AKM: at the cutting edge of science

The Atlantic KET Med consortium are at the cutting edge of both engineering and science in the EUs Atlantic Area. As part of the projects activities, we have contributed to a number of scientific articles that help to expand the understanding of processes and interactions surrounding the use of the Key Enabling Technologies (KETs) for developments relevant to the next generation of medical devices.

These works are fully peer reviewed and published in world class scientific journals.

The abstracts of these articles are outlined below. To get the full text please email info@atlantic-ketmed.eu

 Graphene growth kinetics for CO2 laser carbonization of polyimide

 The study of growth kinetics of graphene on Polyimide upon carbon-dioxide (CO2) laser irradiation enables optimisation of crystal size for maximum electrical conductivity. We report the first study on growth kinetics of graphene produced by laser carbonization of polyimide using the Arrhenius equation. The peak irradiation temperature (Tirr) for each laser fluence was calculated from the photothermal model, solved by Finite Element Analysis in COMSOL software. Studies of the Raman spectra of the laser induced graphene revealed that the crystallite size increases with decreasing scan-speed at constant laser fluence. The barrier activation energy for graphene growth was found to be 0.20 ± 0.03 eV.

Materials Letters 307 (2022) 131097

 The role of fluence in determining the response of thin molybdenum films to ultrashort laser irradiation; from laser-induced crystallization to ablation via photomechanical ablation and nanostructure formation

 The selective processing of Mo at low temperatures is challenging, especially in advanced manufacturing on flexible and heat-sensitive substrate due to its higher melting temperature. The key role of fluence in determining the response of thin Mo films to ultrashort laser irradiation is considered in this study. At low fluences, the electrical properties of Mo are enhanced by a localized laser-induced crystallization mechanism; the electrical mobility of Mo is increased and the contact resistance between Mo-Si interface is reduced. At higher fluences, selective patterning of Mo proceeds without impacting the Si layer and the threshold fluence for ablation increases with the film thickness of Mo. Two fluence dependent ablation mechanisms are observed depending on the Mo film thickness. For thin films of thicknesses 20 nm and 40 nm, selective ablation proceeds only by a photothermal interaction. For 60 nm and 80 nm thick films, selective ablation proceeds by both photomechanical and photothermal interactions at two-separate higher fluence regimes, respectively. Interestingly, between these two ablation regimes, a non-ablative nanostructuring regime occurs. The study provides a concise overview of the process window for implementing the laser-induced modifications to Mo layers with minimal impact to the substrate using single wavelength ultrashort pulse laser.

  • Applied Surface Science 592 (2022) 153315

 Selective laser ablation of molybdenum from aluminium in a multi-layered thin film system

 It is challenging to selectively remove a thin film with a high melting temperature from an underlying one with a low melting temperature where precise stops at layer interfaces are required. In this study, an ultrashort laser pulse is used to cleanly ablate a 40 nm thin molybdenum layer from an aluminium thin film in a commercially relevant heterostructure. We observe clean delamination of molybdenum from aluminium. We observe that columnar nanostructures in Mo are generated well below the damage threshold fluence. Both delamination and nanostructure formation are considered in terms of electromechanical forces. At low fluence, only selective removal of the very near surface of the Mo thin film was observed. At higher fluences, ablation of the Mo film occurred without melting of the underlying Al layer. Our computational results confirm the important role of thermionic electron emission and near surface electron-phonon coupling in nanostructure generation. We introduce a thermal boundary resistance effect in the heat diffusion by electrons to more accurately describe the release of the Mo from the Al layer. The comparison of experimental and simulation results confirms that electromechanical stresses can be an important mechanism in selective thin film ablation.

– Surfaces and Interfaces 26 (2021) 101438

Femtosecond Laser-Induced Crystallization of Amorphous Silicon Thin Films under a Thin Molybdenum Layer

A new process to crystallize amorphous silicon without melting and the generation of excessive heating of nearby components is presented. We propose the addition of a molybdenum layer to improve the quality of the laser-induced crystallization over that achieved by direct irradiation of silicon alone. The advantages are that it allows the control of crystallite size by varying the applied fluence of a near-infrared femtosecond laser. It offers two fluence regimes for nanocrystallization and polycrystallization with small and large crystallite sizes, respectively. The high repetition rate of the compact femtosecond laser source enables high-quality crystallization over large areas. In this proposed method, a multilayer structure is irradiated with a single femtosecond laser pulse. The multilayer structure includes a substrate, a target amorphous Si layer coated with an additional molybdenum thin film. The Si layer is crystallized by irradiating the Mo layer at different fluence regimes. The transfer of energy from the irradiated Mo layer to the Si film causes the crystallization of amorphous Si at low temperatures (∼700 K). Numerical simulations were carried out to estimate the electron and lattice temperatures for different fluence regimes using a two-temperature model. The roles of direct phonon transport and inelastic electron scattering at the Mo−Si interface were considered in the transfer of energy from the Mo to the Si film. The simulations confirm the experimental evidence that amorphous Si was crystallized in an allsolid- state process at temperatures lower than the melting point of Si, which is consistent with the results from transmission electron microscopy (TEM) and Raman. The formation of crystallized Si with controlled crystallite size after laser treatment can lead to longer mean free paths for carriers and increased electrical conductivity.

ACS Appl. Mater. Interfaces 2021, 13, 3779737808

Improvement of electrical properties of ITO thin films by melt-free ultra-short laser crystallization

We describe a novel solid state crystallisation method for optimising a thin film transparent conductive oxide when deposited on flexible polymer substrates. The method is based on ultra-short non-thermal laser sintering of indium tin oxide (ITO) thin films. In this study, we used commercial ITO thin films deposited on a flexible polyethylene terephthalate substrate with a relatively low melting temperature compared with ITO on glass. We demonstrate the use of laser scanning with high pulse overlapping at fluences seven times less than the threshold required for melting/damage of ITO. The results confirm greater than four times enhancement in the mobility of charge carriers of ITO thin films after laser scanning and sheet resistance can be reduced up to 25%. There is no reduction in optical transparency observed in laser treated samples. Surface morphology and x-ray diffraction analyses confirm the improvement in crystallite sizes by laser sintering, resulting in a greater than 37% increase in grain size due to enhanced crystallization. Comparison of experimental and simulation based on a delayed two temperature model confirms that ITO thin film crystallization occurred at about one-third of the melting temperature of ITO.

– J. Phys. D: Appl. Phys. 54 (2021) 185103 (9pp)

Femtosecond Laser Assisted Crystallization of Gold Thin Films

We propose a novel low temperature annealing method for selective crystallization of gold thin films. Our method is based on a non-melt process using highly overlapped ultrashort laser pulses at a fluence below the damage threshold. Three different wavelengths of a femtosecond laser with the fundamental (1030 nm), second (515 nm) and third (343 nm) harmonic are used to crystallize 18-nm and 39-nm thick room temperature deposited gold thin films on a quartz substrate. Comparison of laser wavelengths confirms that improvements in electrical conductivity up to 40% are achievable for 18-nm gold film when treated with the 515-nm laser, and the 343-nm laser was found to be more effective in crystallizing 39-nm gold films with 29% improvement in the crystallinity. A two-temperature model provides an insight into ultrashort laser interactions with gold thin films and predicts that applied fluence was insufficient to cause melting of gold films. The simulation results suggest that non-equilibrium energy transfer between electrons and lattice leads to a solid-state and melt-free crystallization process. The proposed low fluence femtosecond laser processing method offers a possible solution for a melt-free thin film crystallization for wide industrial applications.

– Nanomaterials 2021, 11, 1186.

Non-melt selective enhancement of crystalline structure in molybdenum thin films using femtosecond laser pulses

It is challenging to crystalize a thin film of higher melting temperature when deposited on a substrate with comparatively lower melting point. Trading such disparities in thermal properties between a thin film and its substrate can significantly impede material processing. We report a novel solid-state crystallization process for annealing of high melting point molybdenum thin films. A systematic investigation of laser induced annealing from single pulse to high pulse overlapping is reported upon scanning at fluences lower than the threshold required for the damage/ablation of molybdenum thin films. The approach allows better control of the grain size by changing the applied laser fluence. Atomic force microscopy surface morphology and x-ray diffraction (XRD) analysis reveal significant improvements in the average polycrystalline grain size after laser annealing; the sheet resistance was reduced by 19% of the initial value measured by a Four-point probe system. XRD confirms the enlargement of the single crystal grain size. No melting was evident, although a change in the close packing, shape and size of nanoscale polycrystalline grains is observed. Ultrashort laser induced crystallinity greatly enhances the electrical properties; Hall measurements reinforced that the overall carrier concentration increases after scanning at different laser fluences. The proposed method, based on the aggregation and subsequent growth of polycrystalline and single crystal-grains, leading to enhanced crystallization, has potential to be applicable in thin film processing industry for their wide applications.

J. Phys. D: Appl. Phys. 55 (2022) 115301 (10pp)

Toward Developing Immunocompetent Diabetic Foot Ulcer-on-a-Chip Models for Drug Testing

Bioengineering of skin has been significantly explored, ranging from the use of traditional cell culture systems to  the most recent organ-on-a-chip (OoC) technology that permits skin modeling on physiological scales among other  benefits. This article presents key considerations for developing physiologically relevant immunocompetent diabetic  foot ulcer (DFU) models. Diabetic foot ulceration affects hundreds of millions of individuals globally,  especially the elderly, and constitutes a major socioeconomic burden.When DFUs are not treated and managed in a timely manner, 15–50% of patients tend to undergo partial or complete amputation of the affected limb. Consequently, at least 40% of such patients die within 5 years postamputation. Currently, therapeutic strategies are actively sought and developed. However, present-day preclinical platforms (animals and in vitro models) are not robust enough to provide reliable data for clinical trials. Insights from published works on immunocompetent skinon- a-chip models and bioengineering considerations, presented in this article, can inform researchers on how to develop robust OoC models for testing topical therapies such as growth factor-based therapies for DFUs. We propose that immunocompetent DFU-on-a-chip models should be bioengineered using diseased cells derived from individuals; in particular, the pathophysiological contribution of macrophages in diabetic wound healing, along with the typical fibroblasts and keratinocytes, needs to be recapitulated. The ideal model should consist of the following components: diseased cells embedded in reproducible scaffolds, which permit endogenous ‘‘diseased’’ extracellular matrix deposition, and the integration of the derived immunocompetent DFU model onto a microfluidic platform. The proposed DFU platforms will eventually facilitate reliable and robust drug testing of wound healing therapeutics, coupled with reduced clinical trial failure rates.

TISSUE ENGINEERING: Part C Volume 27, Number 2, 2021

What is the “normal” wound bed temperature? A scoping review and new hypothesis

Wound bed temperature measurement holds the potential to be a safe, easy to use, and low-cost tool to aid objective wound bed assessment, clinical decision making and improved patient outcomes. However, there is no consensus on the normal range of wound bed temperature in chronic wounds. We conducted a scoping review including any study type, from 2010 to 2020 in which chronic wound bed temperature was reported. Thirteen studies including 477 patients met our criteria. Venous ulcers (VLU) accounted for 46.5% (n = 222) of wounds; diabetic foot ulcers (DFU) for 25.4% (n = 121) with pressure ulcers (PU), mixed arterial venous ulcers (MAVLU) and unknown aetiology accounting for the remainder. The weighted mean of means for wound bed temperature was 31.7_C (n = 395) for all wound types; 31.7_C for VLU; 31.6_C for DFU; 33.3_C for PU; 30.9_C for MAVLU; and 32.0_C for those with unknown aetiology. Based on our review, we hypothesise that normal wound bed temperature is within a range of 30.2–33.0_C.

Wound Rep Reg. 2021;29:843–847.