Hélène Serier-Brault

Lucile Thibault

Magalie Legrain

Philippe Deniard

Xavier Rocquefelte

Philippe Leone

Jean-Luc Perillon

Stéphanie Le Bris #

Jean Waku #

Stéphane Jobic

Thermochromism in yttrium iron garnet compounds

Polycrystalline yttrium iron garnets (Y 3 Fe 5 O 12 ) were synthesized by solid-state reaction and characterized by X-ray diffraction, Mössbauer spectroscopy and UV-vis-NIR diffuse reflectance spectroscopy. The green color of the compounds have been assigned to the presence of a O 2- Fe 3+ ligand-to-metal charge transfer together with d-d transitions, the absence of Fe 2+ in samples being verified with Mössbauer measurements. By increasing the temperature to around 300°C, the YIG material shows a thermochromic behavior with a continuous color change from greenish to brownish due to a red-shift of the ligand-to-metal charge transfer. Y 3 Fe 5 O 12 host lattice was also doped with chromium ions and the same red-shift phenomenon was observed although the effect on the crystal structure is opposite. Indeed, Cr 3+ ions were proved to play a major role on the color either by an inductive effect or, more probably, by a direct effect on the electronic structure of the material as discussed in this work.

Introduction

Thermochromic inorganic materials change color with a temperature variation (heating or cooling) and offer potentialities for temperature indicators [START_REF] Nassau | The Physics and Chemistry of Color[END_REF][START_REF] Sone | Inorganic Thermochromism[END_REF] in different kinds of devices such as temperature sensors for safety (kitchen tools, hotplates, fridges…), laser marking or warning signals. The color change can be brutal or continuous as well as reversible or irreversible according to the involved mechanism. An irreversible color change is mainly associated with a chemical transformation such as decarbonation or dehydratation [START_REF] Robertson | [END_REF] for instance. The application of such materials is a little bit restricted and concerns principally the monitoring of temperature sensitive products whereas the reversible thermochromism is used in much more applications.

Each application requests specific characteristics such as color change temperature, color contrast or cyclability, i.e the number of color cycles that can be imposed to the material without any fatigue effect.

For oxide compounds, reversible thermochromism can be explained by a gradual reduction of the semiconductors band-gap 4 with a temperature increase (red-shift), or a crystallographic phase transition and/or an insulator/metal transition [5][6][7][8][9] . The restriction of semiconductors lies in the limited color choice and the likely slight color contrast due to a continuous variation of the color from pale yellow to red. Concerning materials getting phase transition (namely, first order transition), the color contrast can be very pronounced but the volume variation imposed by the structure transition may reduce considerably the material cyclability for an intensive use 8 . To obtain blue, green or pink color, i.e. a color not accessible with a unique charge transfer, transition metal must be present due to the d-d transitions which mainly occur in the visible region. However, these d-d transitions are less affected by a temperature variation than by the crystal field. Thus, the solution to obtain a larger panel of colors for thermochromic materials consists in using the mixing rule based on the combination of a semi-conductor material with a d-d transition-based compound. In that case, it is possible to design various reversible thermochromic effects with a control of the temperature transition and the hue of pigment, and a good cyclability. This mixing rule was already discussed in the literature for Bi 2 O 3 -CoAl 2 O 4 , Bi 2 O 3 -LiCoPO 4 and V 2 O 5 -Cr 2 O 3 blends 10 . However, this technique gets some shortcomings especially concerning the hue of the color which is not saturated enough because of its additive synthesis mechanism.

In this work, instead of mixing two oxides having complementary optical properties, we report the investigation of a well-known compound, the yttrium iron garnet, whose the green color originates from a mixing rule directly within the material. Actually, the green color of the YIG comes from both the combination of d-d transitions and a ligand-to-metal charge transfer with a consequent likely thermochromic behavior. For the first time, the color change of YIG, from green to brown with increasing temperature, is discussed together with the effect of chromium doping on thermochromism.

Synthesis and structure characterization of Y 3 Fe 5 O 12

Yttrium iron garnet (YIG) belongs to the garnet family with the chemical composition 12 and crystallizes in the space group Ia-3d (SG n° 230) 11 . In this structure, yttrium ions occupy a dodecahedral site (24c), a represents the octahedral site of Fe 3+ ions (Wyckoff position 16a) and d the tetrahedral site (Wyckoff position 24d). Each octahedron (tetrahedron) is linked to six (four) others via corner-sharing tetrahedra 11,12 (octahedra) to provide a three-dimension structure. The YIG structure is mainly described in the literature in the cubic space group Ia-3d but Rodic et al 13 . used the trigonal space group R-3 (SG n°148) to account for its magnetic behavior. In order to clarify the situation we carried out density functional theory (DFT) calculations using the WIEN2k code [START_REF] Blaha | 2k: An Augmented Plane Wave+LO Program for Calculating Crystal Properties[END_REF] and the on-site PBE0 hybrid functional [START_REF] Perdew | [END_REF] . Our results show that the magnetic structure described in the space group Ia-3d is more favorable than the one described in the space group R-3, by about 480 meV per formula units. Actually YIG oxide is well-known for its room temperature ferrimagnetic properties that are originated from the antiparallel coupling of the two Fe 3+ sublattices (octahedral and tetrahedral). The iron garnet exhibits also magneto-optical, thermal, electrical and mechanical properties such as radiation damage resistance, high thermal conductivity, high electrical resistivity, controllable saturation magnetization and others 16 .

{Y 3 }[Fe 2 3+ ] a (Fe 3 3+ ) d O
Conventional high-temperature solid-state reactions were performed to synthesize undoped Y 3 Fe 5 O 12 . The purity of the samples was checked by X-ray diffraction. All patterns were refined with a cubic structure (JCPDS n° 83-1027), reflecting that the compounds are single-phased.

Rietveld refinement were carried out on undoped YIG, as represented in Figure 1, and refined lattice parameter was determined to be equal to 12.3736(5) Å at room temperature, in good agreement with the literature 16,17 .

The oxidation state of irons cations were checked by 57 Fe Mössbauer analyses at room temperature. The spectrum of the undoped yttrium iron garnet is reported in Figure 2. This is a typical spectrum of single-phase yttrium iron garnet Y 3 Fe 5 O 12 16,18 . The obtained data can be fit with two sextets, the first magnetic sextet (sub-spectrum 1) corresponds to the iron in the octahedral site in the garnet structure with an isomer shift δ of 0.378 mm/s, a magnetic hyperfine field B Hf equal to 49.1 T and a relative intensity I of 60.5%, while the second sextet (sub-spectrum 2) corresponds to the iron in the tetrahedral site 11,12,16 with δ of 0.146 mm/s, B Hf equal to 39.7 T and I of 39,5%. Accounting for Mössbauer analyses, no Fe 2+ ions are present in the yttrium iron garnet.

Optical properties of Y 3 Fe 5 O 12 : effect of the temperature

Since its discovery in 1956, the optical properties of YIG have been investigated only a few times, mainly by Manning 19 , Wood 20 and Wickersheim 21 , on single crystals. If it is well-know that YIG materials exhibit a green color, we will discuss in more details the origin of the color in this compound although the presence of Fe 3+ ions in oxides mainly leads to red compounds as in Fe 2 O 3, ZnFe 2 O 4 or MgFe 2 O 4 for example 22 . The assignment of absorption spectra of Fe-based compounds is relatively difficult for several reasons: 1) the d-d transitions are spin-forbidden and consequently their intensity is expected to be quite weak. However in many oxides the spin rule is not respected because of spin-orbit coupling; 2) theses d-d bands often overlap the charge transfer edge, specially for Fe 3+ ions; 3) Fe 3+ and Fe 2+ often coexist and their d-d bands are localized in the same wavelength range; 4) the simultaneous presence of Fe 2+ and Fe 3+ can lead to an intervalency phenomenon [START_REF] Marfunin | Physics of Minerals and Inorganic Materials[END_REF] . In our compounds, Mössbauer analyses have confirmed the absence of Fe 2+ ions and consequently, the color of YIG is originated from the presence of Fe 3+ only.

Room temperature optical properties (absorption via Kubelka-Munk transformation) of the undoped YIG sample are reported in the inset of Figure 3. As already notified in the literature 19- 21,24 , the optical properties of YIG are originated from the combination of a charge transfer absorption and the Fe 3+ d-d transitions. The band gap absorption, located around 485 nm (2.57 eV), is associated to a charge transfer from the 2p orbitals of oxygen to the empty 3d orbitals of Fe 3+ ions 20,21,25 (O 2-+ Fe 3+ O -+ Fe 2+ ). The optical spectrum reveals also different absorption bands due to the d-d transitions of Fe 3+ in both tetrahedral and octahedral sites. The bands assignment can be performed from the Tanabe Sugano diagram for the d 5 configuration. In the crystal field theory, the ligand field in the tetrahedral coordination being lower than in the octahedral one, we can thus assert that the band located at 900 nm corresponds to the 6 A 1 4 T 1 transition of iron ions in the octahedral site [19][20][21][START_REF] Wemple | [END_REF] . Furthermore, the bands located around 600 nm and the shoulder at 700 nm can be assigned to the 6 A 1 4 T 2 of Fe 3+ ions in the octahedral site with a contribution of transitions 6 A 1 4 T 1 , 4 T 2 for the tetrahedral coordination. In the same way, a potential contribution of the transition 6 A 1 4 A 1 4 E 1 cannot be fully ruled out even if expected at higher energy.

Accounting to the optical measurements, the greenish color of YIG compounds is originated from the slight absorption in the violet range (charge transfer) as well as the presence of d-d transitions in the orange-red region. Consequently, a temperature increase should cause a charge transfer shift associated with the thermal expansion of the unit cell and an increase of metaloxygen interatomic distances. If the charge transfer shift is large enough, the YIG color can be modified. One can notice that normally the d-d transitions should be almost not affected by a temperature variation 10 . The thermochromism of undoped YIG was investigated and followed by diffuse reflectance spectroscopy from room temperature to 300°C (Figure 3). As expected, the

Optical properties of Cr-doped Y 3 Fe 5 O 12

Y 3 Fe 5 O 12 compounds were also doped with chromium ions (up to 10 mol.%) and the purity of all doped samples was checked by X-ray diffraction. In Figure 6 is depicted the variation of the lattice parameters with the chromium content. The unit cell variation can be attributed to the smaller ionic radii of six-fold coordinated Cr 3+ (0.61 Å) compared to that of six-fold high spin Fe 3+ (0.64 Å) 26 . This result is in good agreement with literature where the preference of Cr 3+ for octahedral site was proved, especially from Mössbauer measurements 11,17,27 .

The introduction of chromium cations in the YIG structure leads to a clear color change of the material from greenish to brownish (Figure 7). This color change is very similar to that induced by a temperature increase in YIG although the chromium doping induces a contraction of lattice contrary to the effect of a temperature increase. The optical properties of doped compounds have been also investigated by diffuse reflectance and compared with the undoped sample, all optical spectra being reported in Figure 7. The addition of chromium causes a visible red-shift of the charge transfer, mainly responsible for the color change, and a reinforcement of the d-d transition located at around 700 nm, the others being affected neither in intensity nor in position. The impact of chromium addition on the charge transfer red-shift can be explained according two ways: 1) the Cr 3+ gets an inductive effect on the Fe-O bonds and consequently, on the O 2- Fe 3+ charge transfer 28,29 or 2) the Cr 3+ has a direct contribution with the generation of a specific O 2- Cr 3+ charge transfer.

In the former case, the red-shift can be explained by an inductive effect of the introduction of chromium ions in the host lattice. If we consider the purely ionic model of Brown and Altermatt 30 to estimate bond valence, we can assume that the substitution of Fe 3+ by Cr 3+ in one octahedron needs an adjustment of bond distances around this octahedron to maintain the ionic bond valence of each ion. Actually, the substitution of one iron ion by a chromium one in a regular FeO 6 octahedron (without any change of metal-oxygen bond lengths) leads to a valence of + 2.70 for Cr to compare with a valence of +3.00 for Fe in the isolated octahedron, which means 10% difference. To counterbalance the valence deviation, metal-oxygen bonds in the octahedron will need to be reduced and will impact the others metal-oxygen bonds (and especially the Fe-O bonds) in the surrounding. Thus, the introduction of many chromium ions would locally create an effect of compression-dilatation of O-Fe bonds within the structure to maintain the ideal valence for all ions, which could explain the red-shift and simulate the effect of temperature.

In the latter case, the red-shift going from undoped Y 3 Fe 5 O 12 to Cr 3+ doped Y 3 Fe 5 O 12 would be correlated to the appearance of new accessible new electronic states within the forbidden gap that play a major role in absorption phenomena. Actually, the band structure of the YIG can be viewed as built upon a valence band (VB) consisting mainly of O-2p orbitals and a conduction band (CB) consisting of Y-4d orbitals. The Fe-3d levels are located between the VB and the CB (Figure 8) and give rise to absorptions at energy lower than 2.4 eV (d-d transitions) as explained previously (see inset in Figure 8). Charge transfer from oxygen towards iron takes place at higher energy (about 2.4 eV) and is associated to the oxidation of O 2-anions into O -ones concomitantly with the reduction of Fe 3+ cations into Fe 2+ ones, O -and Fe 2+ being photogenerated metastable species which recombined instantaneously. Due to strong Coulomb repulsions related to the pairing of two electrons in the same orbital, the 3d 6 configuration of Fe 2+ cations is expected at much higher energy than the 3d 5 configuration of Fe 3+ cations, naturally strongly stabilized due to its half-filled sub-shell. When Cr 3+ cations substitute Fe 3+ ones, extra intrasite d-d transitions occur which may explain the appearance of absorption shoulders and intensity changes in the low energy part of the absorption spectrum (Figure 7).

But the Cr 3+ d-d transitions overlap with those of Fe 3+ and do not modify the hue. In contrast, chromium strongly influences the positioning in energy of the charge transfer. Indeed, the O 2- Cr 3+ charge transfer is expected to take place at a lower energy than the O 2- Fe 3+ one because the 3d 4 high spin electronic configuration of Cr 2+ ions is expected to lye below the 3d 6 high spin configuration of Fe 2+ cations 31 . Consequently, the Cr 3+ /Fe 3+ substitution simulates a temperature increase, the higher the substitution rate, the more red-shifted the charge transfer.

Concluding remarks

In that work, we have reported for the first time the reversible thermochromic behavior of the yttrium iron garnet. The intrinsic optical properties of YIG are originated from O 2--Fe 3+ ligandto-metal charge transfer together with d-d transitions of Fe 3+ ions in octahedral and tetrahedral configuration. By increasing the temperature, the color changes continuously from a greenish to a brownish tone due to a regular red-shift of the absorption threshold associated to the ionic bonds dilatation.

The chromium doping tends to simulate the effect of temperature increase with regular red-shift when raising the Cr 3+ content. The role of Cr 3+ is explained by a modification of the electronic structure and the apparition of a O 2-Cr 3+ charge transfer at lower energy than that of O Room-temperature UV-visible diffuse reflectance spectra were recorded on a Perkin Elmer Lambda 1050 for each finely ground sample. For room-temperature spectra, the spectrophotometer was equipped with an integrating sphere coated with Spectralon®, a highly reflecting fluoropolymer. For measurements versus temperature, the spectrometer was fit with a praying mantis accessory. All spectra were recorded between 200 and 1200 nm with 1 nm step.

The absorption (α/S) values were calculated from the reflectance using the Kubelka-Munk transformation: α/S = (1-R)²/2R where R is the reflectance at the given wavelength, α is the absorption coefficient, and S is the scattering coefficient. The S value is supposed to be particle size independent when grains size is larger than a few micrometers, as in our case.

Mössbauer spectra were obtained with a constant acceleration automatic folding Elscint-type spectrometer using a room temperature 57 Co(Rh) source in transmission geometry and a triangular reference signal. α-Fe was used as reference. The spectra were computed with a least square routine using Lorentzian lines. To obtain an acceptable accuracy, a velocity scale of 11.78 mm/s was used at room temperature.

Table of Contents Synopsis

The effect of temperature increase on the optical properties of yttrium iron garnet has been investigated by diffuse reflectance. This work evidences a sample color change from green to brown by applying a temperature variation. This color change originates from a continuous red-shift of the O 2--Fe 3+ ligand-to-metal charge transfer whereas no impact on the d-d transition is observed. Furthermore, the samples have been doped with Cr 3+ ions and the introduction of chromium cations has the same effect than a temperature increase although its effect on the crystal structure is opposite.

Synopsis

5 .

 5 rise of temperature implies a continuous red-shift of the absorption edge whereas the d-d absorption bands are weakly affected. This regular shift can be explained by a slight elongation of ionic bonds with temperature which is evidenced by an increase of the lattice parameters (Figure4). The dilatation implies a reduction of the electrostatic interaction of the d electrons resulting in a lowering of the position of the d 6 energy level of Fe 2+ . Consequently the O 2-Fe 3+ charge transfer occurs at lower energy. The continuous red-shift is around 0.05 eV/100°C which is quite low compared to the usual shift (0.1 eV/100°C22 ) observed in semiconductors such as ZnO, Bi 2 O 3 ... In order to investigate this aspect from first-principles calculations we have considered 3 models considering the magnetic structure described in the space group Ia-3d with increasing the a parameter of the cubic unit cell, i.e. [a + 0], [a + 0.1] and [a + 0.2] Å, respectively. We found a linear relationship between the charge transfer band position and the cell parameters showing a redshift of -0.05 eV / 0.1 Å. Although this theoretical shift is too small compared to experiments it provides a qualitative confirmation that the volume expansion leads to a redshift of the charge transfer optical transitions. To illustrate the red-shift, pictures were taken for different temperatures and are reported in Figure Thus the evolution of the charge transfer with the temperature leads to a visible color change. Finally, undoped YIG material exhibits a thermochromic behavior at relatively low temperature with a gradual color change from greenish to brownish.The originality of this compound lies in the association of a charge transfer located in the violet range with d-d transitions in the visible region which leads to a greenish initial color that is impossible to obtain with only a charge transfer. Furthermore, the color change with temperature is due to the presence of charge transfer only (abrupt threshold moving from 2.52 eV to 2.39 eV) and not to the d-d transitions whose position does not change with temperature. Thus, in this study, instead of combining a semiconductor and a d-d transition based-compound to obtain a new inorganic thermochromic material, we have an original material where the mixing rule occurs10 directly within the same single-phase material.

6

  hour to finally lead to greenish powder. Each powder was ball milled 10 minutes before characterization.Phase purity and crystal structure were examined by X-ray diffraction (XRD) analysis with a Bruker AXS D8 advanced automatic diffractometer with Cu K-L 3 radiation (germanium monochromator) operating at 40 kV and 40 mA. The XRD patterns were collected in the 10-90 degrees 2θ with a 0.02° step. The structures were refined with the full-matrix least-squares technique using the Jana2006 program[START_REF] Petricek | The Crystallographic Computing System JANA[END_REF] . The errors bars on all parameters were calculated as the following: 3 x the Berar's 33 coefficient x the standard deviation. To record data as a function of temperature, we used a Brüker "D8 Advance" powder diffractometer equipped with an Anton Paar HTK 1200 N high temperature attachment. Data were collected in Bragg-Brentano geometry with a Cu-anode X-ray source (Cu K-L2 , Cu K-L3 ) operated at 40 kV and 40 mA. We used a 1-D position-sensitive detector ("Vantec" detector), with an active area restricted to 3° 2θ to improve the angular resolution of XRD patterns. The Cu Kβ radiation was filtered by means of a Ni foil.

FIGURES CAPTIONFigure 1 .

 1 FIGURES CAPTION

Figure 2 .

 2 Figure 2. Mössbauer spectrum of Y 3 Fe 5 O 12 . Experimental data are represented with open circles

Figure 3 .

 3 Figure 3. Kubelka-Munk transformed reflectivity vs wavelength of undoped Y 3 Fe 5 O 12 at room

Figure 4 .

 4 Figure 4. Evolution of refined lattice parameters of Y 3 Fe 5 O 12 with temperature.

Figure 5 .

 5 Figure 5. Photographs of undoped YIG at different temperatures (The temperature

Figure 6 .

 6 Figure 6. Variation of the lattice parameters of Y 3 Fe 5 O 12 :x% Cr with the molar percentage of chromium.

Figure 7 .

 7 Figure 7. Room-temperature α/S spectrum (Kubelka-Munk transformation) of undoped YIG and Cr 3+ -doped YIG and photographs of the evolution of room-temperature color with introduction of Cr 3+ ions in the YIG structure.

Figure 8 .

 8 Figure 8. Schematic band structure of YIG when doped with chromium ions and Tanabe-Sugano diagram for the d 5 configuration (inset).

  Figure 1

Figure

  Figure 4

  2--Fe 3+ .

	EXPERIMENTAL SECTION
	Conventional high-temperature solid-state reactions were performed to synthesize Y 3 Fe 5 O 12 and
	Y 3 Fe 5-x Cr x O 12 compounds. The constituent oxides Y 2 O 3 (Alfa Aesar, 99.9%), Fe 2 O 3 (Alfa Aesar,
	99.945%, <5 µm powder) and Cr 2 O 3 (Alfa Aesar, 99.997%) were used as starting materials. The
	raw materials were weighed in stoechiometric proportions and ball milled with ethanol in a
	Fritsch Pulverisette 7. Then, the blends were dried at 100°C and annealed at 1350°C in air during

  4

			12.410
		4	12.380
			12.405
			12.400
			12.375
		a (Å) 3	12.390 12.370 a (Å) 12.395	YIG	3% Cr 3+ 6% Cr 3+ 10% Cr 3+
	K/S	2	12.385
			12.380 12.365
		1	12.375	0 Y 3 Fe 5 O 12	1	2	3	4	5	6	7	8	9	10
				0 Y 3 Fe 5 O 12 + 3% Cr 50 3+ Y 3 Fe 5 O 12 + 6% Cr 3+		100		150 mol.% Cr		200		250
				Y 3 Fe 5 O 12 + 10% Cr 3+		Temperature (°C)
		400 0		600		800 Figure 6			1000		1200
							Wavelength (nm)
							Figure 7

(degrees)

AUTHOR INFORMATION

Corresponding Author E-mail: helene.brault@cnrs-imn.fr; stephane.jobic@cnrs-imn.fr Fax: (+33)240-373-995


Gaganpreet Kaur Behrh

Romain Gautier

Camille Latouche

Stéphane Jobic

Hélène Sérier-Brault

Synthesis and Photoluminescence Properties of Ca 2 Ga 2 SiO 7 :Eu 3+ Red Phosphors with an Intense 5 D 0 → 7 F 4 Transition

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Synthesis and photoluminescence properties of Ca 2 Ga 2 SiO 7 :Eu 3+ red phosphors with intense 5 D 0  7 F 4 transition Gaganpreet Kaur Behrh, Romain Gautier, Camille Latouche, Stéphane Jobic, Hélène Serier-Brault* Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes cedex 3, France KEYWORDS : melilite, photoluminescence, europium Supporting Information Placeholder ABSTRACT: Novel melilite type Ca 2 Ga 2 SiO 7 : Eu 3+ red emitting phosphors with different Eu 3+ contents were synthesized via high temperature solid-state reaction. Crystal structure, optical absorption and photoluminescence properties were investigated while DFT calculations were performed on host lattice. The excitation spectra indicate that phosphors can be effectively excited by near-ultraviolet light for a potential application in white lightemitting diodes. Due to the abnormally high intensity emission at about 700 nm arising from the 5 D 0  7 F 4 transition of Eu 3+ , the phosphors Ca 2 Ga 2 SiO 7 : Eu 3+ show a deep red emission with chromaticity coordinates (0.639, 0.358). This high intensity has been attributed to the presence of noncentrosymmetric sites for Eu 3+ ions and is shown to be potentially interesting for applications as light emitting diodes (LEDs) for Solid-State Lighting (SSL) and display devices.

White light emitting diodes (WLEDs) have been attracted much attention due to their considerable impact on issues such as energy consumption and environment. Therefore, WLEDs are thought to have brought a revolution in energy-efficient lighting and are promising to replace conventional light sources such as incandescent and fluorescent lamps in the near future 1,2 . The most common approach to obtain white light is to employ the yellow-emitting YAG:Ce 3+ phosphor and blue InGaN chips. Unfortunately, the drawbacks of this method are a low color rendering index (CRI <80) and a high correlated color temperature (CCT) because of the scarcity of red emission 3,4,5 . Thus, it is meaningful to develop some novel red-emitting phosphors with improved properties for application in WLEDs. Currently, the most extensively studied red emitting rare-earth ion is the trivalent Eu 3+ which is a relevant component of artificial white light with its main emission around 615 nm. If the spectral features of the generated white light need to be similar to that of natural white light, the multiband Eu 3+ emissions are expected to be quite useful because of the better overlap 6 . Currently Eu 3+ -doped luminescent materials are widely used as commercial red phosphors in lighting and displays 7,8 , such as YVO 4 :Eu 3+ , Y 2 O 2 S:Eu 3+ , Y 2 O 3 :Eu 3+ . However, most of the present Eu 3+ -doped phosphors show dominant emissions corresponding to 5 D 0  7 F 1 (~ 593 nm) or 5 D 0  7 F 2 (~ 612 nm) transition of Eu 3+ ions. On the contrary, the intense 5 D 0  7 F 4 (~ 700 nm) emission is relatively rare 7,8 .

Gallate based oxides can act as host materials for light emitting diodes, detectors, photocatalysis, high voltage and multicolor phosphors 9,10 . The calcium gallate Ca 2 Ga 2 SiO 7 belongs to the melilite family and has been much investigated as solid-state laser 11,12 . Very recently, the first study about rare-earth (Eu 2+ , Tb 3+ and Ce 3+ ) doped Ca 2 Ga 2 SiO 7 powder and its photoluminescence for WLEDs has been published 13 . However, till now no investigation has been reported about the luminescence properties of Eu 3+ -doped Ca 2 Ga 2 SiO 7 . Thus, this paper presents the analysis of luminescence properties of Eu 3+ -doped Ca 2 Ga 2 SiO 7 with intense 5 D 0  7 F 2 (~ 610 nm) and 5 D 0  7 F 4 (~ 700 nm) emissions. The compound Ca 2 Ga 2 SiO 7 belongs to the melilite family with the space group P-42 1 m (n°. 113, a = 7.793 Å, c = 5.132Å 14 ). This structure is built upon a regular tetrahedral site fully occupied by Ga 3+ and a much distorted tetrahedral site where Si 4+ and Ga 3+ are randomly distributed. In this framework, the Ca 2+ cations are located in an eightfold coordinated site called a Thomson cube. According to the results presented in Fig. 1, all the obtained samples crystallized in the melilite (ICSD file 027428). When the dopant concentration exceeds 5%, a detailed analysis of the patterns showed very weak diffraction peaks at 31.8° et 32.1°, which indicates the appearance of an unidentified impurity phase. Because of similar ionic radii in an eightfold coordinence, the Eu 3+ ions (r = 1.066 Å 15 ) are expected to substitute Ca 2+ (r = 1.12 Å 15 ). Replacing divalent calcium ion by trivalent europium ion necessitates balancing the extra positive charge. It may easily occur by mean of Ca-vacancies formation or incorporation of interstitial O 2-ions (one per two Eu 3+ ions). In our case, we considered the formation of cationic vacancies by using the formal composition Ca 2-3x/2 Eu x Ga 2 SiO 7 (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1). Absorption properties of the non doped host lattice were investigated to determine the optical band gap. Diffuse reflectance was recorded in finely ground sample and the band gap can be estimated according to the expression proposed by Tauc, Davis and Mott 16 :

[F(R ∞ )h] 1/n = A(h-E g )
where A is a proportional constant, E g is the optical band gap, h is the photon energy in eV, the exponent n denotes the nature of the transition (in our case, we will consider a direct allowed transition, thus n = 1/2), and F(R ∞ ) is Kubelka-Munk function which can be defined as F(R ∞ ) = (1-R)²/2R where R is the reflectance at the given wavelength. The [F(R ∞ )h] 2 was plotted as function of h and was reported in Fig 2 . The optical band gap E g was determined from the value associated with the intersection point of the line tangent to the plotted curve inflection point on the horizontal axis (h = 0). As a result, the band gap of the non doped Ca 2 Ga 2 SiO 7 host lattice is equal to 5.37 eV. DFT calculations were performed on the non doped Ca 2 Ga 2 SiO 7 to estimate the band gap using the VASP package. After optimization 17,18,19 , cell parameters appear to be in a good agreement with respect to experimental ones. Indeed, the a and b optimized lattice parameters are computed around 7.90 Å whereas it is 7.79 Å experimentally. Also, the c parameter is computed at 5.21 Å vs. 5.13 Å in the crystal. Based on these grounds, we now consider the electronic structure (DOS) of the compound. As expected, the Valence Band (VB) is mainly built upon O(2p) orbitals (Fig. 3 + ESI). One should notice that the Ga contribution is non-negligible in this region and especially, at the top of the VB, the "d" contribution is growing whereas the "s" and "p" are decreasing. The bottom of the Conduction Band (CB) is mainly localized on "d" orbitals of calcium. It allows us to assess that the charge transfer is from O(2p) to Ca(d) and Ga(s) orbitals. Finally, the gap (~ 3.4 eV) is underestimated at this level of accuracy (GGA). An additional test computation using the HSE 20 functional together with a 3 x 3 x 4 k-mesh (25 % Hartree-Fock of exact exchange, a range-separation parameter equal to 0.2) tends to fit more accurately the observed gap (≈ 5 eV vs. 5.3 eV experimentally). The photoluminescence excitation (PLE) and emission spectra of Ca 2 Ga 2 SiO 7 : 5% Eu 3+ are shown in Fig. 4. The excitation spectrum of gallosilicate show a broad band at around 267 nm which is due to the charge transfer states (CTS) caused by the transfer of electron from oxygen 2p to the empty 4f orbital of Eu 3+ . In addition, the PLE spectrum gets several sharp lines, which are due to the intra-configurational f-f transitions of Eu 3+ , i.e.,7 F 0  5 D 2 (464 nm), 7 F 0  5 D 3 (413 nm), 7 F 0  5 L 6 (393 nm) and 7 F 0  5 D 4 (361 nm) . Obviously, the most efficient excitation peak is the one originated form 7 F 0  5 L 6 transition (393 nm) which means that this phosphor can be effectively excited by UV LED chip (around 400 nm). The emission spectra of Eu 3+ activated Ca 2 Ga 2 SiO 7 consists in a multi-band emission at about 585, 597, 617, 650 and 700 nm which is assigned to the 5 D 0  7 F J (J = 0, 1, 2, 3 and 4) transitions of Eu 3+ ions, respectively. Among these emission peaks, the two most intense lines are the emissions located at 613 and 700 nm (red region) from 5 D 0  7 F 4 and 5 D 0  7 F 2 transitions. It is well-known that 5 D 0  7 F 0, 1 transition is directed by selection rules for intermediate magnetic-dipole coupling ∆J=0, ±1, and the 5 D 0  7 F 2, 4, 6 are allowed electro-dipole transitions. Generally, when the Eu 3+ ion occupies a crystallographic site with inversion symmetry, its magnetic-dipole transition 5 D 0  7 F 1 orange emission dominates in the emission spectrum, while the electric-dipole transitions 5 D 0  7 F 2, 4 red emission if the Eu 3+ ion is located at an non-inversion center 7,10 . The intense emissions of 5 D 0  7 F 2, 4 (617 and 700 nm) transitions in Fig. 4 indicate that the Eu 3+ ions in Ca 2 Ga 2 SiO 7 host occupy a site with non-inversion symmetry which is coherent with the melilite The evolution of the photoluminescence properties with the doping rate is depicted in Fig. 5 for the phosphors Ca 2 Ga 2 SiO 7 : x% Eu 3+ (with x = 1, 2, 3, 4 and 5%) monitored at 393 nm. The emission intensity increases gradually with the doping rate without getting any concentration quenching. Consequently, the one parameter that prevents to increase the doping rate to enhance emission intensity is the solubility limit of Eu 3+ in the host lattice because over 5% of Eu 3+ ions, a unidentified secondary phase appears on XRD pattern. The Commission Internationale de l'Eclairage (CIE 1931) chromaticity diagram was depicted in Fig. 5 for the Ca 2 Ga 2 SiO 7 : 5% Eu 3+ phosphor excited at 393 nm. For all samples, the color coordinates (x,y) are located in the red region. Referring to the National Television System Committee (NTSC illuminant red= (0.67;0.33)) 21 , the 5%-doped gallium melilite exhibit well comparable pure red CIE color coordinate (0.639, 0.358) which is due to the huge emission intensity of the 5 D 0  7 F 2,4 transitions.

Trivalent europium-doped Ca 2 Ga 2 SiO 7 phosphors were successfully synthesized via solid-state route. Taking into account the charge balance, two Eu 3+ ions substitute three Ca 2+ ions in the samples. In these conditions, the maximal solubility limit is equal to 5% molar. The phosphors show intense emission peaks at 617 and 700 nm from the 5 D 0  7 F 2 and 5 D 0  7 F 4 transitions of Eu 3+ . The dominant emission at 700 nm results in deep red-emission with CIE chromaticity coordinates of (0.637, 0.358). Hence, Ca 2- ions are located in a noncentrosymmetric site, which leads to intense emission peaks at 617 and 700 nm arising from the 5 D 0  7 F 2,4 transitions of Eu 3+ . Moreover, the abnormally dominant emission at 700 nm ( 5 D 0  7 F 4 ) results in deep red-emission with CIE chromaticity coordinates of (0.637, 0.358).

Figure 1 .

 1 Figure 1. Room-temperature powder XRD patterns of the samples Ca 2 Ga 2 SiO 7 : x% Eu 3+ (with x = 0, 1, 2, 3, 4 and 5%).

Figure 2 .

 2 Figure 2. Plot of [F(R ∞ )h] 2 versus h for the non doped Ca 2 Ga 2 SiO 7 .

Figure 3 .

 3 Figure 3. Density of states of the non doped Ca 2 Ga 2 SiO 7 compound.

Figure 4 .

 4 Figure 4. Excitation spectrum monitored at  em =616 nm (blue) and emission spectrum monitored at  exc = 393 nm (red) of Ca 2 Ga 2 SiO 7 : 5% Eu 3+ at room temperature.

  , the presence of unique emission line at 576 nm ( 5 D 0  7 F 2 transition) indicates that Eu 3+ occupies only one site in the lattice, namely the Ca 2+ site of the investigated host lattice.

Figure 5 .

 5 Figure 5. Emission spectra monitored at 393 nm of Ca 2 Ga 2 SiO 7 : x% Eu 3+ (with x = 1, 2, 3, 4 and 5%) at room temperature. Inset: The CIE chromaticity diagram for sample Ca 2-3x/2 Eu x Ga 2 SiO 7 (x = 0.1) excitated at 393 nm.

TABLE OF CONTENTS

 OF Eu 3+ -doped Ca 2 Ga 2 SiO 7 red phosphors were successfully synthesized by solid-state reaction. In the melilite structure, Eu 3+

F 4

3x/2 Eu x Ga 2 SiO 7 (x = 0.1) can be a potential candidate for red LEDs phosphor due to its high luminescence efficiency under UV irradiation and its high color purity.

Funding Sources

This work was supported by the CNRS and the Ministère de l'Enseignement Supérieur et de la Recherche.

AUTHOR INFORMATION

Corresponding Author * helene.brault@cnrs-imn.fr

Author Contributions

All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interests.


Marek Kucharzewski

Ewa Rojczyk
email: ewarojczyk@gmail.com

Katarzyna Wilemska-Kucharzewska

Renata Wilk

Jacek Hudecki

Marek J Los

Novel trends in application of stem cells in skin wound healing

Keywords: stem cells, wound, healing, mesenchymal

The latest findings indicate the huge therapeutic potential of stem cells in regenerative medicine, including the healing of chronic wounds. Main stem cell types involved in wound healing process are: epidermal and dermal stem cells, mesenchymal stem cells (MSCs), endothelial progenitor cells (EPCs) and hematopoietic stem cells (HSCs).

In the therapy of chronic wounds, they can be administrated either topically or using different matrix like hydrogels, scaffolds, dermal substitutes and extracellular matrix (ECM) derivatives. Stem cells are proven to positively influence wound healing by different direct and indirect mechanisms including residing cells stimulation, biomolecules release, inflammation control and ECM remodelling. MSCs are especially worth mentioning as they can be easily derived from bone-marrow or adipose tissue.

Apart from traditional approach of administering living stem cells to wounds, new trends have emerged in recent years. Good healing results are obtained using stem cell secretome alone, for example exosomes or conditioned media. There are also attempts to improve healing potential of stem cells by their co-culture with other cell types as well as by their genetic modifications or pretreatment using different chemicals or cell media. Moreover, stem cells have been tested for novel therapeutic purposes like for example acute burns and have been used in experiments on large animal models including pigs and sheep.

In this review we discuss the role of stem cells in skin wound healing acceleration. In addition, we analyse possible new strategies of stem cells application in treatment of chronic wounds.

Introduction

A chronic wound is a loss of skin or other tissues that do not heal during the time normally needed to heal others wounds. Treatment of these wounds can be long-lasting, requires specific medical skills and constitute a great challenge for contemporary medicine.

The problem affects an estimated twenty million people around the world. This number may significantly increase due to ageing population and escalating incidence of civilization diseases such as obesity and diabetes [START_REF] Bumpus | The ABC's of wound care[END_REF]. Globally, by 2020 the wound care market is projected to surpass 22 billion USD per year [START_REF] Tricco | A systematic review of cost-effectiveness analyses of complex wound interventions reveals optimal treatments for specific wound types[END_REF].

Impaired wound healing increases the number of operative procedures, lengthens hospital stays and worsens a patient's susceptibility to infection [START_REF] Butler | Stem cells and burns: review and therapeutic implications[END_REF]. Thus, many scientists focus on determining different factors affecting the process of wound healing, among others those related to the use of certain drugs, improper diet and exposure to environmental pollution. It is also possible to identify specific diseases that increase the risk of developing non-healing ulcers and to determine the effect of hypoxia on the healing process [START_REF] Burns | Impairments to wound healing[END_REF].

The methods that are currently used in the treatment of chronic wounds are multifactorial and include several steps. First and foremost, direct causes are sought that have made the treatment process difficult and attempts are being made to eliminate them. If, despite this, healing is still very slow or does not progress, the specific methods are used to support the process. However, traditional wound care procedures using debridement, infection control and typical dressings have limited efficacy, especially for treatment of chronic wounds, which is still a challenging task for healthcare experts.

The development of biomedical engineering has increased the interest in working out new therapeutic approaches for wounds difficult to heal. These include for example gene therapy, cell therapy, biological dressings, growth factors delivery and engineered skin equivalents.

Among these new treatment modalities, stem cell-based therapies have gained interest as a part of regenerative medicine.

Currently, a rising number of stem cell therapies has been tested in clinical and preclinical trials in the context of their impact on wound healing [START_REF] Leavitt | Stem cells and chronic wound healing: state of the art[END_REF][START_REF] Tsai | Mesenchymal stem cell in wound healing and regeneration[END_REF].

The use of stem cells in the treatment of chronic wounds is a novel approach that uses processes that naturally occur in wound healing such as the action of growth factors, stimulation of immune processes and regulation of the inflammatory process, as well as improving the blood supply to developing tissue, which speeds up epidermis growth [START_REF] Kolle | Enrichment of autologous fat grafts with ex-vivo expanded adipose tissue-derived stem cells for graft survival: a randomised placebo-controlled trial[END_REF].

The aim of this review is to summarize mechanisms by which different stem cells influence wound healing process and to describe latest strategies used to improve their therapeutic potential in the context of non-healing wounds. Moreover, we will mention some clinical trials and experiments on animal models, that support the use of stem cells, especially mesenchymal stem cells (MSCs), in chronic wound treatment. Finally, we will discuss limits and benefits of stem cell therapy in comparison to traditional ones.

Adult stem cells related to wound healing process

Stem cells, due to their properties -differentiation into other cell types and the potential for unlimited proliferation -are one of the most popular issues of contemporary medicine and biological sciences. According to their ability to differentiate, they can be divided into toti-, pluri-, multi-and unipotent, while according to their source -into embryonic (isolated from the blastocyst), adult (isolated from the mature organism, also known as somatic stem cells) and obtained from the umbilical cord or placenta. Specific type of stem cells are induced pluripotent stem cells (iPSCs), which are generated directly from adult cells by introduction of several genes encoding transcription factors [START_REF] Behr | Stem cells[END_REF].

In the case of injury or minor burn, stem cells take their action and take part, along with the surrounding extracellular matrix (ECM) and growth factors, in regenerative processes.

Apart from epidermal and dermal stem cells (found in the skin), MSCs as well as endothelial progenitor cells (EPCs) and hematopoietic stem cells (HSCs) deserve special attention, as they (directly or indirectly) affect wound healing process and can be potentially used for cell therapy. These three cell types are components of stromal vascular fraction (SVF) -the aqueous fraction derived from enzymatic digestion of lipoaspirate, which itself has been also used for regenerative wound therapy [START_REF] Bora | Adipose tissue-derived stromal vascular fraction in regenerative medicine: a brief review on biology and translation[END_REF][START_REF] Cervelli | Application of enhanced stromal vascular fraction and fat grafting mixed with PRP in post-traumatic lower extremity ulcers[END_REF].

Main sources of stem cell populations involved in wound healing process are presented in Fig. 1, whereas most important functions of stem cells, which positively influence this process are summarized in Fig. 2.

Epidermal and dermal stem cells

Most of skin epidermal stem cells reside within hair follicle bulge, so superficial injuries heal rapidly, without leaving a scar. In this case, stem cell-derived daughter skin cells exit hair follicle and migrate to the basal layer and sebaceous gland of the epidermis. In deeper injuries the scar is formed, whereas epidermal appendages are not regenerated [START_REF] Ito | Stem cells in the hair follicle bulge contribute to wound repair but not to homeostasis of the epidermis[END_REF][START_REF] Li | Epidermal Stem Cells in Skin Wound Healing[END_REF][START_REF] Taylor | Involvement of follicular stem cells in forming not only the follicle but also the epidermis[END_REF][START_REF] Zhang | Therapeutic potential of stem cells in skin repair and regeneration[END_REF]. Hair follicle stem cells (HFSCs) and interfollicular stem cells (two types of epidermal stem cells) are main sources of new keratinocytes essential to reepithelialise the wound. Among them, only interfollicular stem cells reside in the basal layer of the interfollicular epidermis [START_REF] Liang | Somatic epidermal stem cells can produce multiple cell lineages during development[END_REF][START_REF] Zare | Regenerative Medicine: Novel Approach in Burn Wound Healing[END_REF]. Dermal stem cells were characterized relatively late (in the beginning of XXI century). We can distinguish three main subtypes of these cells: neural crest stem cells, mesenchymal stem cell-like dermal stem cells and dermal hematopoietic cells. It is proven, that they show high plasticity an some immunosuppressive properties [START_REF] Chen | Therapeutic Implications of Newly Identified Stem Cell Populations From the Skin Dermis[END_REF].

2.2.Mesenchymal stem cells (MSCs)

The term MSCs refers to multipotent progenitor cells capable of differentiating at least towards the osteoblastic, chondrocytic and adipocytic lineages under in vitro standard differentiation conditions. To be classified as MSCs, they must also express CD73, CD90 and CD105 markers, while not expressing CD14, D19, CD34, CD45, CD11b, CD79a and HLA-DR molecules [START_REF] Dominici | Minimal criteria for defining multipotent mesenchymal stromal cells. The International Society for Cellular Therapy position statement[END_REF]. These cells are similar to fibroblasts [START_REF] Bae | Gene and microRNA expression signatures of human mesenchymal stromal cells in comparison to fibroblasts[END_REF] and can be obtained from foetal tissues (umbilical cord, umbilical cord blood and placenta) and from a number of locations in the adult body, the most important of which are bone marrow and adipose tissue.

Bone marrow is a source of so called bone-marrow derived MSCs (BM-MSCs), whereas MSCs of adipose origin are known as adipose derived stem cells (ADSCs) [START_REF] Aydemir | Mesenchymal stem cells in skin wound healing[END_REF][START_REF] Leavitt | Stem cells and chronic wound healing: state of the art[END_REF]. ADSCs are relatively easy to obtain in comparison to BM-MSCs due to minimization of patient discomfort and elimination of time consuming expansion steps in vitro -MSCs comprise barely 0,01% to 0,001% of all mononuclear cells in bone marrow [START_REF] Li | Isolation and characterization of primary bone marrow mesenchymal stromal cells[END_REF][START_REF] Schneider | Rescue plan for Achilles: Therapeutics steering the fate and functions of stem cells in tendon wound healing[END_REF]. It was shown, that both ADSCs and BM-MSCs have identical transcriptomes that express genes specific for stem cells, e.g. Oct4, Sox2 and Rex1 [START_REF] Izadpanah | Biologic properties of mesenchymal stem cells derived from bone marrow and adipose tissue[END_REF].

During wound healing, MSCs take part in all stages of this process and were proven to persist at the wound site even after the completion of healing [START_REF] Pratheesh | Evaluation of persistence and distribution of intra-dermally administered PKH26 labelled goat bone marrow derived mesenchymal stem cells in cutaneous wound healing model[END_REF]. They coordinate the inflammatory process, modulate fibrosis and reduce scarring, as they show strong immunomodulatory and immunoregulatory characteristics demonstrated by release of cytokines, chemokines and growth factors [START_REF] Hu | The Role of Stem Cells During Scarless Skin Wound Healing[END_REF][START_REF] Mester | Stem Cell Therapy in Wound Healing[END_REF][START_REF] Ren | Mesenchymal stem cell-mediated immunosuppression occurs via concerted action of chemokines and nitric oxide[END_REF]. It is reported, that human MSCs release over 30 biomolecules, that promote wound healing [START_REF] Hwang | Comparison of cytokine expression in mesenchymal stem cells from human placenta, cord blood, and bone marrow[END_REF]. Thus, they not only coordinate inflammation, fibrosis and scarring, but also increase angiogenesis and granulation tissue formation [START_REF] Maxson | Concise review: role of mesenchymal stem cells in wound repair[END_REF], show antimicrobial activity [START_REF] Krasnodembskaya | Antibacterial effect of human mesenchymal stem cells is mediated in part from secretion of the antimicrobial peptide LL-37[END_REF], neutralize reactive oxygen species (ROS) [START_REF] Sato | Nitric oxide plays a critical role in suppression of T-cell proliferation by mesenchymal stem cells[END_REF] and enhance dermal fibroblast function [START_REF] Smith | Mesenchymal stem cells induce dermal fibroblast responses to injury[END_REF]. Moreover, it was shown, that MSCs conditioned medium (via paracrine signalling) stimulates stem cells residing near the damaged tissue to repair the defect [START_REF] Ho | Mesenchymal stem cells and their conditioned medium can enhance the repair of uterine defects in a rat model[END_REF].

MSCs are used clinically both in the autologous system and as an allogenic transplant. This is possible due to the already mentioned immunomodulating properties of these cells, resulting in lack of immune system reaction to the transplantation of MSCs from a donor, as well as the lack of recognition (by allogeneic MSCs) of the recipient as a foreign organism.

Recently, more and more experiments have revealed detailed mechanisms of wound healing promotion by MSCs and their conditioned media. For example, it was shown in vivo and in vitro, that Ell3 (a transcription elongation factor) is one of the factors determining wound healing efficiency of ADSCs conditioned medium [START_REF] Lee | Ell3 Modulates the Wound Healing Activity of Conditioned Medium of Adipose-derived Stem Cells[END_REF]. SDF-1 produced by ADSCs is also very potent in wound healing acceleration, even in the case of healing impairments caused by glucocorticoids [START_REF] Kato | SDF-1 improves wound healing ability of glucocorticoid-treated adipose tissue-derived mesenchymal stem cells[END_REF]. On the other hand, keratinocyte migration and epithelial-mesenchymal transition (EMT)-like process taking place in these cells (crucial for re-epithelialization) were shown to be mediated by β 2 -adrenergic receptor (β 2 -AR), which is able to recognize soluble factors secreted by BM-MSCs [START_REF] Huo | Bone Marrow-Derived Mesenchymal Stem Cells Promoted Cutaneous Wound Healing by Regulating Keratinocyte Migration via beta2-Adrenergic Receptor Signaling[END_REF].

It was also shown on diabetic mice model, that BM-MSCs enhance wound healing among others by decreasing MMPs expression, resulting in improved collagen 1 content [START_REF] Xu | Mesenchymal stem cells correct impaired diabetic wound healing by decreasing ECM proteolysis[END_REF].

To conclude, wound repair promoting MSCs mechanisms of action are relatively well known. They are able to differentiate and transdifferentiate into tissue specific cells, to secrete a wide range of paracrine factors, to regulate immune response and local tissue microenvironment [START_REF] Li | Mechanisms of action of mesenchymal stem cells in cutaneous wound repair and regeneration[END_REF].

Endothelial progenitor cells (EPCs)

Endothelial progenitor cells (EPCs) can be derived as a fraction of bone marrow mononuclear cells (BMMNCs) or peripheral blood mononuclear cells (PBMNCs). In the case of tissue injury they are recruited from bone marrow (circulate in the blood) and then, localize to sites of injury and distribute near endothelial cells thanks to the expression of endothelial and hematopoietic markers [START_REF] Asahara | Isolation of putative progenitor endothelial cells for angiogenesis[END_REF][START_REF] Lau | Exploring the role of stem cells in cutaneous wound healing[END_REF]. EPCs are known to be sensitive to hypoxia, as they respond to hypoxia-inducible factor 1 (HIF-1)-induced stromal cell-derived factor 1 (SDF-1) in conditions of oxygen deprivation [START_REF] Ceradini | Progenitor cell trafficking is regulated by hypoxic gradients through HIF-1 induction of SDF-1[END_REF].

They also contribute to angiogenesis, which makes them a promising target for chronic wound treatment, especially those with ischemic pathologies, like for example diabetic ulcers [START_REF] Capla | Diabetes impairs endothelial progenitor cell-mediated blood vessel formation in response to hypoxia[END_REF][START_REF] Grenier | Resident endothelial precursors in muscle, adipose, and dermis contribute to postnatal vasculogenesis[END_REF].

Hematopoietic stem cells (HSCs)

Hematopoietic stem cells (HSCs) constitute a relatively large fraction of BMMNCs (1%) [START_REF] Li | Isolation and characterization of primary bone marrow mesenchymal stromal cells[END_REF], but they can also be isolated from peripheral and umbilical cord blood.

Potential of HSCs in skin regeneration results i.a. from their high plasticity [START_REF] Kamolz | Cultured human epithelium: human umbilical cord blood stem cells differentiate into keratinocytes under in vitro conditions[END_REF] and participation in angiogenesis process [START_REF] Sackstein | The bone marrow is akin to skin: HCELL and the biology of hematopoietic stem cell homing[END_REF]. They also affect (positively) ECM during wound healing by secreting collagen and downregulating matrix metalloproteinases (MMPs) expression [START_REF] Kanji | Nanofiber-expanded human umbilical cord blood-derived CD34+ cell therapy accelerates murine cutaneous wound closure by attenuating pro-inflammatory factors and secreting IL-10[END_REF]. Moreover, they stimulate proliferation of keratinocytes and fibroblasts, which significantly accelerates wound closure [START_REF] Kim | Human cord blood-derived endothelial progenitor cells and their conditioned media exhibit therapeutic equivalence for diabetic wound healing[END_REF]. Differentiation of HSCs into macrophages is an important event during wound healing. It was shown, that this process is significantly impaired in diabetes, which is mediated by one of DNA methyltransferases [START_REF] Yan | Diabetes impairs wound healing by Dnmt1-dependent dysregulation of hematopoietic stem cells differentiation towards macrophages[END_REF].

Stem cells application in wound healing

In larger burns and chronic wounds, the number and function of stem cells may be impaired. For example, it was shown, that chronic wound environment inhibits ADSCs proliferation rate and changes expression of some genes crucial for wound healing in these cells [START_REF] Koenen | Acute and chronic wound fluids inversely influence adipose-derived stem cell function: molecular insights into impaired wound healing[END_REF]. Moreover, EPCs derived from diabetic patients (that often suffer from non-healing ulcers) demonstrated decreased adherence to human umbilical vein endothelial cells, impaired proliferation and less involvement in tubule formation in comparison to controls [START_REF] Tepper | Human endothelial progenitor cells from type II diabetics exhibit impaired proliferation, adhesion, and incorporation into vascular structures[END_REF]. That is why a concept was created to support healing process by supplying missing stem cells to specific sites. In recent years, more and more studies have shown that stem cells (especially MSCs) can be useful for treating wounds that are difficult to heal. Different subtypes of MSCs are the subject of both in vitro and in vivo studies on this topic. There are also some clinical trials confirming the effectiveness of stem cell therapy in wound healing. Currently, most of them (both published and unpublished) focus on MSCs (⁓ 30%), EPCs (⁓ 25%) and BMMNCs (⁓ 20%). The most popular approach is the use of autologous stem cells (⁓ 90%) [START_REF] Leavitt | Stem cells and chronic wound healing: state of the art[END_REF]. MSCs used for experiments are mainly derived from bone marrow and adipose tissue (previously mentioned BM-MSCs and ADSCs), but beneficial effects on wound healing were also shown for example in case of umbilical cord blood and Wharton's Jelly derived MSCs [START_REF] Jung | Comparison of human umbilical cord blood-derived mesenchymal stem cells with healthy fibroblasts on woundhealing activity of diabetic fibroblasts[END_REF][START_REF] Pourfath | Monitoring wound healing of burn in rat model using human Wharton's jelly mesenchymal stem cells containing cGFP integrated by lentiviral vectors[END_REF].

The overview of some important animal model experiments and clinical trials using stem cells in the context of wound healing is presented in the Table 1.

Modes of stem cells delivery

One of the most important problems restricting the use of stem cell-based therapies is the mode of cell delivery to damaged tissues. The simplest techniques involve topical cell applications (for example as a spray), direct injections to the dermis or muscle as well as systemic delivery into circulation [START_REF] Duscher | Stem Cells in Wound Healing: The Future of Regenerative Medicine? A Mini-Review[END_REF]. However, these methods have many limitations, for example difficulties in tissue targeting and high cell attrition rate (in the case of systemic delivery), low cell survival, additional tissue damages, lack of cell-ECM attachment and hostile wound environment (in the case of local administration). That is why bioscaffold-based stem cell delivery mode has recently gained much attention [START_REF] Garg | Capillary force seeding of hydrogels for adipose-derived stem cell delivery in wounds[END_REF][START_REF] Kean | MSCs: Delivery Routes and Engraftment, Cell-Targeting Strategies, and Immune Modulation[END_REF][START_REF] Rustad | Enhancement of mesenchymal stem cell angiogenic capacity and stemness by a biomimetic hydrogel scaffold[END_REF], in which cells are transferred on a matrix (cellular or acellular) such as hydrogel, scaffold, dermal substitute or ECM derivative [START_REF] Zare | Regenerative Medicine: Novel Approach in Burn Wound Healing[END_REF].

Polymer-based biomaterials are commonly used for stem cell delivery into the wound difficult to heal, as they show high biocompatibility, bioactivity and biodegradability [START_REF] Kohane | Polymeric biomaterials in tissue engineering[END_REF]. They can be prepared using both natural polymers (e.g. collagen, fibrin, elastin, gelatin, silk, hyaluronic acid, chitosan) and synthetic ones (e.g. polyanhydrides, polyethylene glycol, poly(lactic-co-glycolic) acid). Some scaffolds (for example those with collagen) exhibit similar biological actions to cytokines regulating cell growth and immunity [START_REF] Mashiko | Therapeutic effects of a recombinant human collagen peptide bioscaffold with human adipose-derived stem cells on impaired wound healing after radiotherapy[END_REF]. Thermosensitive hydrogels are popular, as they effectively encapsulate the cells increasing the efficiency of their delivery to the wound [START_REF] Kaisang | Adipose-derived stem cells seeded in Pluronic F-127 hydrogel promotes diabetic wound healing[END_REF]Lei et al., 2018). Some modifications of classical scaffolds are also made, which further promotes regenerative wound healing by improving biological activity of stem cells. Most often, these include the addition of fragments of natural proteins like laminin, fibrin, glycosaminoglycan and many others [START_REF] Dash | Stem Cells and Engineered Scaffolds for Regenerative Wound Healing[END_REF][START_REF] Dawson | Biomaterials for stem cell differentiation[END_REF][START_REF] Ghasemi-Mobarakeh | Structural properties of scaffolds: Crucial parameters towards stem cells differentiation[END_REF][START_REF] Kohane | Polymeric biomaterials in tissue engineering[END_REF]. Moreover, addition of silver nanoparticles to the cellular dressing can be an interesting way to prevent bacterial colonization of wounds during healing process [START_REF] Perez-Diaz | Silver-pig skin nanocomposites and mesenchymal stem cells: suitable antibiofilm cellular dressings for wound healing[END_REF].

Dermal substitutes (especially those derived from placental tissues) are also promising, as they include viable cells like fibroblasts, epithelial cells and MSCs, which actively produce tissue reparative paracrine factors. Among these substances there are not only growth factors, but also some anti-inflammatory and antimicrobial molecules, which accelerate the shift from the inflammatory to proliferative phase of wound healing [START_REF] Bieback | Mesenchymal stromal cells from human perinatal tissues: From biology to cell therapy[END_REF][START_REF] Maxson | Concise review: role of mesenchymal stem cells in wound repair[END_REF][START_REF] Parolini | Concise review: isolation and characterization of cells from human term placenta: outcome of the first international Workshop on Placenta Derived Stem Cells[END_REF]. A completely different approach is represented by ECM derivative, so called acellular dermal matrix (ADM). It does not contain living cells, but preserves the ultrastructure and protein content characteristic for native ECM, which improves regenerative activity of stem cells, even those tested on diabetic wound mice model [START_REF] Chu | Dynamic multiphoton imaging of acellular dermal matrix scaffolds seeded with mesenchymal stem cells in diabetic wound healing[END_REF].

New therapeutic purposes of stem cells

Nowadays, stem cells are also tested for new therapeutic purposes, for example to treat corneal wounds and acute burns. ADSCs were administrated to mice in the form of eye drops, which resulted in earlier and better corneal repair [START_REF] Zeppieri | Adipose Derived Stem Cells for Corneal Wound Healing after Laser Induced Corneal Lesions in Mice[END_REF]. In another study, two types of ADSCs (autologous and allogenic) were injected into burn full-thickness wounds created in male Wistar rats. It was shown, that only autologous cells enhanced wound healing, with injections 0,5 cm from the wound edge being more effective in comparison to injections in the wound centre [START_REF] Chang | Autologous and not allogeneic adipose-derived stem cells improve acute burn wound healing[END_REF].

Pretreatment and co-culture of stem cells

Various co-culture and preconditioning systems are being developed, which in some cases accelerates stem cell differentiation and their impact on wound healing process. One of the examples of this approach can be co-culturing ADSCs (on advanced collagen wound matrix) with primary keratinocytes and keratinocyte conditioned medium, which after fifteen days initiated ADSCs differentiation towards keratinocytes [START_REF] Edwards | Differentiation of adipose derived stem cells to keratinocyte-like cells on an advanced collagen wound matrix[END_REF]. Furthermore, ADSCs preconditioned by endothelial cell medium showed increased angiogenic potential, both in vivo (on diabetic mice model) and in vitro as well as enhanced proliferation and endothelial differentiation [START_REF] Fromer | The endothelial cell secretome as a novel treatment to prime adipose-derived stem cells for improved wound healing in diabetes[END_REF]. Similarly, human ADSCs co-cultured with endothelial cells on collagen peptide bioscaffold accelerated wound closure on irradiated mice model [START_REF] Mashiko | Therapeutic effects of a recombinant human collagen peptide bioscaffold with human adipose-derived stem cells on impaired wound healing after radiotherapy[END_REF]. Unexpectedly, MSCs pretreated with S100A8/A9 (protein complex belonging to danger associated molecular patterns recognized by immune system) accelerated murine wound healing to the greater extent, than MSCs alone [START_REF] Basu | A Novel S100A8/A9 Induced Fingerprint of Mesenchymal Stem Cells associated with Enhanced Wound Healing[END_REF]. Some other studies revealed, that treatment using MSCs combined with platelet rich plasma (PRP) -an increased concentration of platelets in small volume of plasma after centrifugation -increases wound healing rates by promoting re-epithelialization and regulating inflammatory processes [START_REF] Mahmoudian-Sani | The effect of mesenchymal stem cells combined with platelet-rich plasma on skin wound healing[END_REF][START_REF] Roubelakis | Platelet-rich plasma (PRP) promotes fetal mesenchymal stem/stromal cell migration and wound healing process[END_REF][START_REF] Stessuk | Platelet-rich plasma (PRP) and adipose-derived mesenchymal stem cells: stimulatory effects on proliferation and migration of fibroblasts and keratinocytes in vitro[END_REF][START_REF] Wang | Platelet rich plasma: myth or reality?[END_REF]. Stem cells can be also pretreated by various chemicals like for example emu oil emulsified in egg lecithin and butylated hydroxytoluene. Such conditioning significantly increased the regenerative potential of ADSCs and even affected their cell cycle, which was proven by many different tests in vitro [START_REF] Arezoumand | The emu oil emulsified in egg lecithin and butylated hydroxytoluene enhanced the proliferation, stemness gene expression, and in vitro wound healing of adipose-derived stem cells[END_REF].

There were also attempts to activate stem cells with bioglass (type of silicate biomaterial) ionic products, which improved healing ability of urine-derived stem cells (USCs) by stimulation of paracrine effects between USCs and recipient cells as well as between fibroblasts and endothelial cells (Zhang et al., 2018b).

Genetic modifications of stem cells

There are also some studies, aim of which is to genetically modify stem cells in order to change their activity to solve a specific problem. For example, TGF-β 3 overexpressing BM-MSCs not only improved the course of wound healing process, but also reduced scar tissue formation, which may be useful in prevention of keloid formation (Li et al., 2018a). Another experiment held on murine epidermal stem cell culture revealed, that overexpression of Cd271 (p75 neurotrophin receptor) increased cell ability to differentiate, proliferate and migrate. The same study also proved, that Cd271 promotes burn wound healing on mice model (Zhang et al., 2018a). Human stem cells have also been subject to some genetic modifications. CA-Akt and v-myc expressing human ADSCs showed higher proliferation potential and higher vascular endothelial growth factor (VEGF) secretion level than control cells, which translated into acceleration of wound healing [START_REF] Song | Genetic modification of human adipose-derived stem cells for promoting wound healing[END_REF]. Other studies have demonstrated the possibility to maintain differentiation and proliferation potential of human MSCs isolated from placenta and bone marrow by overexpression of telomerase reverse transcriptase (TERT) genes [START_REF] Abdallah | Maintenance of differentiation potential of human bone marrow mesenchymal stem cells immortalized by human telomerase reverse transcriptase gene despite [corrected] extensive proliferation[END_REF][START_REF] Zhang | Successful immortalization of mesenchymal progenitor cells derived from human placenta and the differentiation abilities of immortalized cells[END_REF]. Finally, genetic modifications of stem cells derived from patients with chronic wounds also seem very promising. For example, scientists from Austria and Italy successfully regenerated functional epidermis of 49-year-old woman suffering from junctional epidermolysis bullosa using her own epidermal stem cells modified with laminin subunit β3 (LAMB3) gene inserted by retroviral vector [START_REF] Bauer | Closure of a Large Chronic Wound through Transplantation of Gene-Corrected Epidermal Stem Cells[END_REF].

Experiments on large animal models

Mouse or rat animal models are most commonly used to evaluate stem cells delivery effects on various aspects of wound healing. Nevertheless, some experiments on larger animals have been performed recently. It was shown, that Yorkshire pigs injected by ADSCs demonstrated improved healing parameters, which was proven by simple wound measurement as well as by using Real-Time PCR and Western Blotting for collagens, α -smooth muscle actin and CD31 expression assessment [START_REF] James | Adipose stem cells enhance excisional wound healing in a porcine model[END_REF]. Similar results were obtained on diabetic swine model using ADSCs alone, ADSCs differentiated into endothelial cells and various conditioned media [START_REF] Irons | Acceleration of diabetic wound healing with adipose-derived stem cells, endothelial-differentiated stem cells, and topical conditioned medium therapy in a swine model[END_REF]. Moreover, it was shown on Bergamasca sheep model, that MSCs from peripheral blood improved wound healing in many aspects (macroscopic and biochemical), which was assessed 15 and 42 days after lesion creation [START_REF] Martinello | Allogeneic mesenchymal stem cells improve the wound healing process of sheep skin[END_REF].

Stem cells secretome application for wound healing

Interestingly, the research group from Chile and Colombia showed on diabetic mice model, that better therapeutic effect (than with the MSCs themselves) is obtained with the use of their acellular derivative. It suggests, that the tissue repair must be initiated by some bioactive molecules present in stem cell conditioned medium [START_REF] De Mayo | The role of bone marrow mesenchymal stromal cell derivatives in skin wound healing in diabetic mice[END_REF]. Thus, another interesting trend in modern regenerative medicine is to use stem cell secretome (such as conditioned media or purified exosomes) instead of cells themselves. This approach resolves some technical problems connected with living cells such as tumorigenicity, immune compatibility, infection transmission and complicated material storage. It was shown, that adipose tissue extracellular fraction (isolated from lipoaspirate) upregulated dermal and epidermal cell proliferation rate and enhanced migration of dermal fibroblasts [START_REF] Bellei | Adipose tissue-derived extracellular fraction characterization: biological and clinical considerations in regenerative medicine[END_REF]. What is more, MSCs-derived exosomes -extra-cellular nano-vesicles transferring active cargoes between the cells [START_REF] Marote | MSCs-Derived Exosomes: Cell-Secreted Nanovesicles with Regenerative Potential[END_REF][START_REF] Zhang | Exosomes: a novel pathway of local and distant intercellular communication that facilitates the growth and metastasis of neoplastic lesions[END_REF] -are very promising as a novel therapy against delayed wound healing [START_REF] Monsel | Mesenchymal stem cell derived secretome and extracellular vesicles for acute lung injury and other inflammatory lung diseases[END_REF][START_REF] Pashoutan Sarvar | Mesenchymal Stem Cell-Derived Exosomes: New Opportunity in Cell-Free Therapy[END_REF][START_REF] Sun | Safety evaluation of exosomes derived from human umbilical cord mesenchymal stromal cell[END_REF]. They can promote angiogenesis, cell migration, proliferation as well as re-epithelialization process by activating some signalling pathways (like STAT3, AKT, ERK and Wnt/β-catenin) resulting in growth factors expression upregulation [START_REF] Goodarzi | Mesenchymal Stem Cells-Derived Exosomes for Wound Regeneration[END_REF][START_REF] Rani | The Exosome -A Naturally Secreted Nanoparticle and its Application to Wound Healing[END_REF]. Furthermore, exosomes derived from iPSCs positively affected wound healing also on diabetic mice model -they accelerated wound closure and angiogenesis (in vivo) and increased migration and proliferation of fibroblasts isolated from these mice [START_REF] Kobayashi | Editors' Choice Effects of exosomes derived from the induced pluripotent stem cells on skin wound healing[END_REF]. Similarly, ADSCs derived exosomes (particularly those from cells overexpressing Nrf-2 transcription factor) reduced the inflammation within the wound on diabetic mice model, which promoted the healing of diabetic foot ulcers. At the same time, they decreased glucose-induced senescence of EPCs (Li et al., 2018b).

Discussion

Human skin is the largest organ of the human body and its main function is to provide proper protection against external factors and the penetration of pathogens into the body. Its continuity is of the greatest importance and any damage may be a potential gateway to infection. Wounds created within the skin are the result of the impact of various damaging and irritating factors leading to disruption of the skin's continuity, while the process of their treatment is multistage and can be long-lasting. At each stage of wound healing, complications such as bacterial infection and hypertrophic scarring (keloid formation) may occur.

Wounds can be divided into acute and chronic depending on the duration and progress of the healing process.

In the case of acute wounds, it is assumed that the process of their healing passes through certain stages, so basically they can repair themselves in the specific period of time. Time necessary for first occurrence of features characteristic for the healing process should not take longer than 4 weeks -wounds that do not show healing after this time are likely not to heal for another 8 weeks [START_REF] Cardinal | Early healing rates and wound area measurements are reliable predictors of later complete wound closure[END_REF]. We can distinguish three stages of the normal healing process, which in some cases overlap in time: inflammation (with haemostasis at the beginning), proliferation and remodelling [START_REF] Li | Pathophysiology of acute wound healing[END_REF]. Wound closure occurs within about 2 weeks after the injury and depends on the depth of the wound. The most important cells involved in this process are myofibroblasts (arising from fibroblasts) which acquire the ability to contract, so it is possible to achieve a contraction of the wound [START_REF] Demidova-Rice | Acute and impaired wound healing: pathophysiology and current methods for drug delivery, part 1: normal and chronic wounds: biology, causes, and approaches to care[END_REF][START_REF] Li | Pathophysiology of acute wound healing[END_REF].

Wounds in which proper healing is inhibited at one of its stages and do not show the characteristic features of its progression within 4 weeks after injury, are referred to as chronic wounds. Within these wounds, cells with altered phenotype can be noticed. In addition, not all of the wound areas are in the same stage of healing process. All these features make it difficult or impossible to properly heal the wound. There are several characteristics of cells present in the chronic wound. First of all, they show proliferative potential and morphological characteristics similar to senescent cells and express low levels of growth factors receptors, which is characteristic of hypoxic cells [START_REF] Aydemir | Mesenchymal stem cells in skin wound healing[END_REF][START_REF] Leavitt | Stem cells and chronic wound healing: state of the art[END_REF]. At the same time, inflammatory mediators damage some growth factors and the ECM, whereas keratinocytes show poor migration capacity and produce fewer growth factors. An important characteristic is also the reduction in the production of laminin 332 [START_REF] Demidova-Rice | Acute and impaired wound healing: pathophysiology and current methods for drug delivery, part 1: normal and chronic wounds: biology, causes, and approaches to care[END_REF] as well as the decrease in fibroblasts mitogenic potential [START_REF] Zare | Regenerative Medicine: Novel Approach in Burn Wound Healing[END_REF]. These changes impair ECM deposition and the ability of fibroblasts to move and create granulation tissue.

Modern methods used in the treatment of chronic wounds include different types of dressings (gauze, occlusive, hydrogel etc.), application of growth factors, skin grafts and hyperbaric oxygen. Although they are well studied and easy to use, the have many disadvantages and limitations.

The simplest gauze dressings are not appropriate for use on exudate wounds, because they dry out and can cause further damage when replacing them. Similarly, occlusive dressings (which task is to maintain the inflammatory phase of the healing process by reducing the amount of oxygen and activating factors characteristic of cells in hypoxia) should only be used on wounds without exudate. Hydrocolloids should not be used on infected wounds, because they are impermeable to air, whereas hydrogel dressings maintain moisture and are easy to replace, but may cause excessive wound moistening. Alginates can be used to treat wounds with high exudate, however, they are not appropriate for dry wounds [START_REF] Han | Chronic Wound Healing: A Review of Current Management and Treatments[END_REF][START_REF] Wiegand | Clinical efficacy of dressings for treatment of heavily exuding chronic wounds[END_REF].

Application of growth factors is quite promising, because it is clear, that in case of chronic wounds there are serious shortages of growth factors and cytokines, which may be one of the reasons for healing difficulties. Hence, attempts are made to supply these substances from the outside. The most well researched and approved for use by the Food and Drug Administration (FDA) in the USA is platelet-derived growth factor BB (PDGF-BB) mainly used in the treatment of neuropathic ulcers [START_REF] Barrientos | Clinical application of growth factors and cytokines in wound healing[END_REF]. However, the results of research conducted on this factor are not conclusive, because of its minor effects in pressure and venous ulcers. Research is also being carried out on other factors such as VEGF or TGF-β (which levels in chronic wounds are also reduced), but so far, effects of their use are not as positive as in case of PDGF-BB [START_REF] Wu | Transforming growth factorbeta1 fails to stimulate wound healing and impairs its signal transduction in an aged ischemic ulcer model: importance of oxygen and age[END_REF].

The most important advantage of skin grafts is the fact, that they contain biological element derived from human skin (taken from cadavers) or from the skin of animal in combination with the material serving as a scaffold for the cells. Cells used to create the transplant are usually human or animal fibroblasts and keratinocytes. Fibroblasts present in the transplant are the source of growth factors, cytokines and other substances that support wound healing. The disadvantage of this solution (when using animal cells) is a certain risk of rejection of the transplant as well as of the induction of allergic reaction. Moreover, these types of substitutes are very expensiv,e so they are used only in special cases [START_REF] Kanji | Advances of Stem Cell Therapeutics in Cutaneous Wound Healing and Regeneration[END_REF].

Treatment with the hyperbaric oxygen is based on the administration of oxygen at a certain, usually high pressure, which is supposed to accelerate regeneration by proliferation of fibroblasts, to improve the immune system and to stimulate angiogenesis within the wound.

Unfortunately, clinical trials in this field have also proved, that conducting this type of treatment causes many side effects such as myopia, oxygen poisoning as well as seizures and pneumothorax [START_REF] Wu | Wound care: the role of advanced wound healing technologies[END_REF].

The use of stem cells in the treatment of chronic wounds has many advantages, which are absent in case of traditional approaches. Stem cell wound therapy is based primarily on their ability to produce substances, that promote regeneration processes such as cytokines and growth factors. Moreover, these cells support angiogenesis, have great proliferation ability and are well tolerated by the patient (in case of autologous therapy), because they come from his own body. Despite their origin from various tissues, they are able to transform into any other type of cells and to proliferate, accelerating the wound closure and additionally supporting the natural healing process. What is more, stem cells are relatively easy to use in therapy, because thanks to their good adherence to plastic materials, they can be administrated directly on the wound (as a covering material) or systemically through the blood [START_REF] Yoshikawa | Wound therapy by marrow mesenchymal cell transplantation[END_REF]. Stem cells also exhibit paracrine capacities and release many factors such as cytokines, chemokines and growth factors into the medium. When applied to the wound bed, they can stimulate local stem cells (present in the tissue adjacent to the wound) to proliferate and to initiate the healing process [START_REF] Ho | Mesenchymal stem cells and their conditioned medium can enhance the repair of uterine defects in a rat model[END_REF].

It is worth mentioning, that MSCs have some important advantages over other types of stem cells. They are multipotent, release a wide range of biologically active paracrine factors (which is well studied), have documented immunomodulatory properties and can be easily derived not only from bone marrow, but also from adipose tissue. That is why they constitute the main pool of stem cells used in the treatment of chronic wounds.

On the other hand, stem cell therapy for chronic wounds has some shortcomings and limitations. The potential risk of this therapy is the ability of stem cells to became malignant, which is especially true for the iPSCs. In MSCs (which are multipotent) this specific risk seems smaller. Another risk, as in any case of using biological material in therapy, is the probability of transmitting or causing specific diseases, for example opportunistic and disseminated infections. This mainly applies to the use of animal cells or allografts [START_REF] Leavitt | Stem cells and chronic wound healing: state of the art[END_REF].

4.1.Conclusions

Chronic wounds still remain a great concern for many healthcare professionals, despite many current therapeutic options. Experiments on cell cultures, animal models and some clinical trials have proven, that adequately administered stem cells positively affect the biochemistry of healing process, which accelerates the overall time of wound closure. They not only differentiate and proliferate, but also can promote the cells in vivo for migration, immunomodulation and angiogenesis.

Especially MSCs and their secretions are promising therapeutics for use to accelerate wound healing, as they are easy to obtain and already well-studied. However, epidermal stem cells (including HFSCs), EPCs and HSCs also show some wound healing-promoting properties, for example influencing angiogenesis process, keratinocytes migration and ECM content.

Regenerative wound healing can be further enhanced by application of different stem cells modification and co-cultures, as well as by the use of appropriate scaffolds. More research is needed to find out (in detail) the mechanisms of action of stem cells in order to increase their therapeutic effectiveness and ensure the safety of their use in the treatment of patients.

Figures

  Figures and tables (with captions):

Fig. 1 .

 1 Fig.1. Main sources of stem cell populations crucial in wound healing process. abbreviations: SCs, stem cells; MSCs, mesenchymal stem cells; EPCs, endothelial progenitor cells; HSCs, hematopoietic stem cells; SVF, stromal vascular fraction

Table 1 .

 1 Stem cells in wound therapy clinical trials and animal models

Declaration of interest: none

Reference

Cell type

Route of administration

Therapeutic efficacy Potential functional forms

Signaling pathway [START_REF] Jimenez | A pilot clinical study of hair grafting in chronic leg ulcers[END_REF] Hair Follicle Stem Cells (HFSCs)

Direct application -Hair skin graft

Average reduction in ulcer size of 30.8%. Granulation tissue, wound border reactivation and less exudation was present in 7 of 10 patients. HFSC can became wound healing fibroblasts and continue the skin dermis repair. [START_REF] Ma | In vitro characterization of human hair follicle dermal sheath mesenchymal stromal cells and their potential in enhancing diabetic wound healing[END_REF] Hair Cells cultured applied to the wound in pigs and mice. Increase epithelialization speed and area. Showed higher number of new blood vessels in the wound bed. [START_REF] Lv | Effect and mechanism of SHED on ulcer wound healing in Sprague-Dawley rat models with diabetic ulcer[END_REF] Stem Cells from Human Exfoliated Deciduous Teeth (SHED) -human/rat Direct injection Decreased wound area in all animals after 14 days. Decrease was slightly weaker in comparison to MSC. Acceleration of the epithelialization process, granulation tissue creation and new blood vessels development also reduction of inflammatory cells number.