PDLC composites based on polyvinyl boric acid matrix – a promising pathway towards biomedical engineering

ABSTRACT Polymer dispersed liquid crystal (PDLC) systems based on a smectic liquid crystal embedded in polyvinylalcohol-boric acid (PVAB) as biocompatible carrying matrix were prepared and characterised. The smectic liquid crystal contains biologically friendly structural blocks and was designed to have a direct isotropic–smectic transition and a mesophase stability range at human body temperature. The resulted PDLCs were characterised from morphological and thermotropic aspects by polarised light microscopy (POM), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and Raman microspectroscopy, and their surface properties were determined by contact angle measurements and surface energy calculations. It was concluded that the electron-deficient PVAB matrix constrains the ester liquid crystal to grow as spherical droplets with planar anchoring. The droplet diameter was comprised in the range 4–11 µm, with a predominant droplet population around 7 µm and a narrower polydispersity as the amount of the liquid crystal in the polymeric matrix increases. The resulted PDLC films exhibited versatile morphology and surface properties which allow targeting of their application. GRAPHICAL ABSTRACT


Introduction
Polymer dispersed liquid crystal (PDLC) systems are a relative new type of composite materials consisting of the micrometric dispersion of liquid crystal (LC) droplets in a polymer matrix. Thus, they combine the unique optic properties of liquid crystals, different from isotropic compound, with the filmforming ability and mechanical strength of the polymer matrix. The development of PDLC composites actually starts once their potential as active substrate in displays is evidenced. [1] Applications such as smart windows, holographic systems, micro-lens or lasers have maintained researchers' interest in this challenging domain, which is expanding continuously. [2][3][4][5][6][7][8] Today, researchers' attention is given to the potential applications of PDLCs in biomedical engineering and the food industry: smart food packaging, [9] artificial irises, [10,11] bio-sensors for biologically active matter [12] or other envisaged bio-applications. [13] Such innovative bio-applications require the use of biocompatible materials. This is the reason why the ability of biopolymers or biocompatible polymers to act as a polymer matrix for PDLC composites is of current interest. In this context, our attention was focused on polyvinylalcohol-boric acid (PVAB), a biocompatible polymer generally less studied, which has the potential to replace the well-known polyvinylalcohol. PVAB is soluble only in hot water, is transparent and is immiscible with liquid crystals, assuring good optical properties. One of our previous studies demonstrated that PVAB, due to its electron-deficient boron atom, assures selective nematic liquid crystal anchoring with no orientation defect and narrow polydispersity of the LC droplettwo very important attributes of PDLC systems that are usually difficult to achieve. [14] Among PDLC systems, those containing smectic liquid crystals were demonstrated to be more attractive due to their high contrast, energy-saving nature and ability to generate bistabilityproperties which prompt the maintenance of optical states without an electrical field, being ideal for portable electronic devices. Moreover, the smectic A-based PDLC composites demonstrated reversible memory attributed to the stiffness of the layered structure of the mesophase. [15][16][17][18] However, there are only few studies in the literature related to PDLC composites based on smectic liquid crystals, because of the difficulty in obtaining smectic liquid crystals with direct isotropic-smectic transition and wide stability range at room temperature on the one hand, and the difficulty in growing smectic droplets in a polymer matrix on the other. [15][16][17][18][19][20] A literature survey revealed that esters containing liquid crystals are biological friendly compounds successfully used in bio-applications. [21][22][23] Nevertheless, the previously known ester-based smectic liquid crystals have high thermal transitions and thus high temperature mesophase stability range, properties which make them unattractive for PDLCs.
For all the above-mentioned reasons, we decided to obtain new PDLC composites using PVAB as confinement matrix and a novel smectic liquid crystal with a direct smectic-isotropic transition and wide mesophase stability range in the human body temperature domain, designed and synthesised by us. [24] The microencapsulation method was used in order to obtain initial composite films whose microstructure was reset by thermal annealing. Complementary methods were used to characterise the novel PDLCs polarised light microscopy, differential scanning calorimetry, scanning electron microscopy, Raman microspectroscopy, UV-vis and photoluminescence spectroscopy, contact angle measurements and surface energy calculationsin order to access multiple scales involved in PDLC-producing phenomena.
Three important aspects must be underlined here: (i) the smectic liquid crystal used in this study was designed and synthesised by us; (ii) PVAB is a versatile, less well-known and -used biocompatible polymer; and (iii) the key intrinsic results obtained for the understudy PDLC composites can be used as a model for the encapsulation of ester-based drugs into PVAB in order to obtain controlled release and delivery.

Materials
Polyvinyl alcohol boric acid (Mw = 54 000, 4% water content) was purchased from Aldrich and used as received.
Buthyl-p-[p′-n-octyloxy benzoyloxy]benzoate (BBO) was synthesised as smectic A liquid crystal with a direct isotropic-smectic A transition. [24] The benzoate derivative was chosen as it is well known that benzoate derivatives are biologically and pharmacologically bioactive moleculesbeing suitable in the medical and analytical fields. [23]

Techniques
The 1 H-NMR spectrum of BBO was recorded on a BRUKER Avance DRX 400 MHz spectrometer, equipped with a 5 mm direct detection QNP probe with z-gradients. The chemical shifts are reported as δ (ppm) relative to the residual peak of the DMSO solvent.
The textures of the pure liquid crystal and PDLC composites were observed by polarised light microscopy, with an Olympus BH-2 microscope equipped with a Linkam THMS 600/HSF9I heating stage and a TMS91 control unit. The samples were observed during a heating/cooling/heating scan, at a heating/cooling rate of 5°C min -1 .
Differential scanning calorimetric (DSC) measurements were performed on a DSC 200 F3 Maia device (Netzsch, Germany), under nitrogen purge (nitrogen flow 50 mL/min). The device was temperature and sensitivity calibrated with indium, according to standard procedures. Around 5 mg of each sample were loaded in punched and sealed aluminium crucibles and DSC curves were registered on a heating-cooling-heating scan, at a heating/cooling rate of 5 o C min -1 . The transition temperatures were read at the top of the endothermic and exothermic peaks.
Raman spectra were collected on a confocal Raman microscope (WITec alpha300 R), under 532 nm laser excitation, a 0.9 NA objective and a 100 μm pinhole. By using a 600 g/mm grating, the spectral region up to 3700 cm -1 was recorded in a single spectrum, with a resolution of 3 cm -1 . Raman imaging was performed in scanning mode, with a resolution of~31 pixels/μm 2 . Raman images were constructed from the integrated intensity of a given Raman band, after background subtraction. The Raman system is equipped with an integrated polarisation module, by which the polarisation of the excitation beam can be rotated manually.
The microstructure of the composite films was viewed with a field emission scanning electron microscope (Scanning Electron Microscope SEM EDAX -Quanta 200) at an accelerated electron energy of 10 eV. The morphological observation was carried out for (i) the film samples as resulted by casting and for (ii) the films heated at 80°C. The droplet diameters were measured using Image J Software and the obtained values were further used to build the corresponding histograms using OriginPro 8 software.
The static contact angle for the PDLC and BBO film samples was obtained using a CAM-200 instrument from KSV Finland, by the sessile drop method, at room temperature and controlled humidity. The measurement was performed within 10 s after placing 1 μL drop of water on the film surface, and the contact angle was measured by fitting the drop profile using the Young-Laplace equation. [12,25] The contact angle was measured on five random locations of the surface, the average value being considered. To calculate the components of the free surface energy and total free surface energy, the contact angle at equilibrium between the studied surface and three pure liquids twice distilled water, formamide and diiodomethanewas measured. The total surface free energy (γ TOT s ) was calculated using the acid-base approach of van Oss and Good. [12,25]

Smectic liquid crystal
A thermotropic smectic liquid crystal containing ester linkagesnamed buthyl-p-[p′-n-octyloxy benzoyloxy] benzoate] (BBO) was synthesised by an esterification reaction starting from p-hydroxy benzoic acid. [24] The right structure of the liquid crystal was proved by 1 H-NMR and Raman spectroscopy ( Figure 1s) and further by single-crystal X-ray diffraction measurements (1).
As can be seen, the BBO molecules adopt a twisted conformation conferred by the torsion of the ester linkage between the two aromatic rings. The driving force in supramolecular packing appears to be the rigid-flexible self-assembly ( Figure 1). [26] In the crystal structure, the molecules are packed forming ribbons with an intermolecular distance of around 5.7 Å, extended parallel to the a axes. The ribbons are packed two-by-two forming packs which lie in anti-parallel directions along b axes and forming molecular layers. The inter-ribbon distance is quite short, around 3.5 Å, indicating strong lateral forces due to the hydrogen bonding between the neighbouring ester oxygens and aromatic or aliphatic hydrogens, and also between the aromatic carbons and hydrogens. Furthermore, along the c axes, the molecular layers are arranged in the anti-parallel directions of the molecules, with the inter-layer distance around 9 Å. An important role in maintaining crystal integrity appears to be played by the strong lateral forces.
The thermotropic behaviour of the BBO compound, investigated by differential scanning calorimetry and polarised light microscopy, consists of the occurrence of an enantiotropic smectic A mesophase with a direct isotropic-smectic A transition and a wide mesophase stability range. As can be seen from Figure 2, the smectic mesophase was observed during the heating scan by transition of the crystalline state to a striated marbled texture (Figure 2(a)), while during the cooling scan it clearly appeared from the isotropic state ( Figure 2(b)) showing a characteristic fan-shaped texture, [27,28] which transformed into a crystalline state after further cooling ( Figure 2(c)).
Compared to cyano-containing liquid crystals, generally used in PDLC production and which were proved to be cytotoxic, [29,30] the new synthesised compound is created from biologically friendly building blocks, [21,23] offering potential use in biomedical applications.

PDLC composite preparation
PDLC composite films were obtained by encapsulation of the BBO smectic liquid crystal into polyvinyl alcohol boric acid as carrying matrix, by the micro-emulsification method. The components ratio was varied to give four different samples with growing percentage of liquid crystal ranging from 10 to 40 (Table 1). The composites gave freestanding films, with high adhesivity to the glass support. Polarised light microscopy, Raman microspectroscopy, differential scanning calorimetry and scanning electron microscopy were employed in order to determine the morphological characteristics of the new PDLC composites, in terms of droplet shape, size, polydispersity, distribution and configuration, as well as polymer anchoring effect and morphological stability. To further understand the interface forces at the polymer-liquid crystal boundary, and in order to estimate the potential of the composite as a biomaterial, contact angle measurements and surface energy calculations were also performed.

Polarised light microscopy
Polarised light microscopy (POM) is a visual method that allows simple initial determination of liquid crystalline droplet segregation and, besides, provides twodimensional information about their shape, location, size distribution and thermal dynamic in the observed plane. Comparing the pure LC thermotropic behaviour to that of the PDLCs, conclusions regarding the thermodynamic of the phase separation and especially the droplet anchoring principle can be drawn.
The pure smectic LC compound (BBO) exhibited under POM a clear fan-shaped texture, the signature of a smectic A mesophase. This fan-shaped texture is stable up to 28°C when crystallised.
As can be seen from Figure 3(a-c), the PDLC samples with lower amounts of LC showed clear, round, birefringent droplets. As the percentage of liquid   crystal in the polymer matrix increased the droplet density grewan almost continuous fine birefringent texture being observed for the highest amount of LC, due to the overlapping droplets on cross-section of the transparent polymer matrix. [12,31,32] Generally, the colour within the droplets was distributed as concentric rings, similar to the simple polygonal textures formed by smectic A mesophase in thick preparations. [27] During heating no crystalline-smectic transition was observed, suggesting a pre-existing layered organisation of the crystalline state (see crystallographic analysis) which was mantained into the smectic mesophase and could be stabilised by the interphase interaction with the polymer matrix. The droplet isotropisation occurred slowly within a wider temperature range (67-72°C) as compared to the pure liquid crystal (69-70°C), clearly indicating strong interphase forces able to anchor the liquid crystal droplets. Moreover, the isotropic LC droplets remained surrounded by a milky birefringent radial shadow (Figure 3(d)), indicating the organisation of the polymer matrix at the interphase. [12,32] The milky birefringence became more intense when the PDLC films were kept for a further period, indicating that PVAB ordering around the BBO liquid crystal evolves over time (Figure 3(d')). So, it can be concluded that strong interactions between the electron-deficient boron atom of the PVAB and the negatively charged ester bonds of the BBO occurred, resulting in an ordering coupling at the interface between the two phases. Consequently, taking into consideration the structure of the BBO, a parallel disposition of their molecules on the droplet wall was predicted.
During cooling the coloured droplets also appeared gradually. These were of fairly uniform size and distribution in the polymer matrix. No other phase transition was clearly seen, indicating good stabilisation of the organisation of the LC molecules within the droplets.
This thermotropic behaviour suggests that, in the liquid crystalline droplets, a simple polygonal texture characteristic of thick LC samples replaced the fanshaped texture formed by the pure liquid crystal, as a consequence of the thick LC sample embedded in the polymer matrix compared to thin LC samples between two lamellae. Most probably, within the droplets of polygonal texture, the smectic layers were arranged in Dupin cyclides [27] with the focal conics lying with ellipses on the plane of the droplet wall and the summit of the corresponding cone of revolution in the droplet centre. The entire droplet volume is thus filled with these cones. The molecules inside the cones are oriented with long axes parallel with respect to the droplet wall, due to the strong tangential anchoring stabilised by the interphase forces. A topographic image of the BBO distribution inside the smectic droplets is shown in Figure 4.

Raman microspectroscopy
Compared to POM, Raman microscopy provides a more detailed insight into the spatial organisation of the molecules inside liquid crystal droplets. Besides, it is a valuable complementary method to establish the phase separation by mapping chemical composition. [32] The two pure composite components have their own spectroscopic signature based on the different groups which vibrate at different wave numbers. Thus, while the Raman spectrum of the PVAB has the most important vibration bands due to C-H stretching around 2800 cm -1 , the BBO liquid crystal has distinct bands due to the C=C stretching into the aromatic rings at 1600 cm -1 and COO group vibration at 1720 cm -1 (Figure 2s). Based on this different Raman signature, composite mapping was realised when different areas corresponding to a phase separation of the continuous polymer matrix and round liquid crystalline droplets appeared ( Figure 5).
The BBO liquid crystal molecules are strong Raman scatters. The vibration of the characteristic C=C and COO groups of the BBO compound occurs in the molecular plane, and their vibration direction is parallel to the mean director direction. [33] As a consequence, they can be assimilated with orientational director n of the BBO smectic arrangement. The symmetric stretching of the C=C in the aromatic rings and stretching of the COO, respectively, have the ability to induce a major change of polarizability along the long molecular axis of the BBO molecules. As a result, using a linear polarised laser beam, the average orientation of the BBO liquid crystal molecules can be evidenced by simple analysis of the two stretching bands. The intensity of these bands must be maximal when the electromagnetic vector of the incident plane of polarised light (P) lies parallel to the axis of the BBO molecules, and minimal when it lies perpendicular. [32,33] Consequently, the orientational directorwhich is more or less parallel to the long molecular axiscan be found. Figure 5 exhibits as an example, the Raman images of the P2 composite together with Raman spectra registered for different areas (marked as 1, 2 and 3), at different orientations of the excitation laser polarisation (P) as indicated by the double arrow. The Raman maps are plotted from the intensity of the 1600 cm -1 Raman band.
The preferential orientation of the BBO molecules was clearly evidenced by the dependence of the Raman band intensity on the polarisation of the excitation laser. The variation occurred between a maximum to a complete absence of the bands ( Figure 5, case 1), or between a maximum and a minimum ( Figure 5, case 2), indicating spherical droplets tilted under different angles (Figure 2s). Taking into account that maximum intensity is obtained when n // P and minimum when n ⊥ P, it can be assumed that the orientational director of the BBO has a radial orientation of the molecules inside the droplet, with the long axis parallel to the droplet wall, attaining tangential anchoring. A similar behaviour was observed also by plotting the Raman images corresponding to the 1720 cm -1 band. On the contrary, by plotting the Reighley band (laser scattering) or the Raman band at 2900 cm -1 (from the matrix), the dependence on laser polarisation did not present the same behaviour (Figure 3s). This enforces our conclusion that by using polarised Raman imaging the orientation of the molecules inside the droplets was indeed reached. Thus Raman spectroscopy undoubtedly confirmed the theoretical premises and POM observations, demonstratting that the smectic liquid crystal drops were strongly planar/tangentially anchored by the PVAB confining matrix, the drops being filled with focal conic domains in which the molecules have the orientational director planar to the boundary. [34] 3.5. Differential scanning calorimetry DSC is an important technique in PDLC characterisation, giving information about LC segregation, transition dynamic, droplet size polydispersity, and also the liquid crystal/carrying matrix miscibility and morphological stability of the composite materials.
Compared to the pure LC, the DSC curves of the PDLC composites showed no important changes during the heating scans; the crystalline melting and smectic-isotropic transition occurred at the same temperature as the pure liquid crystal (Figure 6(a)). Instead, important changes could be seen in the cooling scans (Figure 6(b)). First, the isotropic-smectic transition appeared at lower temperature and the exothermic peak was larger, indicating a slower occurrence of the smectic mesophase as explained by the awkward organisation of the molecules into Dupin cyclides, constrained by their tangential anchoring to adopt exact positions inside the cones. Secondly, instead of the broad exotherm resulting from the superposition of two peaks (30 and 28°C) in the case of pure liquid crystals, two well-separated exothermic peaks appeared, indicating the separation of two clear transitions: SmA-SmX (31°C) and SmX-crystalline (0°C ), where SmX was attributed to a higher-ordered smectic mesophase. This clearly reflects the ability of the polymer matrix to stabilise the smectic mesophase, a more ordered smectic mesophase being stabilised up to~0°C to the detriment of the crystalline state.
The exact reproduction of the DSC curves on many heating/cooling scans shows the immiscibility of the BBO liquid crystal with the PVAB matrix, and therefore PDLC morphologic stability.
Comparing the DSC curves of the pure PVAB and PDLC composites, one can observe that the secondorder transition around 60°C of the pure PVAB, corresponding to its glass transition (Tg), could not be seen in the case of the PDLCs, even by heating/cooling the sample from room temperature up to 80°C. This absence of the Tg suggests the enhancement of the ordering degree of the PVAB matrix in the PDLC compositesin agreement with POM observations, enforcing the conclusion that there is an ordering coupling at the liquid crystal-polymer matrix interface.

Scanning electron microscopy
To evaluate the microstructure of the PDLC composites, the film samples were scanned at 10 keV, i) as obtained and ii) preheated at 80°C, to reach the isotropic state of the BBO liquid crystal.
The film samples show spherical profiles on the surface, indicating the spherical shape of the liquid crystalline droplets (Figure 7). Compared to other smectic LC-based PDLCs, the ability of the PVAB matrix is remarkable in constraining the smectic LC to adopt a round droplet shape instead of batonnetscharacteristic of SmA mesophase. [14] Taking into consideration the tendency of the BBO liquid crystal to crystallise as needles either from solution (Figure 2(a)) or the molten state (Figure 2(d)), the round droplets reflect the influence of the polymer matrix which exerts enough interface tension to prevent the needle-like growth tendency and to determine a spherical confinement during the liquid-liquid demixing process.
The film samples preheated at 80°C show round holes, their spatial distribution depending on the LC content: the holes are denser as the amount of LC increases (Figure 8(a-d)). Remarkably, the droplet diameter does not change significantly as the percentage of the liquid crystal increases, indicating no significant influence of the liquid crystal/polymer ratio on the interphase forces which drive the droplet formation. As can be seen in Figure 8(a'-d'), which represents the diameter size distribution and standard deviation value, the droplet diameter is in the range 4-11 μm, with a predominant droplet population around 7 μm. It  is observed that the increase in the amount of liquid crystal in the polymeric matrix is accompanied by a decrease in the standard deviation, indicating the narrower dimensional polydispersity of the formed droplets.
Systematic studies of PDLC composites proved that devices with good electro-optic performance were obtained for micrometric droplets with diameter less than 10 μm, which assures a good balance between the transmittance and scattering of the incident light. [35][36][37] Moreover, it was demonstrated that PDLCs containing a higher concentration of liquid crystal droplets enhance the device stability, while micrometric droplets which are able to preserve the intrinsic properties of the pure liquid crystal allow good transmission of bistable states. [13] Taking into consideration all the above, it is observed that the understudy composites meet the morphological features required by the highperformance PDLC materials associated with electrooptical devices, even of bi-stable devices.

Hydrophilicity and surface free energy calculation
The surface properties of a biomaterial are essential features which define its behaviour in a biological environment, and thus its potential for a good hostimplanted integration. Important parameters which are usually considered when the level of biocompatibility of a biomaterial is evaluated are the contact angle and the surface free energy. Exhaustive studies have demonstrated that biomaterials with moderate wettability/hydrophilicitywhich correspond to a water contact angle value in the range 60-90º, elicit the least foreign-body reactions for long-term implantation. [38] On the other hand, in the case of ocular applications (lenses, artificial corneas, etc.) the biomaterial must have a water contact angle value close to that of the natural cornea, which is 50º. [39] Moreover, studies on PVA indicate that the contact angle value varies as a function of the crystallinity degree, offering the possibility of modulating the application. This is an important feature, taking into consideration that POM and DSC measurements indicate the ability to tune the PVAB confinement matrix crystallinity.
In the light of these data, the water contact angle of two pure components and their PDLC composites was measured on the same films at different times after their preparation: 2 weeks and 4 months. As can be seen in Figure 9, the contact angle of the pure BBO and PVAB film was not significantly affected by time, while that of the PDLC composites decreases as the liquid crystal content increased, reaching a difference of 7º in the case of P4. This behaviour is in agreement with the increase in the crystallinity degree of the polymer matrix when the hydroxyl group density on its surface increased. [39] The crystallinity degree increased even more for a higher BBO/PVAB weight ratio, due to the larger surface interphase which determines a more efficient coupling of the ordering at the interface between the two components.
The water contact angle value of the BBO and PVAB pure components indicates moderate hydrophilicity, both materials being potential biocompatible. By combining them into PDLC composites, the water contact angle value is of moderate wettability for smaller amounts of BBO (P1, P2), but is of moderate hydrophilicity for higher BBO amounts (P3, P4). Taking into consideration that higher droplet density is desirable for PDLC applications, these results suggest that P3, P4 composite films are potential candidates for indwelling biomedical applications. On the other hand, the decrease in contact angle of P2 and P3 films at values close to that of the corneal surface indicate them as potential candidates for ocular biomedical applications. Since the decrease in water contact angle is accompanied by an increase in the degree of crystallinity (as proved by POM and DSC measurements), which means improved mechanical properties, their potential value for this kind of application is enhanced.
Surface free energy (SFE) is a surface property that reflects the chemical composition of the material and even the orientation of the molecules, aspects which are important mainly in the case of PDLC systems in which molecules inside the liquid crystal droplets are anchored within the PDLC matrix by interphase forces. Total surface free energy (γ TOT s ), dispersive (γ LW s ), polar (γ AB s ), acidic (γ S + ) and basic (γ S − ) components of the surface free energy were calculated based on the contact angle at equilibrium between PDLC surface and three pure liquids: twice-distilled water, formamide and diiodomethane. [12,25] It is expected that the interphase interaction at the liquid crystal-polymer boundary will determine the BBO configuration in PDLC droplets and, on the other hand, influence the surface free energy, which can be further electrically switched and thus used to design bio-sensors as blood sensors and sperm testers. [40] Moreover, it was demonstrated that biomaterials with surface free energy higher than 22 mN/m favour cellular adhesion and maintain tissue multicellular structure. [38,41] As can be seen from Figure 10(a), pure PVAB film has a sufficiently high value of surface energy of 47.46 mN/m, with an important contribution of dispersive Lisfshitz-van der Waals (γ S LW = 37) and polar (γ a = 10.46) forces given by the significant electrondonating (γ S -= 11.58) and less important electronaccepting (γ S + = 2.36) contributions. As expected, these results indicate a much larger contribution of basic sites (attributed to the electron-rich oxygen atoms) compared to acidic ones (attributed to electron-deficient boron atoms), due to the higher number of rich electron oxygen atoms against electron-deficient boron ones in the PVAB macromolecules.
The BBO liquid crystal based on benzoate units has also a high value of surface energy of 42.8 mN/m, given by a higher contribution of the dispersive Lisfshitz-van der Waals (γ S LW = 40.32) and a less important one of the polar forces (γ a = 2.48) as a result of an almost absent contribution of electron acceptance (γ S + = 0.08), despite an important electron-donating (γ S − = 18.22) contribution. This is in agreement with the liquid crystal structure consisting of rich electron ester groups and extremely poor electron non-polar aliphatic units. Compared to the PVAB matrix, the major surface free energy changes of the PDLCs result from the changes of polar (γ ab ) contributions given by acidic (γ S + ) and basic (γ S − ) parts, the Lifshitz-van der Walls component (γ S LW ) being less affected by BBO liquid crystal addition into the PVAB matrix, meaning that BBO and PVAB interact mainly through hydrogen bonds and Lewis acid-base forces ( Figure 10). Considering the acid-base theory, the boron from PVAB works as a Lewis acid (electron acceptor) whereas the hydroxyl group from PVAB and ester group from LC work as a Lewis base (electron donor).
The polar part γ ab and Lewis acid γ + decrease while Lewis base γ − component increases with increasing the BBO content in the PDLC composites, according with the interfacial interaction between PVAB and BBO which involves the predominant basic sites of BBO (COO) and acidic sites of PVAB (B), which renders more predominant OH basic sides of the PVAB to the interface.
Analysing the surface forces of the films kept for 4 months (Figure 10(b)), it can be seen that the contribution of acidic sites (boron atoms) decreased even more, almost disappearing, accompanied by increasing of the basic sites' contribution (OH). This fact confirms rearrangement of the PVAB macromolecules around the BBO droplets which affect the composite surface. Thus, during this time, more boron acidic sites oriented to the BBO-PVAB interface, leading to increasing of OH basic sites' density accessible at the composite surface. The matrix and LC interact through the electrondeficient boron and electron-enriched ester group while hydroxyl groups from PVAB are oriented towards the surface, and thus the Lewis base component of SFE increases. No significant changes of the non-polar sites were seen, indicating insignificant van der Waals interaction at the interface.
Comparing the surface energy values of the PDLC composites with those of natural cornea, [39] close values of the total surface energy and its apolar and polar components can be seen, indicating once again the potential of the study materials for biological applications.

Conclusions
The obtaining of polymer dispersed liquid crystals based on a new smectogen and polyvinylalcohol-boric acid as polymer matrix revealed the ability of PVAB to constrain the growing of liquid crystal in smectic droplets. PVAB is a biocompatible polymer, soluble only in hot water, transparent, immiscible with liquid crystals, assuring good optical properties. The new smectic liquid crystal has a direct isotropic-smectic transition and forms a smectic A mesophase which is stable over a wide temperature range (69-28°C), superposed with that of the human body. The use of the PVAB as matrix succeeded in embedding of the smectic liquid crystal as planar anchored droplets due to the interfacial attractive forces between the electron-deficient boron and electron-rich ester units. The resultant PDLC films present a uniform distribution of the smectic droplets with narrow size polydispersity and diameter around 7 μmideal dimensional parameters for opto-electronic applications. The two components are completely immiscible, a fact which points to the advantage in keeping their optical properties and precludes the loss of liquid crystal as plasticiser of the matrix. The value of surface free energy of PDLC films is sufficiently high to assure electrical switching and cell adhesion. Moreover, the hydrophilicity of the PDLC films can be tuned by the simple control of PVAB matrix morphology, to meet the requirements of different biological applications. All these findings recommend the studied PDLC as a material for biomedical engineering.