electrospun bioresorbable trileaflet heart valve prosthesis for tissue

research from animal testing to clinical experience
178
Ann Ist Super Sanità 2007 | Vol. 44, No. 2: 178-186
Electrospun bioresorbable trileaflet
heart valve prosthesis for tissue engineering:
in vitro functional assessment of a pulmonary
cardiac valve design
Costantino Del Gaudio(a, b), Alessandra Bianco(b) and Mauro Grigioni(a)
(a)
(b)
Dipartimento di Tecnologie e Salute, Istituto Superiore di Sanità, Rome, Italy
Dipartimento di Scienze e Tecnologie Chimiche, Università degli Studi “Tor Vergata”, Rome, Italy
Summary. Currently implanted prosthetic heart valves, both mechanical or biological ones, are used
to restore the proper blood hemodynamics when the native valves fail. However, these medical devices are not free from drawbacks, such as hemolysis or calcification, also presenting the relevant disadvantage of being unable to growth, repair and remodel. An improvement could be represented by
bioresorbable polymeric tissue-engineered heart valves. In this paper a poly(ε-caprolactone) (PCL)
heart valve prosthesis, realized by means of electrospinning, and its in vitro functional characterization in a pulse duplicator, resembling pulmonary conditions, is presented. Morphological examination revealed polymeric micrometric fibers randomly oriented with an average porosity of about
90%. Pulse duplicator testing highlighted that leaflets opened synchronously and showed a correct
coaptation in the diastolic phase, even if a slight rotation of the leaflets was visualized. In silico
study by numerical simulation of the closed phase predicted the stress distribution within the leaflet,
showing that peak levels are reached at the commissures and sustained by the structure without
failure. The present study highlighted the technical feasibility to produce polymeric bioresorbable
functional heart valves by means of electrospinning. Further studies and design changes are needed
in order to optimize the final scaffold to bear arterial hemodynamic conditions.
Key words: bioresorbable heart valve, electrospinning, functional in vitro testing.
Riassunto (Valvola cardiaca bioriassorbibile elettrofilata per ingegneria dei tessuti: valutazione funzionale in vitro di un modello polmonare). Le protesi valvolari cardiache attualmente impiantate,
sia meccaniche che biologiche, sono in grado di ristabilire la corretta emodinamica e garantire una
qualità di vita soddisfacente quando le valvole native non possono più assolvere alla propria funzione. Tuttavia questi dispositivi presentano diversi limiti (emolisi, calcificazione) e non possono
rimodellarsi in risposta alle modificazioni dell’organismo ospite. Un miglioramento in tal senso
può essere rappresentato dalle valvole cardiache in polimero bioriassorbibile. In questo lavoro si
presenta la realizzazione e caratterizzazione funzionale, parametri del sito polmonare, di una protesi valvolare in policaprolattone (PCL) prodotta mediante elettrofilatura (electrospinning). L’esame
morfologico ha evidenziato una struttura composta di fibre polimeriche casualmente orientate con
una porosità del 90%. Le prove funzionali in un duplicatore di impulsi hanno mostrato l’apertura
sincrona dei lembi valvolari e una corretta apposizione in diastole, anche se una leggera deformazione della valvola è stata evidenziata. La simulazione numerica della distribuzione degli sforzi nei
lembi valvolari ha evidenziato che i valori più alti sono raggiunti a livello delle commissure. Questo
studio ha mostrato la possibilità di produrre valvole cardiache in polimero bioriassorbibile mediante elettrofilatura. Ulteriori studi sono necessari per ottimizzarne la struttura al fine di sopportare il
carico pressorio arterioso.
Parole chiave: valvola cardiaca bioriassorbibile, elettrofilatura, caratterizzazione funzionale in vitro.
INTRODUCTION
Mechanical or biological heart valves are the most
common medical devices currently used to restore
the failure of native valves. However both of them
are affected from several drawbacks that limit the
long-term efficacy. To date a nonthrombogenic, non-
calcific prosthesis, which maintains mechanical and
hemodynamic characteristics and exhibits sufficient
fatigue properties, has not been designed [1]. Tissue
engineering could be an alternative approach to move
towards a heart valve equivalent. A number of studies investigated the possibility to produce tissue engi-
Address for correspondence: Mauro Grigioni, Dipartimento di Tecnologie e Salute, Istituto Superiore di Sanità, Viale Regina
Elena 299, 00161 Rome, Italy. E-mail: [email protected].
Electrospun bioresorbable trileaflet heart valve prosthesis for tissue engineering
neered prostheses, derived from polymers, decellularized scaffolds or biological scaffolds of non-valvular
origin.
Synthetic polymeric scaffolds present several attractive characteristics including the possibility
to deal with bioresorbable biomaterials, to have a
number of assembling techniques available, to control the repeatability of the resulting scaffold and
to have greater control over mechanical properties. However, possible limitations related to these
biomaterials can also be highlighted. Long term
mechanical properties can change as the polymer
degrades and the reaction products can potentially
induce a toxic response [1]. Moreover, a number of
difficulties in the regulation of cell adhesion and
tissue organization can be reported: extracellular
matrix proteins, such as specific ligands to promote
cell attachment to the matrix, are not present in
synthetic polymers [2].
The use of decellularized scaffolds can overcome
some disadvantages over synthetic materials [2, 3], but
the development of an effective technique for decellularizing heart valves and removing cellular debris is a
key-point to be addressed for a successful scaffold, that
is not often achieved [4-6].
Finally, biological scaffolds of non-valvular origin
can be regarded as a valid alternative. These materials (e.g., collagen) contains natural cellular adhesion sites and are less likely to initiate toxic immune
response. Being acellular materials the decellularization process is not required, thus preventing the
consequences of an incomplete decellularization.
However, also these materials have not fulfilled all
the requirements for a proper tissue-engineered
heart valve scaffold (e.g., mechanical properties) [1].
Moreover, collagen type I can elicit platelet adhesion and aggregation; several studies investigated
the role of glycoprotein VI in the interaction with
collagen to promote platelet activation and thrombus formation [7-9].
A brief survey of studies on tissue-engineered heart
valves is resumed in Table 1.
Material
This study presents the early results of a functional poly(ε-caprolactone) (PCL) heart valve made
by electrospinning. PCL is a bioresorbable aliphatic polyester, subjected to enzymatic degradation
through hydrolysis of ester bonds [22]. The slow
degradation period, due to its semicrystalline nature,
allows a) seeded cells to create extracellular matrix
(ECM), b) provide prolonged mechanical reliability
[22, 23], c) elicits low inflammatory response within
the surrounding tissue, due to the lower concentration of released acidic products [24]. Specifically,
PCL degradation is a two-stage process; first, the
molecular weight reduces according approximately
to an exponential law, then polymer fragments are
processed by cellular phagocitosis. PCL is finally
excreted by biliar or gastrointestinal route [25, 26].
Moreover taking into account mechanical proper-
ties, PCL is supposed to be an eligible biomaterial
for cardiovascular tissue engineering applications,
compared to other polyesters (e.g., polyglycolic acid,
polylactic acid) characterized by higher stiffness and
lower compliance.
Production process
Electrospinning is an efficient technique to produce polymeric fibers with diameters ranging from
nano- to micrometers. It is realised by applying high
voltage between a capillary, through which a polymeric solution flows, and a grounded collecting target [27]. Electric field induces charges on the surface
of the pendant drop at the tip of the capillary and
mutual charge repulsion causes a force directly opposite to the surface tension [27]. As the intensity
of the electric field increases, the formation of the
so-called Taylor cone from the polymeric drop occurred and when the electrostatic force overcomes
surface tension and viscoelastic forces (for a critical
value of the external electric field) a charged jet is
forced from its tip [27, 28]. The polymeric jet is then
subjected to instability that determines the stretching of the polymeric jet itself and the evaporation
of the solvent, this process leads to the formation of
a series of dry fine fibers [28]. Fibers are randomly
collected onto a fixed metal grounded target in form
of nonwoven mat or as a fibrous aligned mat on a
rotating target.
Electrospun PCL heart valve was functionally
characterized by means of pulse duplicator in physiological condition resembling the pulmonary site.
High speed cinematography was also carried out to
investigate leaflet dynamic behaviour and to asses
whether a proper leaflet coaptation occurred in the
diastolic phase. Finally a structural numerical simulation was performed to highlight stress distribution
within the leaflet, in the closed position.
MATERIALS AND METHODS
Valve prosthesis experimental procedure
Trileaflet stentless PCL (Sigma-Aldrich, Mn=80000)
valve was realized by means of electrospinning.
The polymer was solved in chloroform (14% w/v)
and successively collected into a glass syringe fitted
with a metallic blunt tip needle (22G). PCL was
electrospun on a rotating custom-made aluminium
trileaflet heart valve-shaped target at 10 cm from
the needle; due to its complex geometry, the rotating speed was 0.3-0.4 rpm in order to favour a homogeneous polymer deposition. Briefly, the valve
prosthesis was designed according to the following
technical procedure, resembling several choices already used on the marketed animal tissue derived
prostheses, such as the sewing procedure made by
using three fixation points where the posts are located in stented prostheses. The coaptation region
was modelled by straight lines linked with an arc,
then each leaflet was realized extruding the above
mentioned region along a spline trajectory, in the
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Costantino Del Gaudio, Alessandra Bianco and Mauro Grigioni
Table 1 | Overview of tissue-engineered heart valves
Scaffold
Production technique
Cell seeding
Testing
Ref.
PGA-P4HB
Heat application welding technique
Human marrow stromal cells
Bioreactor
[10]
PHOH-P4HB
Stereolithography
Bioreactor
[11]
Vascular cells from ovine
carotid artery and jugular vein.
Lamb model
Pulmonary site
[12]
PHO
PGA-P4HB
Heat application
welding technique
Autologous ovine
myofibroblasts and endothelial
cells
Bioreactor conditioning
Lamb model
Pulmonary site
[13]
PCL
Electrospinning
Human myofibroblast
Bioreactor
[14]
PGA-P4HB
Molding
Cells from human vena
saphena magna
Bioreactor
[15]
PGA-PLLA
Melt extrusion to have flat
nonwoven sheets to be assembled
in heart valve scaffolds
MSC from ovine bone marrow
Sheep model
Pulmonary site
[16]
Decellularized ovine
pulmonary valves
Autologous jugular veins
endothelial cells
Bioreactor conditioning
Lamb implantation
[17]
Decellularized porcine
pulmonary valves
Canine bone marrow-derived
cells
Dog implantation
[18]
Decellularized porcine
pulmonary valves
Sheep vascular endothelial
cells
Sheep implantation
[19]
Evaluation of the residual
potential to attract monocytic cells
depending on the origin of the
scaffold
[4]
Decellularized porcine and
human pulmonary valve
conduits
Decellularized porcine
heart valve conduits
HUVEC
Evaluation of endothelial cells
to abolish platelet adhesion
and activation (decellularized
porcine matrix acts as a plateletactivating surface)
[5]
Acellularized allogenic
lamb heart valve conduits
Lamb myofibroblasts
Reseeded and acellularized heart
valve conduits were implanted
into lambs
[2]
Decellularized human
pulmonary valve allogarfts
Endothelial progenitor cells
Implantation into two pediatric
patients
[3]
Bovine type-I collagen
Rapid prototyping
Human aortic valve interstitial
cells
In vitro evaluation of cell response
to collagen concentration (discshaped scaffolds) in static
conditions
[20]
Fibrin
Moulding
Carotid artery-derived cells
Bioreactor
[21]
PGA: polyglycolic-acid; PHOH: poly-3-hydroxyoctanoate-co-3-hydroxyhexanoate; P4HB: poly-4-hydroxybutyrate; PHO: polyhydroxyoctanoate; PCL:
Poly(ε-caprolactone); HUVEC: human umbilical vein endothelial cells; MSC: mesenchymal stem cells.
sagittal plane, from the top of the valve to the annulus. Table 2 summarizes the design parameters.
A steady state flow rate of the polymeric solution
was achieved by means of a syringe pump (KD
Scientific, USA) running at 0.6 ml/h, while a high
voltage power supply (Spellman, UK) assured the
tension of 12 kV for the electrospinning process.
Polymer deposition time was fixed to 1.5 h.
Structural characterization
Morphology of the heart valve prosthesis was investigated by scanning electronic microscopy (SEM).
Thickness measurements of the two commissural
regions of each leaflet were performed to check if
uniform polymer deposition occurred. For this aim
a measuring pressure of 10 g/cm2 was imposed to
the samples, as prescribed by ISO 7198.
Electrospun bioresorbable trileaflet heart valve prosthesis for tissue engineering
Table 2 | Heart valve design parameters
Tissue annulus diameter
19 mm
Valve inner diameter
14.5 mm
Valve height
11.5 mm
Arc radius of central leaflet coaptation area to
reduce peak stress
3 mm
Porosity (ε) was estimated according to the following relationship [29, 30]:
( )

ρ 

0
 ⋅ 100
ε =  1 − ρ 

(Eq. 1)
where ρ is the density as calculated from the weight
to volume ratio. Circular samples were cut out from
the leaflets of each valve to estimate ε. Thickness was
evaluated as previously described, while weight was
evaluated by means of analytical balance (Sartorius
CP124S, resolution 10-4 g) Density value of 1.145 g/
ml (as-purchased polymer) was considered for ρ0.
Functional testing
Electrospun trileaflet stentless heart valve prosthesis was tested in the VSI pulse duplicator (Vivitro
Systems, Inc., Canada), properly modified with a
thin glass window at the top of the testing site to
monitor the valve function, in the opening and in
the closing phases, with a camera [31].
The electrospun valve was located in the testing site
by means of custom-made retention PVC ring. Valve
commissures were loosely sutured to three posts at
120°, fixed on the ring, in order to prevent a possible
collapse in the diastolic phase (Prolene 7-0, Ethicon).
A mechanical no-leakage reference valve was inserted
in the tricuspid site. Functional signals (ventricular,
atrial, arterial pressures and arterial and tricuspid
Fig. 1 | PCL electrospun heart valve.
flows) were acquired. Hydrodynamic behaviour of
the prosthetic valve was investigated using saline solution (0.9% NaCl), as test fluid. A sine waveform
was selected to drive the pump at 60 bpm.
Due to the small size of the prosthesis a specific
investigating protocol was considered. Three stroke
volumes were selected (20, 30 and 40 ml) for a mean
pulmonary pressure of 30 mmHg. Each setting condition was averaged on 16 cardiac cycles and repeated
three times. Mean transvalvular pressure drop, cardiac output and energy loss were evaluated for all the
conditions investigated. Energy loss was computed as
the integral over time of the product of instantaneous transvalvular pressure and flow rate [32].
Kinematics of the prosthetic valve leaflets was
studied using the Kodak Ektapro camera (sampling
rate 250 frames/s), located on top of the testing site.
Numerical study
Prediction of stress distribution on valve leaflet
was determined by means of numerical simulation
(Comsol, Sweden). Because of valve symmetry only
one leaflet was modelled in the closed position (diastolic phase). Due to the particular retention system adopted (see “Functional testing” subsection),
leaflet surface relative to valve stent was considered
fixed while on the arterial side of the leaflet the
transvalvular diastolic pressure was imposed (35
mmHg corresponding to the imposed stroke volume
of 40 ml and mean pressure of 30 mmHg, respectively). The model was discretized by means of tetrahedral elements (about 11000). In this preliminary
stage the material was considered isotropic, being a
reasonable assumption due to the randomly orientation of polymeric fibers, with an elastic modulus
of 6.4 MPa (estimated by means of uniaxial tensile
test on dog-bone shaped specimens) and a Poisson
coefficient of 0.45.
RESULTS
The electrospun valve is showed in Figure 1.
Measured thickness of leaflets was 0.84 ± 0.14
mm, while estimated porosity was 89.13 ± 2.50%.
SEM micrographs showed a random arrangement
of polymeric fibers without beads defect (Figure 2),
with an average fiber diameter in the micrometric
range (3.11 ± 0.43 µm).
The average cardiac cycle for the electrospun valve,
acquired in the experimental session with the pulse
duplicator, is reported in Figure 3, while Figure 4
showed the measured functional valve parameters.
Mean transvalvular pressure drop was in the range
7-12 mmHg while the measured cardiac output was
in the range 1.18-2.35 l/min, suggesting that the
valve was affected by unsignificant leakage flow.
The estimate energy loss was in the range 30-100 mJ,
corresponding about to 28% of the ventricle energy
within a cardiac cycle.
Valve dynamic behaviour was recorded by camera
acquisition. The opening behaviour at the ejection
181
Costantino Del Gaudio, Alessandra Bianco and Mauro Grigioni
Fig. 2 | SEM micrograph of PCL
electrospun heart valve.
peak is reported in Figure 5a. Leaflets opened synchronously, even if a suboptimal opening occurred,
due to the mild setting conditions imposed by the
pulse duplicator. In the diastolic period the valve
did not maintain the correct shape showing a slight
rotation of the leaflets (Figure 5b).
The predicted stress distribution in the diastolic
phase is reported in Figure 6 for maximum loading conditions of the experimental protocol (stroke
volume of 40 ml and mean pressure of 30 mmHg).
Quite uniform stress distribution was predicted
within the leaflet showing an increase in the central
region of the free edge (about 100 kPa). Highest
stresses occurred at the intersection of the free edge
of the leaflet with the resembling stent structure, being in the range 200-250 kPa.
DISCUSSION
Valid alternatives to currently used heart valve
prostheses represent a relevant issue to be addressed
by means of tissue engineering applications. This
study reported the early results of the hydrodynamic
characterization of a trileaflet stentless electrospun
PCL heart valve.
The most immediate need for tissue engineered heart
valves and regenerative technology is in the pediatric
and young adult patients since results of valve replace-
60
150
40
100
20
50
0
0
-20
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (s)
0.7
0.8
0.9
1
Valve flow (ml/s)
Pressure (mmHg)
182
-50
Fig. 3 | Average cardiac cycle curves
(pressures and valve flow) for the
PCL electrospun heart valve for a
stroke volume of 40 ml.
Energy loss (mJ)
Cardiac output (l/min)
Mean pressure drop
(mmHg)
Electrospun bioresorbable trileaflet heart valve prosthesis for tissue engineering
14
12
10
8
6
15
20
25
30
35
40
45
15
20
25
30
35
40
45
2.5
2
1.5
1
150
100
50
0
15
20
25
30
Stroke volume (ml)
35
ment are not as favourable as in older adults [33, 34].
Children undergone to heart valve replacement can
have a positive outcome thanks to prosthetic devices
made by bioresorbable polymer. On this basis, small
size heart valve prosthesis was selected for a pivotal
study and infant pulmonary system [35] conditions
were replicated by means of the pulse duplicator.
Moreover, it has been also reported that a mild
conditioning in bioreactors is the first step for a successful tissue-engineered heart valve [13, 17]. Thus
moderate pulsatile circulation can promote a confluent monolayer of endothelial cells on valve cusps,
playing an important role in the long-term durability and functionality of tissue engineered heart valve
prostheses [17]. The investigated condition also resembled these settings, promoting a step of growing
of native structured valve directly in the patient with
the capability to regenerate during its own life while
the PCL structure is bioresorbed.
SEM investigation revealed PCL fibers randomly
arranged; this is the typical result of the electrospinning process when a fixed or slowly rotating target
A
B
40
45
Fig. 4 | Electrospun functional parameters in terms of measured transvalvular pressure drop, cardiac output
and energy loss for all the stroke volumes investigated (20, 30, 40 ml).
is used to collect polymeric fibers. In particular, the
low speed imposed to the metallic target was selected to favour a homogenous deposition of the polymer to resemble the very complex three-dimensional
geometry of a trileaflet heart valve. Moreover, the
high estimated porosity represents a valuable feature
for a tissue-engineered scaffold, promoting easy diffusion of nutrients to and waste products from the
implant and vascularization as well [36].
The experimental session in the pulse duplicator
highlighted a proper functioning of the herein proposed device also showing a good repeatability for
the test conditions, as suggested by the small standard deviations calculated. Cinematographic analysis revealed synchronously opening of the leaflets
during the ejection period and a good apposition in
the diastolic period, even if a slight rotation was detected which prevented to maintain the correct valve
shape.
However, this promising result needs to be validated under more severe loading conditions, e.g. resembling the aortic site or fatigue testing, and according
Fig. 5 | Opening frame at the systolic
peak (a) and in the diastolic phase (b).
183
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Costantino Del Gaudio, Alessandra Bianco and Mauro Grigioni
250
200
150
100
50
Fig. 6 | Numerical stress distribution within the leaflet in the
diastolic phase (system pressure 30 mmHg) [kPa].
to specific protocols for tissue engineering applications (cell-to-scaffold response). Hydrodynamic
assessment of the heart valve prosthesis is only the
preliminary step, because it should be pointed out
that artificial heart valves do not resemble the complex architecture and organization of native valve
leaflets. Semilunar heart valves are microscopically
composed of three layers: ventricularis (rich in radially aligned elastin fibers), spongiosa (largely
composed of glycosoaminoglycans) and fibrosa
(primarily composed of dense packed collagen fibers, arranged parallel to the cuspal free edge). Each
layer contributes to the definition of a biomechanical characteristic that enables the heart valve to its
specific function. Structural elements within each
layer provide the high anisotropic properties [34,
37]. Thus the ultimate goal of a functional tissue engineered heart valve prosthesis can be reached replicating the structure of a native valve. For this aim
mechanical conditioning in bioreactors of seeded
scaffolds seems to promote anisotropy, suggesting
that repetitive changes in strains can induce desired
organization in the leaflets, enhances tissue formation and thereby improve mechanical strength [15].
The reproducibility in fiber deposition was evaluated by means of uniaxial tensile test on several
specimens (at least four per test type) cut out from
electrospun mats collected with a similar experimental set-up. Average tensile modulus and tensile
strength were 6.4 ± 0.2 MPa and 0.84 ± 0.07 MPa,
respectively, highlighting standard deviations obtained in the production process to be within 10%.
Nevertheless the evaluation of repeatability of hydrodynamic characteristics of electrospun valves is
an issue to be addressed, it should be pointed out
that the in vitro assessment cannot give complete insights on the in vivo functioning. Due to the nature
of the device, i.e. stentless valve with particular sewing procedure, hemodynamic features will depend
on the implantation criteria adopted by the surgeon
to fit to patient’s anatomy. The latter possibility is
generally appreciated by surgeon to adapt the geometry to the pathological aortic root to be treated.
Numerical simulation showed the stress distribution within the leaflet in the closed position, assuming the measured diastolic pressure difference as input variable. Numerical simulations were carried out
under simplified assumptions not strictly resembling
the structural characteristics of the electrospun leaflet (the fibrous morphology was neglected) or the
application of the correct experimental boundary
conditions (leaflet surface connected to the stent
was considered fixed in the numerical simulation).
Although the influence of anisotropy in heart valve
leaflets should be considered for a more realistic
stress analysis, in this study numerical simulation
run under the assumption of isotropy. The aim was
to predict the peak stress locations depending on
the shape given to the leaflet, before cell seeding and
culturing: highest values were found where the free
edge is connected to the resembling stent structure.
The obtained results are to be intended as a starting point in view of design optimization of leaflet
geometry in order to minimizing critical stress concentrations. As previously reported, the anisotropic
nature of tissue engineered heart valve is generally
developed by means of dynamic culture systems,
i.e. bioreactors. For instance, Driessen et al. [38]
presented the prediction of the evolution of stress
distribution at selected time points after culturing
non-woven polyglycolic acid, coated with poly-4hydroxybutyrate, heart valve scaffold in a diastolic
pulse duplicator, finding a monotonically stress increase caused by the decrease of construct thickness.
Numerical analysis represents an improvement in
the prediction of stress distribution or in the design
of a novel device, but it should be also underlined
that the obtained results need to be validated with
experimental data in order to deal with a realistic
model. Differences arise comparing numerical results of tissue engineered leaflets and native porcine
leaflets, showing that the mechanical behaviour of
engineered leaflets is less nonlinear, less anisotropic
and lower coaptation occurred due the absence of
large radial strain [38]. However, computational
analysis of the mechanical response of a closed
valve is a relevant issue to be addressed, because
peak stresses in the fully closed position contribute
to tears and perforations of the leaflets [39].
Electrospinning was previously implemented as a
multistep procedure to realize a heart valve with the
Electrospun bioresorbable trileaflet heart valve prosthesis for tissue engineering
aortic root. Experimental parameters differed from
the ones here considered and a not homogenous
deposition of the polymer onto the target was obtained. It has been reported that the scaffold failed
due to weakness of the bellies of the leaflets and the
aortic ring, where the construct was very thin and
tore easily in in vitro testing session by means of
pulse duplicator [14].
Results here presented showed the technical feasibility to produce a functional bioresorbable heart valve
by means of electrospinning. A correct functional response was shown to the testing protocol by means of
pulse duplicator. Nevertheless under the mild setting
conditions considered, the valve did not undergone
to structural damage. Future development of this
study is represented by the optimization of the electrospinning parameters, valve design criteria and the
selection of the most suitable polymer, co-polymer or
blend to be used for tissue engineering heart valve application.
Acknowledgements
The authors wish to thank Dr. F. Nanni (Dipartimento di Scienze
e Tecnologie Chimiche, Università degli Studi “Tor Vergata”,
Rome, Italy) for SEM analysis.
Received on 28 October 2007.
Accepted on 19 February 2008.
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