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Experimental investigations of the fluid dynamics of vitreoretinal surgery
Vitrectomy is a surgical procedure by which the vitreous humour is removed from the vitreous
chamber of the eye and is replaced with a tamponade fluid. Vitrectomy is performed
by means of a system consisting of several components, including the vitreous cutter,
which aspirates the vitreous body, a viscoelastic fluid that occupies the vitreous chamber of the
eye. The vitreous cutter port geometry may have great influence in vitrectomy surgery from
the fluidics and safety standpoints. For this reason, there has been an increase interest from
ophthalmic companies in optimising the fluidics within the vitreous chamber during surgical
maneuvers, in order to avoid retinal tractions and intraocular pressure (IOP) variations.
Once removed, the vitreous is often replaced with Silicone Oils (SOs), which, however, inevitably
tend to emulsify in the aqueous solution present in the eye, limiting the chances of
using SOs as long term vitreous substitutes. Many months are indeed often required for complete
healing of retinal damages. Moreover, formation of emulsions triggers local inflammatory
processes, which can cause further complications.
This thesis is an experimental work related to the physics of ocular surgical procedures
and it consists of two main parts. The first part concerns the fluid mechanics of the vitrectomy
procedure. In particular, experiments with different approaches have been performed on the
fluidics of various vitrectomy systems, with the objective of identifying ideal working conditions
that minimise the risks of retinal damages, while keeping the surgical procedure efficient.
The second part is related to the characterisation of the interfacial rheology between SOs and
aqueous solutions containing endogenous proteins released by the ocular tissue as a response to
an inflammatory state. Aim of this part is to investigate to what extent biomolecules present in
the aqueous solution can modify the rheological properties of the interface between the two fluids
and, in turn, facilitate the formation of an emulsion. This work is carried out in collaboration
with Dr. Libero Liggieri at CNR-ICMATE.
Each part consists of several chapters, which address various projects within these two topics
and are briefly summarised below.
PART I. PERFORMANCE ASSESSMENT OF VITRECTOMY DEVICES
An important issue associated with vitrectomy is the risk of generation of retinal tractions by the
vitreous cutter. It has been demonstrated that an intermittent flow generates fluid accelerations,
with consequent pressure variations within the vitreous chamber, which may result in pulsatile
tractions on the retina, with significant risk of iatrogenic retinal damage. Rossi et al. (2014b)
proposed a conceptual diagram according to which, for a safe and efficient vitrectomy, optimisation
of fluidics can be achieved by maximising the flow rate and minimising the acceleration
around the cutter port.
Another important aspect of the surgical procedure is prevention of large IOP changes within
the eye. In particular, ocular hypertension or ocular hypotony can have very important clinical
consequences. In order to maintain a normal IOP value, the balance between the aspiration and
the irrigation pressures during vitrectomy plays a very important role. In chapter 1 we reviewed
the literature concerning fluid dynamics of vitreoretinal surgery.
In chapter 2 we evaluated the performance of vitreous cutters in vitro in terms of flow rate,
using vitreous phantoms with different rheological properties (artificial vitreous, AV). On each
fluid solution we performed rheological tests in order to obtain a good control of the properties
of the working fluid and thus guarantee reproducibility of the experiments. This allowed us to
compare the efficiency of the instruments in a highly controlled environment and to assess how
their behavior is affected by fluid properties.
Our results showed that all cutters are higly sensitive to fluid rheology and this is particularly
true for double blade cutters, which, however, always produce larger flow rates compared to
single blade ones. These findings imply that large flow rate fluctuations should be expected when
a vitreous cutter is operated in a medium with heterogeneous mechanical properties (Nepita
et al., 2021).
In chapters 3 and 4 we show two experimental studies aimed at assessing the performance
of two vitrectomy systems, characterised by different technologies (guillotine and ultrasound
probes), tested both with balanced salt solution (BSS) and AVs. The following types of experiments
have been performed in a cubic testing chamber.
- Flow rate measurements: mean flow rate has been measured using a digital image tracking
process for all cutters, changing the operational parameters within a wide range (cutting frequency,
aspiration pressure, etc.). Results for guillotine cutters show that for viscoelastic fluids
the averaged flow rate only slightly decreases as the cutting frequency increases. The ultrasound
cutters produce larger flows regardless the stroke (the amplitude of the ultrasound vibrations of
the needle).
- Flow fields measurements: two dimensional PIV experiments have been performed with
the goal of characterising the flow fields generated by all cutters. For each set of parameters, a
sequence of images has been acquired on two different planes: \u201cfrontal view\u201d and \u201clateral view\u201d.
Moreover, for a specific solution, two areas of interest have been used for the acquisition: a \u201clarge
field\u201d of view, aimed at describing the entire circulation induced by the cutters at the scale of
the experimental box, and a \u201csmall field\u201d of view, i.e. a zoom around the cutter tip, to describe
more accurately flow structures around the cutter port. The flow induced by ultrasound cutters
is strongly three dimensional and time dependent. Flow structures are generated at different
spatial scales, from large scales similar to the experimental domain to smaller scales close to
the cutter port (Stocchino et al., 2020).
- Temperature measurements: a specific task was dedicated to measure temperature variations
induced by ultrasound cutters as a function of the stroke, maintaining the vacuum pressure
equal to zero. The two dimensional variation of the temperature field has been monitored
with an infrared thermal camera. The acquired thermal images were used to obtain the temperature
variations in time at a particular spot, or to generate two dimensional temperature maps.
The temperature slightly grows but this effect is likely to be clinically irrelevant, since infusion
of a different fluid will mitigate it. Moreover, the cutter head reaches maximum values close to
body temperature, meaning that it will not transmit much heat to the needle and eventually to
the vitreous during surgery (Stocchino et al., 2020).
- Pressure measurements: for this specific type of experiments a model of the human vitreous
chamber at real scale and with a realistic geometry has been employed. The eye model has been
designed considering both infusion and aspirations lines, thus mimicking the real surgical procedure.
The main goal of this part of the research activity is to perform pressure measurements
at two different positions of the vitreous chamber (anterior and posterior): for this purpose two
high precision pressure transducers (Viatran model 422) have been used.
Results show that dynamic variations of the pressure due to the motion of the cutter blade
are always small compared to the average pressure in the eye. Pressure fluctuations related to
fluid motion are felt more intensely by the pressure sensor placed on the back of the vitreous
chamber, opposite with respect to the irrigation line. This is probably due to the generation of
a jet that impinges the chamber wall opposite to its position. When the aspiration phase begins
the pressure in the eye model drops, which is related to hydraulic head (pressure) losses along
the pipelines of the system. This pressure drop is typically reduced (sometimes very efficiently)
when the vitrectomy system can actively compensate pressure variations. Results suggest that
maximum irrigation pressure that can be used for compensating pressure losses during aspiration
should be selected depending on several parameters such as the minimum irrigation
pressure and the aspiration pressure, in order to optimise the desired compensation.
PART II. STUDY OF SILICONE OIL EMULSIFICATION IN RELATION TO VITRECTOMY
The tendency of SOs to emulsify in water is influenced by the chemico-physical properties of
the fluids and interface and by external factors, such as temperature and mechanical energy
provided to the system. Exposition of the SO to endogenous molecules (proteins, lipids, etc.) in
the vitreous chamber, the presence of which is favored by the post-surgical inflammatory state
of ocular tissues, is also considered to be important for the generation of an emulsion. The mechanisms
at the basis of the effects of these molecules on the emulsification process depend on
their capability to adsorb at the water-oil interface, modifying the mechanical interfacial properties,
such as the interfacial tension (IT) and the dilational viscoelasticity (DV), i.e interfacial
rheological response to surface variations.
In chapters 5 and 6 we give an overview of the possible mechanisms for SO emulsion formation
and stabilisation, with a particular focus on biomolecules adsorption processes at the
SO-aqueous interface.
In chapter 7 we propose an experimental study aimed at achieving a chemico-physical characterisation
of the interfacial properties between SOs and aqueous solutions and at investigating
the effect of surfactant biomolecules, such as albumin and \u3b3-globulins, possibly responsible
for emulsion formation. In particular, measurements of IT on pendant drops of the protein aqueous
solution in the SO have been performed. These measurements have been carried out by a
Drop Shape Tensiometer (PAT1-Sinterface, Berlin). This technique allows one to quantify the
DV by measuring the IT response to small sinusoidal perturbations of frequency imposed on the
interfacial area.
Results show that adsorption of these proteins at the interface reduces the IT to values compatible
with the generation of an emulsion and provides values of the DV compatible with a good
stability of the resulting emulsions (Nepita et al., 2020). This study allowed us to understand
the effect of individual, key blood proteins on the mechanical properties of the interface. In order
to evaluate the overall effect of the presence of these proteins, we conducted a second series
of experiments adopting whole human serum blood (WHSB) samples obtained from donors. The results obtained with the serum dissolved in the solution showed that, already in concentrations
of 1-2 orders of magnitude smaller than that of single proteins, the IT significantly decreases,
up to values compatible with the formation of an emulsion, indicating therefore possible synergistic
effects with other components of the serum, in addition to the proteins previously studied.
Moreover, the DV modulus increases up to rather high values, compatible with the stabilisation
of the emulsions against the coalescence.
Drugs used as intraoperative tools in vitrectomy can also act as surfactants. An example
is the intraocular injection of an anti-inflammatory steroid, composed of micro crystal particles.
To assess whether the drug has a surfactant role at the interface, we conducted a first series
of experiments to quantify the IT and the DV properties. We performed the measurements on
the aqueous phase of the dispersion (supernatant), which separates after the sedimentation of
crystals. The experiments with the supernatant neither show a significant decrease of IT that
can be associated with the formation of an emulsion, nor lead to significant DV values. However,
it is known that dispersions of partially hydrophobic solid particles can give rise to particularly
stable emulsions, known as Pickering emulsions (Binks, 2002).
In order to verify the capability of biomolecules to form emulsions stable over the timescale
of months, comparative emulsification tests have been performed and shown in chapter 8. Emulsions
were produced using to the Double Syringe method (Gaillard et al., 2017). Although this
emulsification method may be very different from what occurs in practice, it is significant for
investigating the stability of the emulsion formed and is, therefore, used as a simple screening
test. For all samples containing blood proteins or the steroid drug, the presence of an emulsion
layer was detected, the thickness of which decreases with increasing dilution, and which was
stable over long times. On the contrary, the sample containing the Dulbecco buffer alone was already
separated a few minutes after the emulsion formation. Thus, we can conclude that blood
proteins are able to form and stabilise emulsions. Moreover, the steroid drug is able to stabilise
Pickering-type emulsions.
Eye movements are the main source of mechanical energy applied to the interface between
SO and aqueous solution inside the eye. A set of measurements in a realistic domain have been
performed. A qualitative and a quantitative analysis of the risults are reported in chapter 9 and
10, respectively. In particular we have used a Plexiglass model of a vitreous chamber with realistic
geometry. The experimental setup is equipped with an optical diagnostics for monitoring the
emulsion, a mechanical system controlled by a computer to simulate eye saccadic movements
and thermalised at 35\ub1C. Once filled with SO and aqueous solution, the model is mounted on a
support connected to a computer-controlled motor. In this study, harmonic rotations were considered.
The temporal evolution of the emulsion has been studied through acquisition of images
with a digital camera. In the case of emulsions characterised by very small droplets, a microscope
(Leica Microsystems, Switzerland) has been used to determine their distribution and size.
The main result obtained from these experiments consists in the formation and persistence of
emulsions even when a low concentration of albumin has been dissolved in the aqueous solution
(1% of the physiological concentration in blood). Moreover, the majority of the drops had a very
small diameter: in the range of 0 \u2013 10 \u3bcm. Droplets with these characteristic dimensions are
considered more dangerous from the clinical point of view, since they are able to favor the onset
of ocular complications
Absorbing new subjects: holography as an analog of photography
I discuss the early history of holography and explore how perceptions, applications, and forecasts of the subject were shaped by prior experience. I focus on the work of Dennis Gabor (1900â1979) in England,Yury N. Denisyuk (b. 1924) in the Soviet Union, and Emmett N. Leith (1927â2005) and Juris Upatnieks (b. 1936) in the United States. I show that the evolution of holography was simultaneously promoted and constrained by its identification as an analog of photography, an association that influenced its assessment by successive audiences of practitioners, entrepreneurs, and consumers. One consequence is that holography can be seen as an example of a modern technical subject that has been shaped by cultural influences more powerfully than generally appreciated.
Conversely, the understanding of this new science and technology in terms of an older one helps
to explain why the cultural effects of holography have been more muted than anticipated by forecasters
between the 1960s and 1990s
Stimulus-responsive Injectable Polysaccharide Scaffolds for Soft Tissue Engineering Prepared by O/W High Internal Phase Emulsion
This thesis describes work on the development of several novel stimuli-responsive
porous hydrogels prepared from oil-in-water (o/w) high internal phase emulsion
(HIPE) as injectable scaffolds for soft tissue engineering. Firstly, by copolymerising
glycidyl methacrylate (GMA) derivatised dextran and N-isopropylacrylamide
(NIPAAm) in the aqueous phase of a toluene-in-water HIPE, thermo-responsive
polyHIPE hydrogels were obtained. The temperature depended modulus of these
porous hydrogels, as revealed by oscillatory mechanical measurements, indicated
improvements of the mechanical properties of these hydrogels when heated from
room temperature to human body temperature, as the polyNIPAAm copolymer
segments starts to phase separate from the aqueous phase and causes the hydrogel to
form a more compact structure within the aqueous phase of the polyHIPE. Secondly
ion responsive methacrylate modified alginate polyHIPE hydrogels were prepared.
The physical dimensions, pore and pore throat sizes as well as water uptakes of these
ion responsive hydrogels can be controllably decreased in the presence of Ca2+ ions
and are fully recovered after disruption of the ionic crosslinking using a chelating
agent (sodium citrate). These ion-responsive polyHIPE hydrogels also possess good
mechanical properties (modulus up to 20 kPa). Both of these polyHIPE hydrogels
could be easily extruded through a hypodermic needle while breaking into small
fragments (about 0.5 to 3.0 mm in diameter), but the interconnected porous morphology was maintained after injection as revealed by SEM characterisation.
Furthermore, the hydrogel fragments produced during injection can be crosslinked
into a coherent scaffold under very mild condition using Ca2+ salts and alginate
aqueous solution as the ionically crosslinkable adhesive.
In order to increase the pore size of these covalently crosslinked polyHIPE hydrogels
and also find a biocompatible nontoxic emulsifier as substitution to traditional
surfactants, methyl myristate-in-water and soybean oil-in-water HIPEs solely
stabilised by hydroxyapatite (HAp) nanoparticle were prepared. These Pickering-
HIPEs were used as template to prepare polyHIPE hydrogels. Dextran-GMA, a water
soluble monomer, was polymerised in the continuous phase of the HAp Pickering
HIPEs leading to porous hydrogels with a tunable pore size varying from 1.5 ÎŒm to
41.0 ÎŒm. HAp is a nontoxic biocompatible emulsifier, which potentially provides
extra functions, such as promoting hard tissue cell proliferation.
HIPE-templated materials whose porous structure is maintained solely by the
reversible physical aggregation between thermo-responsive dextran-b-polyNIPAAm
block polymer chains in an aqueous environment (for this type of HIPE templated
material we coined the name thermo-HIPEs) were prepared. No chemical reaction is
required for the solidification of this porous material. This particular feature should
provide a safer route to injectable scaffolds as issues of polymerisation/crosslinking
chemistry or residual initiator fragments or monomers potentially being cytotoxic do not arise in our case, as all components are purified polymers prior to HIPE formation.
Thermo-HIPEs with soybean oil or squalene as dispersed oil phase were prepared.
Also in this HIPE system it was possible to replace the original surfactant Triton
X405 with colloidal HAp nanoparticles or pH/thermo-responsive polyNIPAAm-co-
AA microgel particles. The pore sizes and the mechanical properties of colloidal
particles stabilised thermo-HIPEs showed improvement compared with thermo-HIPEs
stabilised by Triton X405.
In summary new injectable polyHIPEs have been prepared which retain their pore
morphology during injection and can be solidified by either a thermal or ion (Ca2+) or
chelating ion (Ca2+) stimulus. The materials used are intrinsically biocompatible and
thus makes these porous injectable scaffolds excellent candidates for soft tissue
engineering
Methods for Image Recognition of Charged Particle Tracks in Track Detector Data Automated Processing
Exploration of a double emulsion formulation strategy to increase alendronic acid permeability
Alendronic acid (ALD) is a drug used to treat osteoporosis and has low oral bioavailability of 0.7 %. Furthermore, ALD has a complicated dosing regimen and can cause oesophageal irritation. ALD is ionised in water causing it to be less permeable in the gastrointestinal tract. To improve the permeability and absorption of ALD in the gastrointestinal tract, encapsulation of ALD within a double emulsion formulation was examined. Double emulsions were fabricated using a single-step homogenisation process using ALD, soybean oil and two surfactants, Span 80 and Tween 80. Fabricated double emulsions were characterised for ALD encapsulation and imaged using a light microscope so that emulsion morphology and size could be analysed. Double emulsions with greater than 90% ALD encapsulation were selected for in vitro permeation studies with Caco-2 and HDLEC cells to assess the potential permeability of ALD in intestinal lymphatic cells. ALD permeation improved by approximately 40% when double emulsion was used on Caco-2 cells pre-treated with oleic acid and taurocholic acid (OCTC), compared to non-pre-treated cells. Similarly, when the double emulsions were evaluated using HDLEC cells, the permeation of ALD was approximately 65% to 73%, with formulation B18 displaying the highest permeation. Interestingly, both studies revealed the integrity of the tight junction was not affected by the ALD double emulsion formulation. These studies also suggested improvement of ALD permeation could be from the absorption of double emulsion could be either via transcellular or paracellular pathways. These findings reveal that there may be potential to formulate ALD with improved permeation gastrointestinal tract
The emergence of liquid mixing from spinning magnetic nanoparticules in rotating magnetic fields : visualisation and characterisation towards a better understanding of the underlying phenomena
Ce travail vise Ă approfondir lâutilisation de nanoparticules magnĂ©tiques (NPM) en suspensions colloĂŻdales en tant quâagitateurs nanomĂ©triques pouvant manipuler le mĂ©lange Ă lâĂ©chelle micromĂ©trique lorsque soumis Ă des champs magnĂ©tiques. Dâabord, les structures cohĂ©rentes Ă©mergeant de la rotation de NPM sont observĂ©es par microscopie et sont quantifiĂ©es Ă lâaide dâune analyse multi-rĂ©solution basĂ©e sur les transformĂ©es en ondelettes, dĂ©montrant lâeffet significatif des paramĂštres Ă©tudiĂ©s sur la gĂ©nĂ©ration de structures cohĂ©rentes vorticales Ă lâintĂ©rieur du fluide. La sĂ©paration des agitateurs nanoscopiques de la phase continue Ă mĂ©langer en utilisant des Ă©mulsions de nanofluides magnĂ©tiques est ensuite Ă©valuĂ©e en comparant le couple exercĂ© par ces Ă©mulsions en champs magnĂ©tiques avec celui de suspensions colloĂŻdales Ă©quivalentes. Le mĂ©lange effectif similaire indique, pour des champs magnĂ©tiques rotatifs, que le momentum dĂ©veloppĂ© par les NPM en rotation est en mesure de se transmettre aux gouttelettes ainsi quâĂ la phase continue des Ă©mulsions.The present work further explores the use of MNPs in colloidal suspensions as nano-scale devices to manipulate mixing at the micro-scale when the whole system is subject to magnetic fields. First, the coherent structures emerging from rotating MNPs are visually observed using a microscopic imaging protocol and are quantitatively analyzed with a multi-resolution wavelet transforms based technique. The results demonstrate the significant effect of nanofluid composition and magnetic field parameters on the inception of coherent vortical structures inside the fluid. Secondly, the separation of the stirring MNPs from the continuous phase by using magnetic nanofluid emulsions is evaluated by comparing the torque exerted by MNPs in emulsions with the one in equivalent colloidal suspensions under magnetic fields. The very similar effective mixing indicates, for rotating magnetic fields, that the momentum developed by spinning MNPs inside emulsion droplets is transferable not only to the droplets but also to the surrounding liquid
Development and evaluation of a test program for Y-site compatibility testing of total parenteral nutrition and intravenous drugs
Published version. Source at http://doi.org/10.1186/s12937-016-0149-x.Background: There is no standardized procedure or consensus to which tests should be performed to judge compatibility/ incompatibility of intravenous drugs. The purpose of this study was to establish and evaluate a test program of methods suitable for detection of physical incompatibility in Y-site administration of total parenteral nutrition (TPN) and drugs.
Methods: Eight frequently used methods (dynamic light scattering, laser diffraction, light obscuration, turbidimetry, zeta potential, light microscopy, pH-measurements and visual examination using Tyndall beams), were scrutinized to elucidate strengths and weaknesses for compatibility testing. The responses of the methods were tested with samples containing precipitation of calcium phosphate and with heat destabilized TPN emulsions. A selection of drugs (acyclovir, ampicillin, ondansetron and paracetamol) was mixed with 3-in-1 TPN admixtures (OlimelÂź N5E, KabivenÂź and SmofKabivenÂź) to assess compatibility (i.e. potential precipitates and emulsion stability). The obtained compatibility data was interpreted according to theory and compared to existing compatibility literature to further check the validity of the methods.
Results: Light obscuration together with turbidimetry, visual inspection and pH-measurements were able to capture signs of precipitations. For the analysis of emulsion stability, light obscuration and estimation of percent droplets above 5 ÎŒm (PFAT5) seemed to be the most sensitive method; however laser diffraction and monitoring changes in pH might be a useful support. Samples should always be compared to unmixed controls to reveal changes induced by the mixing. General acceptance criteria are difficult to define, although some limits are suggested based on current experience. The experimental compatibility data was supported by scattered reports in literature, further confirming the suitability of the test program. However, conflicting data are common, which complicates the comparison to existing literature.
Conclusions: Testing of these complex blends should be based on a combination of several methods and accompanied by theoretical considerations
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