Substrate Compliance Research Articles

A chemo-mechanical model for growth and mechanosensing of focal adhesion

Focal adhesion (FA), the complex molecular assembly across the lipid membrane, serves as a hub for physical and chemical information exchange between cells and their microenvironment. Interestingly, studies have shown that FAs can grow along the direction of contractile forces generated by actomyosin stress fibers and achieve larger sizes on stiffer substrates. In addition, the cellular traction transmitted to the substrate was observed to reach the maximum near the FA center. However, the biomechanical mechanisms behind these intriguing findings remain unclear. To answer this important question, here we first developed a one-dimensional (1D) chemo-mechanical model of FA where key features like adhesion plaque deformation, active contraction by stress fibers, force-dependent association/dissociation of integrin bonds connecting two surfaces, and substrate compliance have all been considered. Within this formulation, we showed that the rigidity-sensing capability of FAs originates from the deformability of stress fibers while the force-dependent breakage of integrin bonds leads to the appearance of the traction peak at the FA center. Furthermore, by extending the model into three-dimensional as well as incorporating assembly/dis-assembly kinetics of adhesion proteins, we also demonstrated how anisotropic stress/strain field within the adhesion plaque will be induced by the presence of contractile forces which leads to the directional growth of the FA.

An investigation of the electrical dynamics in electroactive polymer transducers with resistive electrodes

To pursue a variable-capacitance working principle, transducers based on soft electroactive polymers (EAPs) need deformable electrodes that match the compliance and stretchability of the EAP polymeric substrates. A variety of manufacturing procedures are available to create conductive materials that can achieve this, including solutions that can provide remarkably low resistivity. However, the simplest and most feasible options often involve the use of particle-filled (e.g. carbon-filled) polymer composites, which, while easy to produce, tend to exhibit relatively high resistivity. This high level of resistivity, combined with the inherent capacitance of EAP transducers, introduces dynamic effects in the devices electrical activation, which may affect performance. This paper investigates the impact of electrode resistivity on the electrical dynamics of EAP devices, combining continuum models and experimental validations. We use a continuum generalisation of known resistive-capacitive (RC) transmission line models to accurately predict voltage gradients on the surfaces of electrostatic transducers subject to rapidly varying voltages. We then present an experimental validation by measuring the spatial voltage distributions over carbon-based polymeric electrodes of dielectric elastomer (DE) transducers, and find a good agreement with our model predictions. We use our validated model to provide general estimates of the typical charging time and limit working frequency ranges of DE devices as a function of their dimensional scale and electrode sheet resistance. Our model provides useful indications for designing compliant electrodes in EAP transducers given target performance, or to understand the working limits of devices with given geometry and dielectric-electrode properties.

The Influence of the Mechanical Compliance of a Substrate on the Morphology of Nanoporous Gold Thin Films.

Nanoporous gold (np-Au) has found its use in applications ranging from catalysis to biosensing, where pore morphology plays a critical role in performance. While the morphology evolution of bulk np-Au has been widely studied, knowledge about its thin-film form is limited. This work hypothesizes that the mechanical compliance of the thin film substrate can play a critical role in the morphology evolution. Via experimental and finite-element-analysis approaches, we investigate the morphological variation in np-Au thin films deposited on compliant silicone (PDMS) substrates of a range of thicknesses anchored on rigid glass supports and compare those to the morphology of np-Au deposited on glass. More macroscopic (10 s to 100 s of microns) cracks and discrete islands form in the np-Au films on PDMS compared to on glass. Conversely, uniformly distributed microscopic (100 s of nanometers) cracks form in greater numbers in the np-Au films on glass than those on PDMS, with the cracks located within the discrete islands. The np-Au films on glass also show larger ligament and pore sizes, possibly due to higher residual stresses compared to the np-Au/PDMS films. The effective elastic modulus of the substrate layers decreases with increasing PDMS thickness, resulting in secondary np-Au morphology effects, including a reduction in macroscopic crack-to-crack distance, an increase in microscopic crack coverage, and a widening of the microscopic cracks. However, changes in the ligament/pore widths with PDMS thickness are negligible, allowing for independent optimization for cracking. We expect these results to inform the integration of functional np-Au films on compliant substrates into emerging applications, including flexible electronics.

Device for Measuring Contact Reaction Forces during Animal Adhesion Landing/Takeoff from Leaf-like Compliant Substrates.

A precise measurement of animal behavior and reaction forces from their surroundings can help elucidate the fundamental principle of animal locomotion, such as landing and takeoff. Compared with stiff substrates, compliant substrates, like leaves, readily yield to loads, presenting grand challenges in measuring the reaction forces on the substrates involving compliance. To gain insight into the kinematic mechanisms and structural-functional evolution associated with arboreal animal locomotion, this study introduces an innovative device that facilitates the quantification of the reaction forces on compliant substrates, like leaves. By utilizing the stiffness-damping characteristics of servomotors and the adjustable length of a cantilever structure, the substrate compliance of the device can be accurately controlled. The substrate was further connected to a force sensor and an acceleration sensor. With the cooperation of these sensors, the measured interaction force between the animal and the compliant substrate prevented the effects of inertial force coupling. The device was calibrated under preset conditions, and its force measurement accuracy was validated, with the error between the actual measured and theoretical values being no greater than 10%. Force curves were measured, and frictional adhesion coefficients were calculated from comparative experiments on the landing/takeoff of adherent animals (tree frogs and geckos) on this device. Analysis revealed that the adhesion force limits were significantly lower than previously reported values (0.2~0.4 times those estimated in previous research). This apparatus provides mechanical evidence for elucidating structural-functional relationships exhibited by animals during locomotion and can serve as an experimental platform for optimizing the locomotion of bioinspired robots on compliant substrates.

Vertical climbing in free-ranging bonobos: An exploratory study integrating locomotor performance and substrate compliance.

Ecological factors and body size shape animal movement and adaptation. Large primates such as bonobos excel in navigating the demanding substrates of arboreal habitats. However, current approaches lack comprehensive assessment of climbing performance in free-ranging individuals, limiting our understanding of locomotor adaptations. This study aims to explore climbing performance in free-ranging bonobos and how substrate properties affect their behavior. We collected data on the climbing performance of habituated bonobos, Pan paniscus, in the Bolobo Territory, Democratic Republic of Congo. We analyzed 46 climbing bouts (12 ascents, 34 descents) while moving on vertical substrates of varying diameter and compliance levels. This study assessed the average speed, peak acceleration, resting postures, and transitions between climbing and other locomotor modes. During climbing sequences and transitions, bonobos mitigate speed variations. They also exhibit regular pauses during climbing and show higher speeds during descent in contrast to their ascent. Regarding the influence of substrate properties, bonobos exhibit higher speed when ascending on thin and slightly flexible substrates, while they appear to achieve higher speeds when descending on large and stiff substrates, by using a "fire-pole slide" submode. Bonobos demonstrate remarkable abilities for negotiating vertical substrates and substrate properties influence their performance. Our results support the idea that bonobos adopt a behavioral strategy that aligns with the notion of minimizing costs. Overall, the adoption of high velocities and the use of low-cost resting postures may reduce muscle fatigue. These aspects could represent important targets of selection to ensure ecological efficiency in bonobos.

Elasto-compliance of harmonically stimulated soft micro-gaps during electro-magneto-kinetic flows.

In this article, we develop and solve an analytical model to understand the elasto-hydrodynamic force response of a deformable, soft substrate, under dynamic loading; wherein the microfluidic gap between the substrate and load is subjected to electro-magneto-hydrodynamic interactions. As a simple physical system, we model the coupled fluid-structure-interaction characteristics when a rigid, small cylinder is permitted to impinge harmonically on an infinitely large elastic, soft substrate, and an oscillatory, squeeze flow establishes in the micro-gap formed between the two. We discuss the different observations and mechanics in terms of the governing Dukhin, Hartmann, and electroviscous numbers. The influence of electromagnetic stimuli on the flow, and its implications vis-à-vis the substrate compliance is the focus of the article. We reveal that for pure magnetohydrodynamics of the gap electrolyte, the transverse magnetic field and the induced streaming potential resist outward squeeze flow, thereby generating substantial amplifications in the force response. On the contrary, the effect of electro-magneto-hydrodynamics on the force response was strongly affected by the orientation and intensity of the transverse electric field. Notably, such variations depended significantly on the electrokinetic parameters, oscillation frequency, and substrate stiffness, whose effects were intertwined with the transverse and streaming potential electric fields. Further, possibilities of squeeze flow reversal due to opposing electromagnetic forces is observed, which may again modulate the compliance of the substrate in different mannerisms.

Damping the jump of coalescing droplets through substrate compliance.

Sessile droplets coalescing on superhydrophobic surfaces result in spontaneous droplet jumping. Here, through coalescence experiments and fluid-structure interaction simulations for microliter droplets, we demonstrate that such droplet jumping can be damped if the underlying substrate is designed to be compliant. We show that a compliant superhydrophobic substrate with synergistic combinations of low stiffness and inertia deforms rapidly during the coalescence process to minimize the substrate reaction, thus diminishing the jumping velocity. A spring-mass system model for coalescing water droplets is proposed that successfully captures droplet motion and substrate deformation for a wide range of compliant superhydrophobic substrates. These insights can be leveraged to improve the process efficiency in multiple applications, such as designing compliant superhydrophobic substrates for minimizing the scattering of small, nanoliter-sized droplets during atmospheric water harvesting. Lastly, experiments on an exemplar butterfly wing show that droplet jumping velocity reduction can also manifest on natural superhydrophobic substrates due to their inherent compliance.

The influence of the variability of the support of the mortar base plate on the quality of the results obtained in the process of its numerical design

Due to the high costs associated with the purchase of ammunition and firing in certified training ground centers, tests of retaining plate deformations are increasingly replaced by computer simulations using numerical models. Computer programs usually use a single-parameter subsoil model (Winkler-Zimmermann) for calculations, which requires providing the subgrade susceptibility coefficient. The subgrade compliance coefficient is intended to determine the mutual reaction of the subgrade and the structure due to the pressure exerted on the soil by the retaining slab, which settles. When designing slabs in computer programs, it is assumed that the substrate compliance coefficient is constant. Determining the impact of the soil on the retaining slab is important when analyzing its deformations. The subject of the work was the analysis of the influence of ground support on the results obtained during modeling of the retaining slab. In order to obtain data for FEM analysis and validation, the actual strains occurring on the thrust plate were measured using strain gauge rosettes. The plate deformations were measured during field shooting tests. In order to vary the influence of supporting the slab on the ground and obtain reliable stress values on the slab surface, a method of successive iterations was proposed. Calculations are performed using this method until the error is smaller than the assumed one.

Polymer Complex Multilayers for Drug Delivery and Medical Devices.

Polymer complex multilayers (PCMs) can be engineered into various structures with tunable properties via layer-by-layer (LBL) assembly driven by noncovalent forces. Due to their ease of preparation, capability of integrating multiple functional components, and excellent substrate compliance, biocompatible PCMs as coating materials or individual entities have attracted extensive attention in biomedical applications. This Spotlight on Applications presents recent progress on PCMs applied for drug delivery and medical devices. We provide several examples to address the importance of using PCM platforms to achieve controlled drug delivery including stimuli-triggered release, sustained release, and spatiotemporal sequential release. The effects of PCM coatings on the bioresponse regulation and performance enhancement of implantable devices are also highlighted. Moreover, the design and fabrication of flexible electrical and optical elements modified with LBL PCMs have been discussed, which demonstrates the great potential to advance emerging wearable devices for disease monitoring and health management.

Retention analysis of droplets over compliant substrates

Droplet dynamics over substrates hold numerous applications including fuel cell technologies, anti-icing, and cleaning technologies. However, a fundamental understanding of droplet retention over compliant substrates has not received much attention. This paper presents a detailed investigation of the effect of substrate compliance over contact angles and maximum retentive/adhesive force applied over the droplet by a substrate (of varying degrees of compliance). We have defined a non-dimensional number called "Substrate Compliance (Cs)" as Cs=γPt+ρVgtF, which can act as the parameter representing the degree of compliance of the substrate. An analytical model is developed to determine the maximum retention force applied over the droplet, which could predict the retention force offered by the substrates (with 0≤Cs≤12000) satisfactorily with a maximum deviation of 11.52 % from experimental results. Further, the effect of substrate compliance over contact angle hysteresis (CAH), the shape of the droplet, and the retention force has been investigated. CAH of droplets over a compliant substrate is found to be higher (≥200 % at inclination angle of 30o) with more elongation and higher aspect ratios as compared to a non-compliant substrate. Also, a free-hanging thin Polydimethylsiloxane (PDMS) membrane (a compliant substrate) could retain ∼146−276% higher droplet volumes compared to a PDMS-coated glass substrate. In the end, we have developed and employed an Artificial Neural Network (ANN) model to predict the composition of PDMS membranes using experimental data, which is successful with an overall R=0.90.

On Combined Influence of Substrate Curvature and Compliance on Parameters of Coating Delamination from a Cylindrical Base

On Combined Influence of Substrate Curvature and Compliance on Parameters of Coating Delamination from a Cylindrical Base

Why does the metabolic cost of walking increase on compliant substrates?

Walking on compliant substrates requires more energy than walking on hard substrates but the biomechanical factors that contribute to this increase are debated. Previous studies suggest various causative mechanical factors, including disruption to pendular energy recovery, increased muscle work, decreased muscle efficiency and increased gait variability. We test each of these hypotheses simultaneously by collecting a large kinematic and kinetic dataset of human walking on foams of differing thickness. This allowed us to systematically characterize changes in gait with substrate compliance, and, by combining data with mechanical substrate testing, drive the very first subject-specific computer simulations of human locomotion on compliant substrates to estimate the internal kinetic demands on the musculoskeletal system. Negative changes to pendular energy exchange or ankle mechanics are not supported by our analyses. Instead we find that the mechanistic causes of increased energetic costs on compliant substrates are more complex than captured by any single previous hypothesis. We present a model in which elevated activity and mechanical work by muscles crossing the hip and knee are required to support the changes in joint (greater excursion and maximum flexion) and spatio-temporal kinematics (longer stride lengths, stride times and stance times, and duty factors) on compliant substrates.

The effects of soft and rough substrates on suction-based adhesion.

The northern clingfish (Gobiesox maeandricus) has a suction-based adhesive disc that can stick to incredibly rough surfaces, a challenge for stiff commercial suction cups. Both clingfish discs and bioinspired suction cups have stiff cores but flexible edges that can deform to overcome surface irregularities. Compliant surfaces are common in nature and technical settings, but performance data for fish and commercial cups are gathered from stiff surfaces. We quantified the interaction between substrate compliance, surface roughness and suction performance for the northern clingfish, commercial suction cups and three biomimetic suction cups with disc rims of varying compliance. We found that all cups stick better on stiffer substrates and worse on more compliant ones, as indicated by peak stress values. On compliant substrates, surface roughness had little effect on adhesion, even for commercial cups that normally fail on hard, rough surfaces. We propose that suction performance on compliant substrates can be explained in part by effective elastic modulus, the combined elastic modulus from a cup-substrate interaction. Of all the tested cups, the biomimetic cups performed the best on compliant surfaces, highlighting their potential to be used in medical and marine geotechnical fields. Lastly, we discuss the overmolding technique used to generate the bioinspired cups and how it is an important tool for studying biology.

Mechanical stress modulates NLRP3 pathway in macrophages

Abstract NLRP3 inflammasome activation releases the pro-inflammatory cytokine IL-1b and induces pyroptotic cell death. Dysregulated NLRP3 activation escalates inflammatory diseases such as acute lung injury (ALI), ventilator-induced lung injury (VILI), liver fibrosis, and idiopathic pulmonary fibrosis (IPF). These inflammatory disorders are associated with increased mechanical stress due to changing tissue compliance/stiffness. Resident macrophages sense this mechanical signal via a process known as “mechanotransduction”. However, the underlying mechanism of how mechanotransduction regulates the NLRP3 inflammasome is not fully known. In this study, we are investigating that how NLRP3 pathway is regulated in lung resident macrophages during mechanical stress. To alter substrate compliance, macrophages are cultured on collagen-coated gels with various physiologically relevant stiffness (elastic moduli; measured in kiloPascal (kPa)) prior to NLRP3 activation. We have found that the NLRP3 inflammasome is sensitive to the substrate stiffness. NLRP3 induced IL-1β and pyroptosis-inducing gasdermin-D production were highest on the softest substrate (0.2 kPa) and decreased on a stiffer substrate (64 kPa). However, NLRC4 inflammasome activation was not mechano-responsive, suggesting ASC adaptor mediated cross signaling in mechanotransduction. Further analysis of gene expression using high-throughput RNA-sequencing, revealed altered expression of inflammatory bio-markers on macrophages when substrate stiffness was varied. Further examination of molecular interactions between adhesion-based mechanotransduction and NLRP3 signaling will help in understanding inflammatory diseases such as lung fibrosis. AAI career in Immunology Fellowship, R01-AI104732 and R56 AI104732

High‐speed terrestrial substrate transitions: How a fleeing cursorial day gecko copes with compliance changes that are experienced in nature

Abstract Animal movement is often largely determined by abiotic conditions of the environment, including substrate properties. While a large body of work has improved our understanding of how different substrate properties can impact locomotor performance and behaviour, few of these studies have investigated this relationship during transitions within a single locomotor event. In nature, terrestrial animals frequently encounter substrate transitions, or changes in substrate level, incline, texture and/or compliance during a single bout of movement, which can be sudden for high‐speed animals. These animals often adjust their posture and kinematics during transitions, and in some cases lose forward velocity. We examined the occurrence and effect of non‐elastic compliance transitions in Rhoptropus afer, a cursorial day gecko known for its ability to sprint rapidly for several metres at a time. We recorded substrate use during provoked escapes in the field and conducted locomotor trials on a trackway that mimicked natural structural habitat conditions with transitions from a rigid surface into sand and from sand back to a rigid surface. During escapes, R. afer used substrates of different compliance (i.e. rock, gravel and sand) and transitioned to and from the more compliant surfaces with even frequency, which matched substrate and compliance availability estimates. In laboratory experiments, sprint speed was not significantly affected by acute changes in compliance, which was likely facilitated by an increased body angle and duty factor upon entering the sand, and potentially a high yield strength of the sand relative to applied forces. We speculate that this species' ability to maintain speed during compliance transitions underlies their apparent indiscriminate substrate use during escapes, and that this behaviour may offer a selective advantage for evading larger terrestrial predators. Using field data to inform and contextualize laboratory experiments thus yields important insights as to how animals accommodate acute changes in substrate conditions encountered during critical locomotor events. A free Plain Language Summary can be found within the Supporting Information of this article.

Characterization of bridge substructures explored by leveraging structural identification of a scaled bridge model

Characterization and evaluation of bridge substructures, especially when as-built information is missing, are needed to assess the vulnerability of bridges to natural hazards, other possible causes of bridge failure, and as prerequisites for bridge substructure and foundation reuse. Structural identification has been investigated as a tool for characterizing and evaluating bridge substructures. Structural identification offers advantages over conventional methods especially by integrating information from structural testing and local NDE approaches.During field investigations the authors observed significant challenges to assessing substructure characteristics such as embedded depth, even for simple piers on spread footing foundations. It is found that impact excitation on bridge superstructure or even directly on substructure aiming for conventional flexural modes in the lower frequency band is not sufficient for a reliable identification of the substructure and its surrounding soil.To understand better the challenges limiting reliable structural identification of bridge substructures, a phenomenological scaled bridge model with strip footing, mimicking an actual bridge on shallow foundations, was designed, fabricated and utilized for controlled laboratory testing. The scaled bridge model could be modified to comprehend the sensitivity of bridge substructure modal properties to: i) stiffness of bridge bearings (causing coupling between superstructure and substructure) and ii) foundation substrate compliance. A finite element representation of the physical model, with a Multiphysics Simulation Finite Element Software, was calibrated and was then exploited to perform further sensitivity studies which confirmed the potential of high frequency axial and flexural modes of the substructures to: iii) identify shallow foundation embedded depth, and iv) characterize the soil substructure interaction.Throughout the paper, different challenges affecting structural identification for characterization and evaluation of bridge substructures and possible countermeasures are discussed. Findings of the study indicate that a high frequency bandwidth covering the axial vibrational modes of the substructure is necessary if structural identification is used for estimating shallow foundation embedded depth. Such a bandwidth is not possible in conjunction with ambient monitoring, pointing to the necessity of a controlled multi-inputmultioutput (MIMO) dynamic testing.

The principles of directed cell migration.

Cells have the ability to respond to various types of environmental cues, and in many cases these cues induce directed cell migration towards or away from these signals. How cells sense these cues and how they transmit that information to the cytoskeletal machinery governing cell translocation is one of the oldest and most challenging problems in biology. Chemotaxis, or migration towards diffusible chemical cues, has been studied for more than a century, but information is just now beginning to emerge about how cells respond to other cues, such as substrate-associated cues during haptotaxis (chemical cues on the surface), durotaxis (mechanical substrate compliance) and topotaxis (geometric features of substrate). Here we propose four common principles, or pillars, that underlie all forms of directed migration. First, a signal must be generated, a process that in physiological environments is much more nuanced than early studies suggested. Second, the signal must be sensed, sometimes by cell surface receptors, but also in ways that are not entirely clear, such as in the case of mechanical cues. Third, the signal has to be transmitted from the sensing modules to the machinery that executes the actual movement, a step that often requires amplification. Fourth, the signal has to be converted into the application of asymmetric force relative to the substrate, which involves mostly the cytoskeleton, but perhaps other players as well. Use of these four pillars has allowed us to compare some of the similarities between different types of directed migration, but also to highlight the remarkable diversity in the mechanisms that cells use to respond to different cues provided by their environment.

Elastic wetting: Substrate-supported droplets confined by soft elastic membranes

While classical wetting is well captured by the famous Young's equation and classical bulge and blister models are readily available, there is limited understanding of a micro- or nano-scale droplet being covered by an ultrasoft elastic membrane. We call this phenomenon elastic wetting to feature the interplay between the liquid's surface tension and the membrane's elastic deformation. Examples of elastic wetting include cell blebs and 2D material bubbles, where the membrane thickness ranges from microns to sub-nanometers. In this work, we study the equilibrium of elastic wetting and solve for the profiles and the pressure-volume relations of the membrane-confined droplets. We show that in elastic wetting, the pressure across the membrane/droplet interface can be described by a simple superposition of the Young-Laplace equation and the nonlinear membrane equation. Furthermore, nonlinear elasticity, geometric nonlinearity, and surface tension, together with membrane-substrate adhesion, interweave at the contact line, leading to rich membrane-confined droplet configurations. Finally, we examine the effect of substrate compliance on elastic wetting and find that the rigid substrate assumption approximates well for most of the existing experiments in the literature. Our results provide fundamental mechanistic insights into the various phenomena of elastic wetting as well as viable means to extract physical parameters including the bubble pressure and the interface energies.

Milliscale Substrate Curvature Promotes Myoblast Self‐Organization and Differentiation

Biological tissues comprise complex structural environments known to influence cell behavior via multiple interdependent sensing and transduction mechanisms. Yet, and despite the predominantly nonplanar geometry of these environments, the impact of tissue-size (milliscale) curvature on cell behavior is largely overlooked or underestimated. This study explores how concave, hemicylinder-shaped surfaces 3-50mm in diameter affect the migration, proliferation, orientation, and differentiation of C2C12 myoblasts. Notably, these milliscale cues significantly affect cell responses compared with planar substrates, with myoblasts grown on surfaces 7.5-15mm in diameter showing prevalent migration and alignment parallel to the curvature axis. Moreover, surfaces within this curvature range promote myoblast differentiation and the formation of denser, more compact tissues comprising highly oriented multinucleated myotubes. Based on the similarity of effects, it is further proposed that myoblast susceptibility to substrate curvature depends on mechanotransduction signaling. This model thus supports the notion that cellular responses to substrate curvature and compliance share the same molecular pathways and that control of cell behavior can be achieved via modulation of either individual parameter or in combination. This correlation is relevant for elucidating how muscle tissue forms and heals, as well as for designing better biomaterials and more appropriate cell-surface interfaces.

InAs heteroepitaxy on nanopillar-patterned GaAs (111)A

Heteroepitaxy on nanopatterned substrates is a means of defect reduction at semiconductor heterointerfaces by exploiting substrate compliance and enhanced elastic lattice relaxation resulting from reduced dimensions. We explore this possibility in the InAs/GaAs(1 1 1)A system using a combination of nanosphere lithography and reactive ion etching of the GaAs(1 1 1)A substrate for nano-patterning of the substrate, yielding pillars with honeycomb and hexagonal arrangements and varied nearest neighbour distances. Substrate patterning is followed by MBE growth of InAs at temperatures of 150–350 °C and growth rates of 0.011 nm/s and 0.11 nm/s. InAs growth in the form of nano-islands on the pillar tops is achieved by lowering the adatom migration length by choosing a low growth temperature of 150 °C at the growth rate 0.011 nm/s. The choice of a higher growth rate of 0.11 nm/s results in higher InAs island nucleation and the formation of hillocks concentrated at the pillar bases due to reduced adatom migration length. A common feature of the growth morphology for all explored conditions is the formation of hillocks or pyramids with sometimes well-defined facets due to the presence of a concave surface curvature at the pillar bases acting as adatom sinks. This InAs growth at the pillar bases and between pillars is investigated in detail in dependence of MBE growth conditions.