Soft, Peelable Organogels from Partially Hydrolyzed Poly(vinyl acetate) and Benzene-1,4-diboronic Acid: Applications to Clean Works of Art

New sustainable polymers and oligomers for Cultural Heritage conservation

The development of "green" chemistry materials with enhanced properties is a central topic in numerous applicative fields, including the design of polymeric systems for the conservation of works of art. Traditional approaches in art restoration comprise polymer thickeners and viscous dispersions to partially control solvents in the removal of soil or aged varnishes/coatings from artifacts. Alternatively, polymeric gel networks can be specifically designed to grant full control of the cleaning action, yielding safe, time- and cost-effective restorations. The selection of polymers and oligomers in gel design is crucial to tune solvent upload, retention, and controlled release over the sensitive artistic surfaces. Starting from an overview of traditional polymer formulations and state-of-the-art gel systems for cleaning works of art, we provide here the design of a new class of gels, focusing on the selection of oligomers to achieve gels with tailored hydrophilicity/hydrophobicity. We evaluated the oligomers Hydrophilic-Lipophilic Balance (HLB) by developing, for the first time, a novel methodology combining SEC and DOSY NMR analysis, which was tested on a library of "green" oligoesters synthesized by polycondensation and poorly explored in the literature. Oligomers with moderate polydispersity were chosen to validate the new protocol as a robust tool for designing polymeric gels even on industrial scale. The methodology is more time-effective than traditional methods, and gives additional insights on the oligomers physico-chemical nature, evaluating their compatibility with different solvents. Then, we used the selected oligoesters with castor oil to obtain a new class of organogels able to upload solvents with varying polarity, which effectively removed different types of unwanted layers typically found in painting restoration. These results validate the oligomers screening approach and the new class of gels as promising chemical processes/materials in art preservation. The methodology can potentially allow evaluation of HLB also for small molecules (e.g., surfactants), opening for the formulation of polymers solutions/gels beyond Cultural Heritage conservation, as in pharmaceutics, cosmetics, food industry, tissue engineering, agriculture, and others.

Organogels: "GelVolution" in Topical Drug Delivery - Present and Beyond

Topical drug delivery holds immense significance in dermatological treatments due to its non-invasive nature and direct application to the target site. Organogels, a promising class of topical drug delivery systems, have acquired substantial attention for enhancing drug delivery efficiency. This review article aims to explore the advantages of organogels, including enhanced drug solubility, controlled release, improved skin penetration, non-greasy formulations, and ease of application. The mechanism of organogel permeation into the skin is discussed, along with formulation strategies, which encompass the selection of gelling agents, cogelling agents, and additives while considering the influence of temperature and pH on gel formation. Various types of organogelators and organogels and their properties, such as viscoelasticity, non-birefringence, thermal stability, and optical clarity, are presented. Moreover, the biomedical applications of organogels in targeting skin cancer, anti-inflammatory drug delivery, and antifungal drug delivery are discussed. Characterization parameters, biocompatibility, safety considerations, and future directions in optimizing skin permeation, ensuring long-term stability, addressing regulatory challenges, and exploring potential combination therapies are thoroughly examined. Overall, this review highlights the immense potential of organogels in redefining topical drug delivery and their significant impact on the field of dermatological treatments, thus paving the way for exciting prospects in the domain.

Non-Aqueous Poly(dimethylsiloxane) Organogel Sponges for Controlled Solvent Release: Synthesis, Characterization, and Application in the Cleaning of Artworks

Polydimethylsiloxane (PDMS) organogel sponges were prepared and studied in order to understand the role of pore size in an elastomeric network on the ability to uptake and release organic solvents. PDMS organogel sponges have been produced according to sugar leaching techniques by adding two sugar templates of different forms and grain sizes (a sugar cube template and a powdered sugar template), in order to obtain materials differing in porosity, pore size distribution, and solvent absorption and liquid retention capability. These materials were compared to PDMS organogel slabs that do not contain pores. The sponges were characterized by Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) and compared with PDMS slabs that do not contain pores. Scanning electron microscopy (SEM) provided information about their morphology. X-ray micro-tomography (XMT) allowed us to ascertain how the form of the sugar templating agent influences the porosity of the systems: when templated with sugar cubes, the porosity was 77% and the mean size of the pores was ca. 300 μm; when templated with powdered sugar, the porosity decreased to ca. 10% and the mean pore size was reduced to ca. 75 μm. These materials, porous organic polymers (POPs), can absorb many solvents in different proportions as a function of their polarity. Absorption capacity, as measured by swelling with eight solvents covering a wide range of polarities, was investigated. Rheology data established that solvent absorption did not have an appreciable impact on the gel-like properties of the sponges, suggesting their potential for applications in cultural heritage conservation. Application tests were conducted on the surfaces of two different lab mock-ups that simulate real painted works of art. They demonstrated further that PDMS sponges are a potential innovative support for controlled and selective cleaning of works of art surfaces.

Removal of Calcareous Concretions from Marine Archaeological Ceramics by Means of a Stimuli-Responsive Hydrogel

The presence of calcareous concretions on the surface of marine archaeological ceramics is a frequently observed phenomenon. It is necessary to remove these materials when the deposits obscure the feature of ceramics. Unfortunately, calcareous concretions provide distinctive documentation of the burning history of ceramics. The interaction of acid solution or detachment of the deposit layers in physical ways leads to the loss of archeological information. To prevent the loss of archeological information and to achieve precise and gentle concretion removal, responsive hydrogel cleaning systems have been developed. The hydrogels synthesized are composed of networks of poly(vinyl acetate)/sodium alginate that exhibit desirable water retention properties, are responsive to Ca²⁺ ions, and do not leave any residues after undergoing cleaning treatment. Four distinct compositions were selected. The study of water retention properties involved quantifying the weight changes. The composition was obtained from Fourier transform infrared spectra. The microstructure was obtained from scanning electron microscopy. The mechanical properties were obtained from rheological measurements. To demonstrate both the efficiency and working mechanism of the selected hydrogels, a representative study of mocked samples is presented first. After selecting the most appropriate hydrogel composite, a cleaning process was implemented on the marine archaeological ceramics. This article demonstrates the advantages of stimuli-responsive hydrogels in controlling the release of acid solution release, thereby surpassing the limitations of traditional cleaning methods.

Nanostructured bio-based castor oil organogels for the cleaning of artworks

HYPOTHESIS

Organic solvents are often used for cleaning highly water-sensitive artifacts in modern/contemporary art. Due to the toxicity of most solvents, confining systems must be formulated to use these fluids in a safe and controlled way. We propose here castor oil (CO) organogels, obtained thorough cost-effective sustainable polyurethane crosslinking. This methodology is complementary to previously demonstrated hydrogels, when conservators opt for organic solvents over aqueous formulations.

EXPERIMENTS

The gels were characterized via Small-angle Neutron Scattering and rheology before and after swelling in two organic solvents commonly adopted in cleaning paintings. The removal of a photo-aged acrylic-ketonic varnish was evaluated under visible and ultraviolet light, and with FTIR 2D imaging.

FINDINGS

The new gels are dry systems that can be easily stored and loaded with solvents before use. Their nanoscale organization, viscoelasticity and cleaning action are controlled changing the amount of crosslinking, the polymeric backbone, and the loaded solvents. The fluids are confined in the nanosized polymeric mesh of the gels, which are highly retentive, granting controlled release over delicate paint layers, and transparent, allowing monitoring of the cleaning process. These features, along with their sustainable synthesis, candidate the CO organogels as feasible solutions for cultural heritage preservation, expanding the palette of advanced tools for conservators over traditional thickeners.

Cross-Linking of Sugar-Derived Polyethers and Boronic Acids for Renewable, Self-Healing, and Single-Ion Conducting Organogel Polymer Electrolytes

This report describes the synthesis and characterization of organogels by reaction of a diol-containing polyether, derived from the sugar d-xylose, with 1,4-phenylenediboronic acid (PDBA). The cross-linked materials were analyzed by infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), scanning electron microscopy (FE-SEM), and rheology. The rheological material properties could be tuned: gel or viscoelastic behavior depended on the concentration of polymer, and mechanical stiffness increased with the amount of PDBA cross-linker. Organogels demonstrated self-healing capabilities and recovered their storage and loss moduli instantaneously after application and subsequent strain release. Lithiated organogels were synthesized through incorporation of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) into the cross-linked matrix. These lithium-borate polymer gels showed a high ionic conductivity value of up to 3.71 × 10^-3 S cm^-1 at 25 °C, high lithium transference numbers (t ₊ = 0.88-0.92), and electrochemical stability (4.51 V). The gels were compatible with lithium-metal electrodes, showing stable polarization profiles in plating/stripping tests. This system provides a promising platform for the production of self-healing gel polymer electrolytes (GPEs) derived from renewable feedstocks for battery applications.

Self-Shaping Microemulsion Gels for Cultural Relic Cleaning

Cleaning is a foundational and essential operation of protection and restoration of cultural relics, which is also the key step of follow-up works. To overcome the problems of uncontrollable diffusion of cleaning solvents and poor coverage of the cleaning solvent carriers on rough surfaces, here, we propose a strategy of using a self-shaping microemulsion gel that is prepared via emulsifying oleophilic solvents into the specific shear-thinning hydrogel structures. The gel can adaptively cover rough surfaces during the cleaning process coupled with avoidance of unnecessary diffusion of the cleaning solvents, and the mechanical reinforcement of in situ polymerized double-network gels enables its easy peeling off from the surfaces without leaving determinable residues. As a representative demonstration, Paraloid B72, a widely used material for the repair and reinforcement of cultural relics, is employed as a model discolored coating, which can be effectively removed from the rough surface of simulated cultural relics after treatment with the resulting gels. Convincingly, the strategy of constructing agarose/polyacrylamide hybrid double-network gels with shear-thinning and self-shaping performances for the cleaning of cultural relics not only improves the convenience and accuracy of operation but also exhibits an efficient cleaning effect, which will greatly expand the application of microemulsion gels in the cleaning of rough surfaces of cultural relics.

Removal of a Past Varnish Treatment from a 19th-Century Belgian Wall Painting by Means of a Solvent-Loaded Double Network Hydrogel

Polymeric materials have been used by painting conservator-restorers as consolidants and/or varnishes for wall paintings. The application of these materials is carried out when confronting loose paint layers or as a protective coating. However, these materials deteriorate and cause physiochemical alterations to the treated surface. In the past, the monumental neo-gothic wall painting ‘The Last Judgment’ in the chapel of Sint-Jan Berchmanscollege in Antwerp, Belgium was treated with a synthetic polymeric material. This varnish deteriorated significantly and turned brown, obscuring the paint layers. Given also that the varnish was applied to some parts of the wall painting and did not cover the entire surface, it was necessary to remove it in order to restore the original appearance of the wall painting. Previous attempts carried out by conservator-restorers made use of traditional cleaning methods, which led to damage of the fragile paint layers. Therefore, gel cleaning was proposed as a less invasive and more controllable method for gently softening and removing the varnish. The work started by identifying the paint stratigraphy and the deteriorated varnish via optical microscopy (OM), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy. A polyvinyl alcohol–borax/agarose (PVA–B/AG) hydrogel loaded with a number of solvents/solvent mixtures was employed in a series of tests to select the most suitable hydrogel composite. By means of the hydrogel composite loaded with 10% propylene carbonate, it was possible to safely remove the brown varnish layer. The results were verified by visual examinations (under visible light ‘VIS’ and ultraviolet light ‘UV’) as well as OM and FTIR spectroscopy.

Biologically Derived Gels for the Cleaning of Historical and Artistic Metal Heritage

In the general global rise of attention and research to seek greener attitudes, the field of cultural heritage (CH) makes no exception. In the last decades, an increasing number of sustainable and biologically based solutions have been proposed for the protection and care of artworks. Additionally, the safety of the target artwork and the operator must be kept as core goals. Within this scenario, new products and treatments should be explored and implemented in the common conservation praxes. Therefore, this review addressing metal heritage is aimed to report biologically derived gel formulations already proposed for this specific area as reliable tools for cleaning. Promising bio-gel-based protocols, still to be implemented in metal conservation, are also presented to promote their investigation by stakeholders in metal conservation. After an opening overview on the common practices for cleaning metallic surfaces in CH, the focus will be moved onto the potentialities of gel-alternatives and in particular of ones with a biological origin. In more detail, we displayed water-gels (i.e., hydrogels) and solvent-gels (i.e., organogels) together with particular attention to bio-solvents. The discussion is closed in light of the state-of-the-art and future perspectives.

Organogel Coupled with Microstructured Electrospun Polymeric Nonwovens for the Effective Cleaning of Sensitive Surfaces

Hydrogels and organogels are widely used as cleaning materials, especially when a controlled solvent release is necessary to prevent substrate damage. This situation is often encountered in the personal care and electronic components fields and represents a challenge in restoration, where the removal of a thin layer of aged varnish from a painting may compromise the integrity of the painting itself. There is an urgent need for new and effective cleaning materials capable of controlling and limiting the use of solvents, achieving at the same time high cleaning efficacy. In this paper, new sandwich-like composites that fully address these requirements are developed by using an organogel (poly(3-hydroxybutyrate) + γ-valerolactone) in the core and two external layers of electrospun nonwovens made of continuous submicrometric fibers produced by electrospinning (either poly(vinyl alcohol) or polyamide 6,6). The new composite materials exhibit an extremely efficient cleaning action that results in the complete elimination of the varnish layer with a minimal amount of solvent adsorbed by the painting layer after the treatment. This demonstrates that the combined materials exert a superficial action that is of utmost importance to safeguard the painting. Moreover, we found that the electrospun nonwoven layers act as mechanically reinforcement components, greatly improving the bending resistance of organogels and their handling. The characterization of these innovative cleaning materials allowed us to propose a mechanism to explain their action: electrospun fibers play the leading role by slowing down the diffusion of the solvent and by conferring to the entire composite a microstructured rough superficial morphology, enabling to achieve outstanding cleaning performance.

Boronate sol-gel method for one-step fabrication of polyvinyl alcohol hydrogel coatings by simple cast- and dip-coating techniques

The self-assembly of polyvinyl alcohol (PVA) and benzene-1,4-diboronic acid (DBA) is employed as a sol–gel method for one-step fabrication of hydrogel coatings with versatile functionalities. A mixture of PVA and DBA in aqueous ethanol is prepared as a coating agent. The long pot life of the mixture allows for the coating of a wide range of materials with hydrogel films by simple cast- and dip-coating techniques. The resultant films show negligible dissolution in water and the intrinsic hydrophilicity of PVA provides the films with functional properties, such as improved antifogging property and resistance to protein and cell fouling. The self-assembling process shows adaptive inclusion properties toward nanoscale materials, such as metal–organic coordination polymers and inorganic nanoparticles, affording composite films. Furthermore, the coating film exhibits a unique secondary functionalization reactivity toward boronic acid-appended fluorescent dyes, through which a variety of materials are converted into fluorescent materials.

The self-assembly of polyvinyl alcohol (PVA) and benzene-1,4-diboronic acid (DBA) is employed as a sol–gel method for one-step fabrication of hydrogel coatings with versatile functionalities.

Maximizing Contact of Supersoft Bottlebrush Networks with Rough Surfaces To Promote Particulate Removal

Efficient removal of particulates from a rough surface with a soft material through a "press and peel" method (i.e., an adhesion and release approach) depends on good conformal contact at the interface; a material should be sufficiently soft to maximize contact with a particle while also conforming to rough surface features to clean the entire substrate surface. Here, we investigate the use of bottlebrush networks-extremely soft elastomers composed of macromolecules with polymeric side chains-as materials for cleaning model substrates of varying roughness. Formed through free-radical polymerization of mono- and dimethacrylate functionalized polysiloxanes, these solvent-free supersoft elastomers exhibit moduli comparable to those of solvated gels, allowing for a lower moduli regime of elastomers to be used in contact experiments than previously possible. By varying the macromonomer to cross-linker ratio, we study the effect of modulus on conformal contact and cleaning for materials that are as soft as gels while minimizing/negating physical and/or chemical concerns that using a traditional material may involve (e.g., changes in component concentrations, solvent evaporation, and syneresis). We study cleaning efficacy by quantifying the conformal contact between soft materials and rough substrates via a contact adhesion-based measurement. These results give insight into the correlation between shear modulus and conformal contact with surfaces of varying feature height. Not only does a decrease in shear modulus leads to improved conformal contact with rough surfaces, but also it facilitates adhesion to particulates situated on the rough surface, thus aiding removal. We highlight this property control with a case study illustrating the removal of an artificial soil mixture from a rough, acrylic surface via peeling rather than rubbing, which can cause damage to delicate surfaces.

Cleaning oil paintings: NMR relaxometry and SPME to evaluate the effects of green solvents and innovative green gels

NMR relaxometry together with SPME allow evaluation of the effects of new green gels for the cleaning of paintings.

A new family of urea-based low molecular-weight organogelators for environmental remediation: the influence of structure

Gelation processes grant access to a wealth of soft materials with tailorable properties, in applications as diverse as environmental remediation, biomedicine and electronics. Several classes of self-assembling gelators have been studied and employ non-covalent bonds to direct assembly, but recently attention has come to focus on how the overall shape of the gelator molecule impacts its gelation. Here we study a new sub-family of low molecular weight organogelators and explore how steric rearrangement influences their gelation. The gels produced are characterised with X-ray diffraction and small-angle neutron scattering (SANS) to probe their ex situ and in situ gelation mechanisms. The best examples were then tested for environmental remediation applications, gelling petrol and oils in the presence of water and salts.

Time-Dependent ATR-FTIR Spectroscopic Studies on Solvent Diffusion and Film Swelling in Oil Paint Model Systems

In the restoration of paintings, solvent diffusion and swelling of polymeric oil paint binding media are important factors to consider. Common cleaning methods with organic solvents or aqueous solutions could lead to undesirable physicochemical changes in the paint in the long term, though the extent of this effect is not yet clear. We used tailored nonporous model systems for aged oil paint to measure paint swelling and solvent diffusion for a wide range of relevant solvents. Using dynamic mechanical analysis (DMA), the glass transition temperature of our model systems was found to be close to room temperature. Subsequently, with a custom sample cell and time-dependent attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, we were able to accurately track swelling and diffusion processes in the polymer films. To quantify the spectroscopic data, we developed a model that completely describes the solvent migration process, including significant film swelling and non-Fickian solvent diffusion. The relation between solvent properties, the diffusion coefficient, and the swelling capacity proved to be rather complex and could not be explained using a single solvent parameter. However, it was found that strongly swelling solvents generally diffuse faster than weakly swelling solvents and that pigmentation does not significantly influence solvent diffusion. These results contribute to a better understanding of transport phenomena in paintings and support the development of improved paint restoration strategies.