.05 newtons; in baby shampoo solution, Aquasure achieved the lowest median of all at 0.04 newtons. Surface roughness measurements told a parallel story at first glance: the uncoated surface had an arithmetic mean roughness of 11.5 micrometers, while RT 601 polished it down to a remarkable 0.2 micrometers and Aquasure and Plasti Dip also achieved large reductions. Yet the force data revealed that roughness alone could not explain handling performance, a finding that became the intellectual pivot of the study.

The decisive insight came from analyzing the entire force path rather than single summary numbers. Teflon spray and Plasti Dip produced smooth, uniform force curves with few peaks, whereas Aquasure, despite its equally low median force, showed pronounced local fluctuations that a clinician’s fingertips would perceive as irregularity. Micro-computed tomography added a third dimension to the evaluation: X-tec preserved the largest free lumen diameter, while Teflon spray achieved the most homogeneous coating with the lowest standard deviation. At the other extreme, RT 601 narrowed the channel most severely and deposited uneven material accumulations that appeared as local hotspots in the diameter heat maps. A coating can thus lower friction while silently distorting the very anatomy the model is meant to reproduce.

Short-term stability testing exposed further nuances. Plasti Dip, Hard Coat Finish, and Aquasure were the most stable, with median forces rising by only 0.01 newtons over sixty cycles. Teflon spray, although it started with the lowest force of all at 0.02 newtons, showed an abrupt increase after roughly forty cycles, jumping by 0.03 newtons overall. The authors interpret this sudden change as more likely reflecting localized coating degradation or partial loss than gradual wear, though the mechanism remains unproven and is flagged as a priority for future investigation. In a training context, where a single model may serve many repetitions, such durability questions matter as much as peak performance.

The final arbiter was clinical judgment. Two interventional neuroradiologists with ten and thirteen years of experience evaluated the coated models inside the full HANNES simulator, navigating a Marathon microcatheter and Traxcess guidewire transfemorally through the vascular tree under standardized pulsatile conditions, and rating each model on a five-point Likert scale. The uncoated model scored poorly, with both physicians reporting increased resistance and frequent, noticeable stick-slip effects. Aquasure and Teflon spray received consistently high marks of four to five points across feel, probing, friction, and resistance realism, with stick-slip effects rated rare or barely noticeable. Plasti Dip was let down not by its friction but by occluded distal vessels, and X-tec divided the evaluators over unrealistically high proximal resistance.

The study’s overall verdict is that choosing a coating is an exercise in balancing tribological performance, geometric fidelity, ease of application, and cost. Teflon spray emerged as the best all-round option: applied in seconds with no pretreatment, inexpensive in practice, homogeneous on micro-CT, and endorsed by both clinicians, though its durability beyond about forty cycles needs work. Aquasure achieved the lowest forces and unanimous clinical approval but demands a substantially more complex procedure involving curing agents and careful handling. The authors caution that their results are exploratory, based on single samples per coating and two physician evaluators, and call for larger multicenter validation, testing across different advancement speeds and printed materials, and systematic study of long-term coating behavior. Even so, the message is clear: a targeted coat of the right chemistry can close the gap between a printed plastic tube and the feel of a living artery, bringing risk-free surgical simulation one realistic step closer.

Subject of Research: Surface coatings to reduce catheter friction in PolyJet 3D-printed small vascular models for neurointerventional training

Article Title: Reducing catheter advancement forces in PolyJet printed small vascular models for neurointerventional training through surface coatings

Article References: Sobirey, E., Schmiech, J., Bechstein, M., Flottmann, F., Wagner, M., Fiehler, J., & Krause, D. (2026). Reducing catheter advancement forces in PolyJet printed small vascular models for neurointerventional training through surface coatings. Frontiers in Medical Engineering, 4, Article 1925336. https://doi.org/10.3389/fmede.2026.1925336

Image Credits: AI Generated

DOI: 10.3389/fmede.2026.1925336

Keywords: PolyJet printing, neurointerventional training, vascular models, surface coatings, tribology, catheter forces, 3D printing, middle meningeal artery, micro-CT, Teflon spray, surgical simulation, friction reduction

The review followed the PRISMA-ScR reporting guidelines for scoping reviews, searching five databases between January 2010 and February 2026 for studies that exposed freshwater fish to silver nanoparticles through water and compared at least two surface conditions under equivalent circumstances. Twenty-one studies met those strict criteria, and together they paint a picture of a field dominated by a handful of model organisms. Zebrafish, Danio rerio, appears in virtually every endpoint category examined, from lethality and malformations to oxidative stress, DNA damage, neurobehavioral changes, histopathology and gene expression. Japanese medaka, Oryzias latipes, is the second most represented species, while rainbow trout and a few neotropical fish round out the dataset. Across all of these species, oxidative stress and silver bioaccumulation were the most broadly assessed endpoints, while genotoxicity and behavioral effects remained largely confined to the two main laboratory models.

To understand why surface chemistry matters so much, it helps to grasp what happens to a silver nanoparticle the moment it enters freshwater. Unlike seawater, where high ionic strength screens the electrostatic repulsion between particles and triggers almost instant agglomeration, freshwater allows nanosilver to remain colloidally stable for longer periods. That stability keeps the particles suspended in the water column where pelagic fish encounter them, but it also means the particles continue to oxidize and release silver ions, the single most important driver of nanosilver toxicity. Over time, agglomeration and sedimentation progressively shift the particles toward the benthic compartment, transferring the exposure burden from open-water fish to bottom-dwelling organisms. Temperature, fluctuating water levels and algal exudates have all been shown to accelerate ion release, while sulfidation and chlorination in natural waters lock silver into far less soluble compounds such as silver sulfide and silver chloride.

Manufacturers fight this instability by wrapping particles in capping agents, and the choice of wrapper turns out to be a decisive toxicological variable. The most common coatings in chemically synthesized nanosilver are citrate, polyvinylpyrrolidone, polyethylene glycol, gum arabic, silicate and polyethyleneimine, with citrate, PVP and PEI dominating the fish literature. The comparative evidence reveals a striking lack of consistency about which coating is worst. In zebrafish embryos, one study of five differently stabilized formulations concluded that the toxicity mechanism depends entirely on the stabilizer: in some cases the coating itself was the poison, in others the nanoparticle, and in others a combination of both. Another zebrafish study found that adding a coating actually increased toxicity compared with naked particles, because a positively charged hydroxyethylcellulose shell reacted more strongly with the negatively charged chorion surrounding the embryo. A third investigation of fourteen different preparations ranked PVP-coated particles as the most toxic, largely because PVP and thiol coatings released the most silver ions.

Rainbow trout told a different story. When researchers compared citrate, branched PEI, PVP and silicate coatings at an environmentally realistic concentration of 50 micrograms per liter, PVP proved the most bioavailable but not the most toxic. Instead, citrate and branched PEI, both charged coatings, produced the greatest genotoxicity and inflammation, reinforcing surface charge as a primary driver of biological damage. In Japanese medaka, gum arabic-coated particles were the most lethal despite releasing little ionic silver, pointing to a genuinely particle-specific toxicity pathway. A follow-up study with ten formulations confirmed gum arabic as the most dangerous coating and traced the mechanism to disruption of sodium regulation through inhibition of the Na+/K+-ATPase pump at the ionocytes of the gill epithelium, rather than to oxidative stress. The same coating can therefore occupy entirely different positions in a toxicity ranking depending on the species, life stage and endpoint being measured.

Particle size compounds the picture. In adult zebrafish exposed to citrate- or PVP-coated particles of 20 and 100 nanometers, size contributed more to toxicity than coating type, with smaller particles consistently more damaging. Yet even here the coating mattered: at equivalent sizes, citrate-coated particles outperformed PVP-coated ones across oxidative stress, DNA damage and apoptosis endpoints, even though PVP-exposed fish accumulated more silver in their tissues. That dissociation between internalized silver and harm is one of the review’s most important insights, because it demonstrates that total tissue burden is a poor proxy for toxicological risk. What matters is not how much silver enters the fish but in what chemical form, released where, and at what rate, a conclusion that challenges the assumptions underlying many standard hazard assessments.

Once nanosilver enters a natural water body, its synthetic coating is almost immediately overlaid by a second layer of adsorbed molecules, the eco-corona, composed of humic and fulvic substances, proteins, carbohydrates and extracellular polymeric substances. This environmental corona resets the particle’s biological interface, typically increasing its hydrodynamic diameter, rendering its surface more negative and blocking the oxidation sites where silver ions are generated. The review found that natural organic matter, especially humic acid, consistently mitigates nanosilver toxicity in fish. In zebrafish larvae, humic acid reduced mortality in a concentration-dependent manner by acting as a barrier to ion release. In adult zebrafish, humic acid actually increased dissolved silver over time by complexing ions through its carboxylic groups, yet mortality and behavioral impairments were still alleviated, showing that toxicity mechanisms extend well beyond simple ion release. An ex vivo gill model in two neotropical species confirmed reduced bioaccumulation, oxidative stress and ion release in the presence of humic acids, extending the protective effect beyond standard laboratory models.

Environmental transformations over time add further layers of complexity. Aged silver nanoparticles proved significantly more toxic than freshly prepared ones in medaka embryos, because ten days of aging increased ionic silver release in linear proportion to mortality. Humic acid counteracted this effect, but only when it had been pre-incubated with the particles, highlighting that contact time is essential for eco-corona formation and its protective action. Sulfidation, the natural conversion of particle surfaces to silver sulfide, acted as what one research team called a natural antidote: mortality in zebrafish embryos decreased progressively with increasing sulfidation levels, and sulfidated particles alleviated hepatic oxidative stress and brain acetylcholinesterase suppression in adult fish. Even the absence of a coating proved informative, as uncoated particles aggregated on the outer chorion surface without penetrating the embryo, yet still caused harm through ions generated at the chorion surface, confirming that the embryonic envelope functions as a filter for particles but not for dissolved silver.

The review closes with a candid assessment of the field’s blind spots. Most studies employed concentrations in the milligram-per-liter range, far above reported environmental levels, and only one included study used a genuinely environmentally relevant concentration of 20 micrograms per liter. The assumption that green-synthesized nanoparticles, made with plant extracts as reducing and capping agents, are inherently safer was directly contradicted by a zebrafish study in which green-synthesized particles were markedly more toxic than PVP-coated counterparts, releasing more ionic silver despite similar sizes, while a comparison in silver carp was confounded by lead contamination in the chemical reference material. Taxonomic coverage remains narrow, dominated by zebrafish and medaka, and endpoints skew toward embryo lethality rather than reproduction, neurobehavior or histopathology. The author argues that hazard assessments based solely on pristine, as-synthesized nanoparticles likely overestimate environmental risk, and that future experiments must track surface transformations dynamically, use realistic exposure levels and broaden both species and endpoints if protective water quality criteria for this emerging nanopollutant are to rest on solid ground.

Subject of Research: How surface coatings, eco-corona formation and environmental transformations modulate silver nanoparticle toxicity in freshwater fish

Article Title: Surface coatings and eco-corona as modulators of silver nanoparticle toxicity in freshwater fish

Article References: Ale, A. (2026). Surface coatings and eco-corona as modulators of silver nanoparticle toxicity in freshwater fish. Discover Toxicology, 3(1), Article 14. https://doi.org/10.1007/s44339-026-00060-2

Image Credits: AI Generated

DOI: 10.1007/s44339-026-00060-2

Keywords: silver nanoparticles, nanotoxicology, freshwater fish, eco-corona, surface coatings, zebrafish, humic acid, sulfidation, oxidative stress, silver ion release, green synthesis, ecotoxicology