On the role of surface composition and curvature on biointerface formation and colloidal stability of nanoparticles in a protein-rich model system

Silica Nanoparticles: A Promising Vehicle for Anti-Cancer Drugs Delivery

The prevalence and death due to cancer have been rising over the past few decades, and eliminating tumour cells without sacrificing healthy cells remains a difficult task. Due to the low specificity and solubility of drug molecules, patients often require high dosages to achieve the desired therapeutic effects. Silica nanoparticles (SiNPs) can effectively deliver therapeutic agents to targeted sites in the body, addressing these challenges. Using SiNPs as vehicles for anti-cancer drug delivery has emerged as a promising strategy due to their unique structural properties, biocompatibility, and versatility. This review explores the various aspects of SiNPs in cancer therapy, highlighting their synthesis, functionalization, and application in delivering chemotherapeutic agents, photosensitizers, and nucleic acids. SiNPs offer advantages such as high drug loading capacity, controlled release, and targeted delivery, enhancing therapeutic efficacy and reducing systemic toxicity. Moreover, this review aims to provide an in-depth understanding of the current state and prospects of SiNPs in revolutionizing cancer treatment and improving patient outcomes.

Multifunctional Polymer Restraint of the Agglomeration of SnO2 Nanocrystals for Efficient and Stable Planar Perovskite Solar Cells

The aggregation of SnO2 nanocrystals due to van der Waals interactions is not conducive to the realization of a compact and pinhole-free electron transport layer (ETL). Herein, we have utilized potassium alginate (PA) to self-assemble SnO2 nanocrystals, forming a PA-SnO2 ETL for perovskite solar cells (PSCs). Through density functional theory (DFT) calculations, PA can be effectively absorbed onto the surface of SnO2. This inhibits the agglomeration of SnO2 nanocrystals in solution, forming a smoother pinhole-free film. This also changes the surface contact potential (CPD) of the SnO2 film, which leads to a reduction in the energy barrier between the ETL and the perovskite layers, promotes effective charge transfer, and reduces trap density. Consequently, the power conversion efficiency (PCE) of PSCs with a PA-SnO2 ETL increased from 19.24% to 22.16%, and the short-circuit current (JSC) was enhanced from 23.52 to 25.21 mA cm-2. Furthermore, the PA-modified unpackaged device demonstrates better humidity stability compared to the original device.

Estimation of SPIO Nanoparticles Uptakes by Macrophages Using Transmission Electron Microscopy

Due to their interesting size-dependent magnetic characteristics and relative biocompatibility, magnetic superparamagnetic iron oxide (SPIO) nanoparticles have been widely exploited as probes for cell and subcellular structure identification, as well as medication and gene delivery. A thorough understanding of the mechanics of the interaction between nanoparticles and macrophages is vital in managing dynamic processes in nanomedicine. In this study, the interaction behavior and uptake of SPIO nanoparticles by M1- and M2-type macrophages were investigated. Mice monocytes were differentiated into M1 and M2 macrophages, and the uptake of SPIO nanoparticles was studied using a TEM microscope. A high resolution image of 1 nm resolution, an image processing technique, was developed to extract the SPIO-NPs from tomographic TEM microscopic images. Lysosomes appear to be the zones of high concentrations of SPIO inside macrophages. Lysosomes were first selected in each image, and then segmentation by the Otsu thresholding method was used to extract the SPIO-NPs. The Otsu threshold method is a global thresholding technique used to automatically differentiate SPIOs from the background. The SPIO-NPs appear in red colors, and the other pixels in the image are considered background. Then, an estimation of the SPIO-NP uptakes by lysosomes is produced. Higher uptake of all-sized nanoparticles was observed in M1- and M2-type macrophages. An accurate estimation of the number of SPIO-NPs was obtained. This result will help in controlling targeted drug delivery and assessing the safety impact of the use of SPIO-NPs in nanomedicine for humans.

Performance of nanoparticles for biomedical applications: The in vitro/in vivo discrepancy

Nanomedicine has a great potential to revolutionize the therapeutic landscape. However, up-to-date results obtained from in vitro experiments predict the in vivo performance of nanoparticles weakly or not at all. There is a need for in vitro experiments that better resemble the in vivo reality. As a result, animal experiments can be reduced, and potent in vivo candidates will not be missed. It is important to gain a deeper knowledge about nanoparticle characteristics in physiological environment. In this context, the protein corona plays a crucial role. Its formation process including driving forces, kinetics, and influencing factors has to be explored in more detail. There exist different methods for the investigation of the protein corona and its impact on physico-chemical and biological properties of nanoparticles, which are compiled and critically reflected in this review article. The obtained information about the protein corona can be exploited to optimize nanoparticles for in vivo application. Still the translation from in vitro to in vivo remains challenging. Functional in vitro screening under physiological conditions such as in full serum, in 3D multicellular spheroids/organoids, or under flow conditions is recommended. Innovative in vivo screening using barcoded nanoparticles can simultaneously test more than hundred samples regarding biodistribution and functional delivery within a single mouse.

Hybrid Hierarchical Heterostructures of Nanoceramic Phosphors as Imaging Agents for Multiplexing and Living Cancer Cells Translocation

Existing fluorescent labels used in life sciences are based on organic compounds with limited lifetime or on quantum dots which are either expensive or toxic and have low kinetic stability in biological environments. To address these challenges, luminescent nanomaterials have been conceived as hierarchical, core–shell structures with spherical morphology and highly controlled dimensions. These tailor-made nanophosphors incorporate Ln:YVO₄ nanoparticles (Ln = Eu(III) and Er(III)) as 50 nm cores and display intense and narrow emission maxima centered at ∼565 nm. These cores can be encapsulated in silica shells with highly controlled dimensions as well as functionalized with chitosan or PEG5000 to reduce nonspecific interactions with biomolecules in living cells. Confocal fluorescence microscopy in living prostate cancer cells confirmed the potential of these platforms to overcome the disadvantages of commercial fluorophores and their feasibility as labels for multiplexing, biosensing, and imaging in life science assays.

Multifunctional Polymer-Regulated SnO2 Nanocrystals Enhance Interface Contact for Efficient and Stable Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) have rapidly developed and achieved power conversion efficiencies of over 20% with diverse technical routes. Particularly, planar-structured PSCs can be fabricated with low-temperature (≤150 °C) solution-based processes, which is energy efficient and compatible with flexible substrates. Here, the efficiency and stability of planar PSCs are enhanced by improving the interface contact between the SnO₂ electron-transport layer (ETL) and the perovskite layer. A biological polymer (heparin potassium, HP) is introduced to regulate the arrangement of SnO₂ nanocrystals, and induce vertically aligned crystal growth of perovskites on top. Correspondingly, SnO₂ -HP-based devices can demonstrate an average efficiency of 23.03% on rigid substrates with enhanced open-circuit voltage (V_OC ) of 1.162 V and high reproducibility. Attributed to the strengthened interface binding, the devices obtain high operational stability, retaining 97% of their initial performance (power conversion efficiency, PCE > 22%) after 1000 h operation at their maximum power point under 1 sun illumination. Besides, the HP-modified SnO₂ ETL exhibits promising potential for application in flexible and large-area devices.

Following in Situ the Degradation of Mesoporous Silica in Biorelevant Conditions: At Last, a Good Comprehension of the Structure Influence

Mesoporous silica nanoparticles (MSNs) have seen a fast development as drug delivery carriers thanks to their tunable porosity and high loading capacity. The employ of MSNs in biomedical applications requires a good understanding of their degradation behavior both to control drug release and to assess possible toxicity issues on human health. In this work, we study mesoporous silica degradation in biologically relevant conditions through in situ ellipsometry on model mesoporous nanoparticle or continuous thin films, in buffer solution and in media containing proteins. In order to shed light on the structure/dissolution relationship, we performed dissolution experiments far from soluble silicate species saturation. Via a complete decorrelation of dissolution and diffusion contributions, we proved unambiguously that surface area of silica vectors is the main parameter influencing dissolution kinetics, while thermal treatment and open mesoporous network architecture have a minor impact. As a logical consequence of our dissolution model, we proved that the dissolution lag-time can be promoted by selective blocking of the mesopores that limits the access to the mesoporous internal surface. This study was broadened by studying the impact of the organosilanes in the silica structure, of the presence of residual structuring agents, and of the chemical composition of the dissolution medium. The presence of albumin at blood concentration was found affecting drastically the dissolution kinetics of the mesoporous structure, acting as a diffusion barrier. Globally, we could identify the main factors affecting mesoporous silica materials degradation and proved that we can tune their structure and composition for adjusting dissolution kinetics in order to achieve efficient drug delivery.

Physico–Chemical Interaction between Clay Minerals and Albumin Protein according to the Type of Clay

Clay minerals are widely utilized in pharmaceutical and dermatological sciences as a gastrointestinal medicine or skin remediation agent. In order to verify the feasibility of clays as an injection, pill, or topical agent, it is important to study their interactions with biological components, such as proteins. In this study, we utilized a protein fluorescence quenching assay and circular dichroism spectroscopy to evaluate general aspects of protein denaturation and conformational change, respectively. Three different clays; layered double oxide (LDO), montmorilonite (MMT) and halloysite nanotube (HNT), were treated with albumin and the physico-chemical effect on the protein’s conformation was investigated. MMT was shown to influence the conformational change the most, owing to the large accessible adsorption site. HNT showed meaningful circular dichroism (CD) band collapse as well as fluorescence quenching in the protein, suggesting a potential harmful effect of HNT toward the protein. Among the three tested clays, LDO was determined to affect protein structure the least in terms of three-dimensional conformation and helical structure.

Monitoring characteristics and genotoxic effects of engineered nanoparticle-protein corona

Engineered nanoparticles (ENPs) possess different physical and chemical properties compared to their bulk counterparts. These unique properties have found application in various products in the area of therapeutics, consumer goods, environmental remediation, optical and electronic fields. This has also increased the likelihood of their release into the environment thereby affecting human health and ecosystem. ENPs, when in contact with the biological system have various physical and chemical interactions with cellular macromolecules including proteins. These interactions lead to the formation of protein corona around the ENPs. Consequently, living systems interact with the protein-coated ENP rather than with a bare ENP. This ENP-protein interaction influences uptake, accumulation, distribution and clearance and thereby affecting the cytotoxic and genotoxic responses. Although there are few studies which discussed the fate of ENPs, there is a need for extensive research in the field of ENPs, to understand the interaction of ENPs with biological systems for their safe and productive application.

Modulation of serum albumin protein corona for exploring cellular behaviors of fattigation-platform nanoparticles

Albumin is the most abundant protein in blood, and is the most frequently identified protein in the protein corona of nanoparticles (NPs). Thus, albumin plays an important role in modulating NPs' physicochemical properties and bioavailability. In this study, the effect of bovine serum albumin (BSA) on gelatin-oleic nanoparticles' (GONs) physicochemical properties and cellular uptake were evaluated. Coumarin-6 was used as indicator to track the cellular uptake of GONs. The binding of BSA onto the GON surface increased the size, slightly reduced the negative net charge of the GON, and improved GON stability. The presence of BSA in cell culture media reduced the cellular uptake of BSA-uncoated GONs on human embryonic kidney cells 293 (HEK 293) and human adenocarcinoma alveolar basal epithelial cells (A549) in the media without FBS addition. Pre-coated BSA corona decreased cellular uptake of GONs in A549 cells in the media, with and without supplemented with 10% fetal bovine serum (FBS) but drastically increased cellular uptake on HEK 293 cells. BSA could be used to modulate protein corona as an endogenous ligand in NP design simply by mixing or incubating BSA with NPs before in vivo administration to inhibit or induce cellular uptake in specific cell types.

Protein corona: a new approach for nanomedicine design

After administration of nanoparticle (NP) into biological fluids, an NP-protein complex is formed, which represents the "true identity" of NP in our body. Hence, protein-NP interaction should be carefully investigated to predict and control the fate of NPs or drug-loaded NPs, including systemic circulation, biodistribution, and bioavailability. In this review, we mainly focus on the formation of protein corona and its potential applications in pharmaceutical sciences such as prediction modeling based on NP-adsorbed proteins, usage of active proteins for modifying NP to achieve toxicity reduction, circulation time enhancement, and targeting effect. Validated correlative models for NP biological responses mainly based on protein corona fingerprints of NPs are more highly accurate than the models solely set up from NP properties. Based on these models, effectiveness as well as the toxicity of NPs can be predicted without in vivo tests, while novel cell receptors could be identified from prominent proteins which play important key roles in the models. The ungoverned protein adsorption onto NPs may have generally negative effects such as rapid clearance from the bloodstream, hindrance of targeting capacity, and induction of toxicity. In contrast, controlling protein adsorption by modifying NPs with diverse functional proteins or tailoring appropriate NPs which favor selective endogenous peptides and proteins will bring promising therapeutic benefits in drug delivery and targeted cancer treatment.

Nanoparticles as drug carriers: current issues with in vitro testing

Incorporation of nanotechnology in the field of drug delivery has created exciting opportunities for the purposeful design of nanocarriers with potentials such as targeted delivery or controlled release of the incorporated cargo, improvement of bioavailability and reduction of therapeutic side-effects. Prior to in vivo administration, nanocarriers should undergo a set of in vitro evaluation procedures to ensure their stability, safety, conformity and ability to fulfill the desired mission. In this paper, current issues with in vitro evaluation techniques used for nanocarrier characterization (assessment of particle size, surface charge, drug release and toxicity) will be discussed. Furthermore, sufficiency of in vitro evaluation procedures for the prediction of in vivo scenarios and the necessary considerations to improve the correlation between the two settings will be debated.

Surface Functionalization of Nanoparticles with Polyethylene Glycol: Effects on Protein Adsorption and Cellular Uptake

Here we have investigated the effect of enshrouding polymer-coated nanoparticles (NPs) with polyethylene glycol (PEG) on the adsorption of proteins and uptake by cultured cells. PEG was covalently linked to the polymer surface to the maximal grafting density achievable under our experimental conditions. Changes in the effective hydrodynamic radius of the NPs upon adsorption of human serum albumin (HSA) and fibrinogen (FIB) were measured in situ using fluorescence correlation spectroscopy. For NPs without a PEG shell, a thickness increase of around 3 nm, corresponding to HSA monolayer adsorption, was measured at high HSA concentration. Only 50% of this value was found for NPs with PEGylated surfaces. While the size increase clearly reveals formation of a protein corona also for PEGylated NPs, fluorescence lifetime measurements and quenching experiments suggest that the adsorbed HSA molecules are buried within the PEG shell. For FIB adsorption onto PEGylated NPs, even less change in NP diameter was observed. In vitro uptake of the NPs by 3T3 fibroblasts was reduced to around 10% upon PEGylation with PEG chains of 10 kDa. Thus, even though the PEG coatings did not completely prevent protein adsorption, the PEGylated NPs still displayed a pronounced reduction of cellular uptake with respect to bare NPs, which is to be expected if the adsorbed proteins are not exposed on the NP surface.

Electrolyte effect on the phase behavior of silica nanoparticles with lysozyme and bovine-serum-albumin proteins

Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) studies have been carried out to investigate the effect of an electrolyte on the phase behavior of anionic silica nanoparticles with two globular proteins-cationic lysozyme [molecular weight (MW) 14.7 kDa] and anionic bovine serum albumin (MW 66.4 kDa). The results are compared with our earlier published work on similar systems without any electrolyte [I. Yadav, S. Kumar, V. K. Aswal, and J. Kohlbrecher, Phys. Rev. E 89, 032304 (2014)]. Both the nanoparticle-protein systems transform to two phase at lower concentration of protein in the presence of an electrolyte. The autocorrelation function in DLS suggests that the diffusion coefficient (D) of a nanoparticle-protein system decreases in approaching two phase with the increase in protein concentration. This variation in D can be attributed to increase in attractive interaction and/or overall increase in the size. Further, these two contributions (interaction and structure) are determined from the SANS data. The changes in the phase behavior of nanoparticle-protein systems in the presence of an electrolyte are explained in terms of modifications in both the repulsive and attractive components of interaction between nanoparticles. In a two-phase system individual silica nanoparticles coexist along with their fractal aggregates.

Functionalized O-carboxymethyl-chitosan/polyethylenimine based novel dual pH-responsive nanocarriers for controlled co-delivery of DOX and genes

Dual pH-sensitive CDPD sequentially dissociates the outermost layer, CPN, and shows pH-triggered programmable release of DOX and genes.

Surface treatment of silica nanoparticles for stable and charge-controlled colloidal silica

An attempt was made to control the surface charge of colloidal silica nanoparticles with 20 nm and 100 nm diameters. Untreated silica nanoparticles were determined to be highly negatively charged and have stable hydrodynamic sizes in a wide pH range. To change the surface to a positively charged form, various coating agents, such as amine containing molecules, multivalent metal cation, or amino acids, were used to treat the colloidal silica nanoparticles. Molecules with chelating amine sites were determined to have high affinity with the silica surface to make agglomerations or gel-like networks. Amino acid coatings resulted in relatively stable silica colloids with a modified surface charge. Three amino acid moiety coatings (L-serine, L-histidine, and L-arginine) exhibited surface charge modifying efficacy of L-histidine > L-arginine > L-serine and hydrodynamic size preservation efficacy of L-serine > L-arginine > L-histidine. The time dependent change in L-arginine coated colloidal silica was investigated by measuring the pattern of the backscattered light in a Turbiscan™. The results indicated that both the 20 nm and 100 nm L-arginine coated silica samples were fairly stable in terms of colloidal homogeneity, showing only slight coalescence and sedimentation.

Optical imaging of absorption and distribution of RITC-SiO2 nanoparticles after oral administration

Purpose

In this study, we investigated the absorption and distribution of rhodamine B isothiocyanate (RITC)-incorporated silica oxide nanoparticles(SiNPs) (RITC-SiNPs) after oral exposure, by conducting optical imaging, with a focus on tracking the movement of RITC-SiNPs of different particle size and surface charge.

Methods

RITC-SiNPs (20 or 100 nm; positively or negatively charged) were used to avoid the dissociation of a fluorescent dye from nanoparticles via spontaneous or enzyme-catalyzed reactions in vivo. The changes in the nanoparticle sizes and shapes were investigated in an HCl solution for 6 hours. RITC-SiNPs were orally administered to healthy nude mice at a dose of 100 mg/kg. Optical imaging studies were performed at 2, 4, and 6 hours after oral administration. The mice were sacrificed at 2, 4, 6, and 10 hours post-administration, and ex vivo imaging studies were performed.

Results

The RITC-SiNPs were stable in the HCl solution for 6 hours, without dissociation of RITC from the nanoparticles and without changes in size and shape. RITC-SiNPs flowed into the small intestine from the stomach and gradually moved along the gut during the experiment. In the ex vivo imaging studies, optical signals were observed mostly in the lungs, liver, pancreas, and kidneys. The orally administered RITC-SiNPs, which were absorbed in the systemic circulation, were eliminated from the body into the urine. The 20 nm RITC-SiNPs showed higher uptake in the lungs than the 100 nm RITC-SiNPs. The distribution of the 100 nm RITC-SiNPs in the liver was higher than that of the 20 nm RITC-SiNPs, but the differences in the surface charge behavior were imperceptible.

Conclusion

We demonstrated that the movement of RITC-SiNPs after oral exposure could be traced by optical imaging. Optical imaging has the potential to provide valuable information that will help in understanding the behavior of SiNPs in the body following exposure.

In vitro interaction of colloidal nanoparticles with mammalian cells: What have we learned thus far?

The interfacing of colloidal nanoparticles with mammalian cells is now well into its second decade. In this review our goal is to highlight the more generally accepted concepts that we have gleaned from nearly twenty years of research. While details of these complex interactions strongly depend, amongst others, upon the specific properties of the nanoparticles used, the cell type, and their environmental conditions, a number of fundamental principles exist, which are outlined in this review.

The protein corona protects against size- and dose-dependent toxicity of amorphous silica nanoparticles

Besides the lung and skin, the gastrointestinal (GI) tract is one of the main targets for accidental exposure or biomedical applications of nanoparticles (NP). Biological responses to NP, including nanotoxicology, are caused by the interaction of the NP with cellular membranes and/or cellular entry. Here, the physico-chemical characteristics of NP are widely discussed as critical determinants, albeit the exact mechanisms remain to be resolved. Moreover, proteins associate with NP in physiological fluids, forming the protein corona potentially transforming the biological identity of the particle and thus, adding an additional level of complexity for the bio-nano responses. Here, we employed amorphous silica nanoparticles (ASP) and epithelial GI tract Caco-2 cells as a model to study the biological impact of particle size as well as of the protein corona. Caco-2 or mucus-producing HT-29 cells were exposed to thoroughly characterized, negatively charged ASP of different size in the absence or presence of proteins. Comprehensive experimental approaches, such as quantifying cellular metabolic activity, microscopic observation of cell morphology, and high-throughput cell analysis revealed a dose- and time-dependent toxicity primarily upon exposure with ASP30 (Ø = 30 nm). Albeit smaller (ASP20, Ø = 20 nm) or larger particles (ASP100; Ø = 100 nm) showed a similar zeta potential, they both displayed only low toxicity. Importantly, the adverse effects triggered by ASP30/ASP30L were significantly ameliorated upon formation of the protein corona, which we found was efficiently established on all ASP studied. As a potential explanation, corona formation reduced ASP30 cellular uptake, which was however not significantly affected by ASP surface charge in our model. Collectively, our study uncovers an impact of ASP size as well as of the protein corona on cellular toxicity, which might be relevant for processes at the nano-bio interface in general.

Topography-driven bionano-interactions on colloidal silica nanoparticles

We report here that the surface topography of colloidal mesoporous silica nanoparticles (MSNs) plays a key role on their bionano-interactions by driving the adsorption of biomolecules on the nanoparticle through a matching mechanism between the surface cavities characteristics and the biomolecules stereochemistry. This conclusion was drawn by analyzing the biophysicochemical properties of colloidal MSNs in the presence of single biomolecules, such as alginate or bovine serum albumin (BSA), as well as dispersed in a complex biofluid, such as human blood plasma. When dispersed in phosphate buffered saline media containing alginate or BSA, monodisperse spherical MSNs interact with linear biopolymers such as alginate and with a globular protein such as bovine serum albumin (BSA) independently of the surface charge sign (i.e. positive or negative), thus leading to a decrease in the surface energy and to the colloidal stabilization of these nanoparticles. In contrast, silica nanoparticles with irregular surface topographies are not colloidally stabilized in the presence of alginate but they are electrosterically stabilized by BSA through a sorption mechanism that implies reversible conformation changes of the protein, as evidenced by circular dichroism (CD). The match between the biomolecule size and stereochemistry with the nanoparticle surface cavities characteristics reflects on the nanoparticle surface area that is accessible for each biomolecule to interact and stabilize any non-rigid nanoparticles. On the other hand, in contact with variety of biomolecules such as those present in blood plasma (55%), MSNs are colloidally stabilized regardless of the topography and surface charge, although the identity of the protein corona responsible for this stabilization is influenced by the surface topography and surface charge. Therefore, the biofluid in which nanoparticles are introduced plays an important role on their physicochemical behavior synergistically with their inherent characteristics (e.g., surface topography).

Small-angle neutron scattering study of differences in phase behavior of silica nanoparticles in the presence of lysozyme and bovine serum albumin proteins

The differences in phase behavior of anionic silica nanoparticles (88 Å) in the presence of two globular proteins [cationic lysozyme (molecular weight (MW) 14.7 kD) and anionic bovine serum albumin (BSA) (MW 66.4 kD)] have been studied by small-angle neutron scattering. The measurements were carried out on a fixed concentration (1 wt %) of Ludox silica nanoparticles with varying concentrations of proteins (0-5 wt %) at pH = 7. It is found that, despite having different natures (opposite charges), both proteins can render to the same kind of aggregation of silica nanoparticles. However, the concentration regions over which the aggregation is observed are widely different for the two proteins. Lysozyme with very small amounts (e.g., 0.01 wt %) leads to the aggregation of silica nanoparticles. On the other hand, silica nanoparticles coexist with BSA as independent entities at low protein concentrations and turn to aggregates at high protein concentrations (>1 wt %). In the case of lysozyme, the charge neutralization by the protein on the nanoparticles gives rise to the protein-mediated aggregation of the nanoparticles. The nanoparticle aggregates coexist with unaggregated nanoparticles at low protein concentrations, whereas, they coexist with a free protein at higher protein concentrations. For BSA, the nonadsorbing nature of the protein produces the depletion force that causes the aggregation of the nanoparticles at higher protein concentrations. The evolution of the interaction is modeled by the two Yukawa potential, taking account of both attractive and repulsive terms of the interaction in these systems. The nanoparticle aggregation is found to be governed by the short-range attraction for lysozyme and the long-range attraction for BSA. The aggregates are characterized by the diffusion limited aggregate type of mass fractal morphology.