Properties Of Biological Materials Research Articles

Biological Material Interfaces as Inspiration for Mechanical and Optical Material Designs.

The extraordinary properties of biological materials often result from their sophisticated hierarchical structures. Through multilevel and cross-scale structural designs, biological materials offset the weakness of their individual building blocks and enhance performance at multiple length scales to match the multifunctional needs of organisms. One essential merit of hierarchical structure is that it can optimize the interfacial features of the "building blocks" at different length scales, from the molecular level to the macroscale. Understanding the roles of biological material interfaces (BMIs) on the determination of properties and functions of biological materials has become a growing interdisciplinary research area in recent years. A pivotal aim of these studies is to use BMIs as inspiration for developing bioinspired and biomimetic materials and devices with advanced structures and functions. Given these considerations, this review aims to comprehensively discuss the structure-property-function relationships of BMIs in nature. We particularly focus on the discussion of BMIs and their inspired materials from mechanical and optical perspectives because these two directions are the most well-investigated and closely related. The challenges and directions of design and fabrication of BMI-inspired mechanical and optical materials are also discussed. This review is expected to garner interest from advanced material communities as well as environmental, nanotechnology, food processing, and engineering fields.

Measurement of Dielectric Properties of Biological Materials Using Co-Axial Fork-Type Probe

This paper delineates a technique for characterizing the dielectric properties of biological materials. A novel design of co-axial fork-type probe with tilted spikes on either side of the central coaxial line to enhance directivity is proposed. The simulation studies were carried out in CST microwave studio to evaluate far-field radiation, S-parameters, and Smith chart. To compute the dielectric parameters of biological sample, a shift in resonant frequency and quality factor of the probe were observed. A theoretical model is developed to calculate the permittivity and conductivity of samples. The model was developed using the simulation analysis and validated on 27 samples. Values of dielectric parameters were obtained from the simulations and compared with standard reported values. The experimental measurements were carried out on 23 samples at room temperature over the frequency range from 5.5 to 7.5 GHz. The repeatability of the measured results has been analyzed by doing different runs, and in this paper, we have observed standard deviation in permittivity is up to two units, which confirms that measurements are consistent and repeatable. To understand the enzyme activity on proteins, the dielectric constant of mix solution of protein (such as egg white, egg yolk, plant protein, and insulin) and papain enzyme at the ratio of 10:1 was investigated through the proposed microwave technique. The decrement in dielectric constant was observed when proteins were added with papain enzyme. This paper on dielectric properties may provide an insight to understand the interaction of proteins with the enzyme. In addition, a microwave surface imaging approach is also used to characterize the dielectric properties of inhomogeneous and composite biological substances.

Measuring light transport properties using speckle patterns as structured illumination

The measurement of light absorption and scattering properties of biological materials has several diagnostic and therapeutic applications. We can measure these properties for skin without contact using structured illumination and imaging. However, building simple handheld devices remains challenging due to motion artefacts and moving targets. To overcome this limitation, we project random speckle patterns instead of discrete spatial frequencies on the target. Since random patterns are spatially broadband, they capture more information per image, enabling frame-by-frame analysis. In this paper, we describe the statistics of objective speckles and demonstrate how the optical system is designed for spatially bandlimited illumination. Next, we use a diverse set of liquid tissue phantom to validate the method. We successfully demonstrate that a calibrated instrument can independently predict the two primary light transport properties of a homogeneous turbid system. This work is a starting point for analysing skin and other heterogeneous biological media in the future.

Mechanical Characterization of 3D Ovarian Cancer Nodules Using Brillouin Confocal Microscopy.

The mechanical interaction between cells and their microenvironment is emerging as an important determinant of cancer progression and sensitivity to treatment, including in ovarian cancer (OvCa). However, current technologies limit mechanical analysis in 3D culture systems. Brillouin Confocal Microscopy is an optical non-contact method to assess the mechanical properties of biological materials. Here, we validate the ability of this technology to assess the mechanical properties of 3D tumor nodules. OvCa cells were cultured in 3D using two established methods: (1) overlay cultures on Matrigel; (2) spheroids in ultra-low attachment plates. To alter the mechanical state of these tumors, nodules were immersed in PBS with varying levels of sucrose to induce osmotic stress. Next, nodule mechanical properties were measured by Brillouin microscopy and validated with standard stress-strain tests: Atomic Force Microscopy (AFM) and a parallel plate compression device (Microsquisher). Finally, the nodules were treated with a chemotherapeutic commonly used to manage OvCa, carboplatin, to determine treatment-induced effects on tumor mechanical properties. Brillouin microscopy allows mechanical analysis with limited penetration depth (~ 92µm for Matrigel method; ~ 54µm for low attachment method). Brillouin microscopy metrics displayed the same trends as the corresponding "gold-standard" Young's moduli measured with stress-strain methods when the osmolality of the medium was increased. Nodules treated with carboplatin showed a decrease in Brillouin frequency shift. This validation study paves the way to evaluate the mechanics of 3D nodules, with micron-scale three-dimensional resolution and without contact, thus extending the experimental possibilities.

Interaction of Radiofrequency Radiation with Biological Systems A Comprehensive Update on Recent Challenges

Rapid advancement of radiofrequency (RF)-driven technologies has greatly affected our everyday lives. Increasing evidence led by in-vitro, in-vivo studies, epidemiological and clinical trials indicates that RF interacts considerably well with biological systems in various ways depending on different exposure parameters and properties of biological materials. Besides their innumerable benefits in different sectors of commercial and military fields, they can induce alterations in many physiological functions of the body, which may culminate into adverse human health consequences. The present article explicitly addresses the RF-based technologies and their applications, fundamentals of RF energy interaction with biological systems, exposure parameters, and dosimetry studies along with thermal and non-thermal effects on different vital organs at molecular and cellular levels. Further, this article outlines the limitations of RF-induced biological effect studies, status of risk assessment, safety levels and its future perspectives.

Nonlinear contact mechanics for the indentation of hyperelastic cylindrical bodies

The mechanical properties of biological materials are commonly found through the application of Hertzian theory to force-displacement data obtained through micro-indentation techniques. Due to their soft nature, biological specimens are often subjected to large indentations, resulting in a nonlinear deformation behavior that can no longer be accurately described by Hertzian contact. Useful models for studying the large deformation response of cylindrical specimens under indentation are not readily available, and the morphologies of biological materials are often closer to cylinders than spheres (e.g., cellular processes, fibrin, collagen fibrils, etc.). In this study, a computational model is used to analyze the large deformation indentation of an incompressible hyperelastic cylinder in order to provide a generalized formulation that can be used to extract mechanical properties from indentation into soft cylindrical bodies. The effects of specimen size and indentation depth are examined in order to quantify the deformation at which the proposed force-displacement relationship remains accurate.

The use of dielectric honey features for overheating diagnostics

During processing operations, honey can be exposed to excessive heat. That adversely affects the quality of product (its biological activity). The aim of this work was to test the possibilities of the use of dielectric properties (relative permittivity and dielectric loss coefficient) for honey overheating detection. Nine honey types obtained directly from beekeepers were investigated. Honey was heated at the temperatures 60, 70, 80, and 90 °C for 24 hours and then cooled down to 25–26 °C. At that temperature, dielectric properties of biological material under study were determined. In the frequency range 400 Hz to 4 kHz, the significant influence of thermal treatment temperature on both dielectric parameters was observed. Both dielectric parameters decrease as the heating temperature increases. The statistical analysis suggested that dielectric parameters can be used for distinguishing honey overheated from not overheated. Therefore, they can be potentially useful for honey quality assessment according to its overheating, e.g. in co-packing process. The proper frequency range for these parameters measurement is 1–4 kHz.

Predicting the effect of aging and defect size on the stress profiles of skin from advancement, rotation and transposition flap surgeries

Predicting mechanical stress contours on skin resulting from local tissue rearrangement surgeries is needed to design optimal treatment plans and avoid wound healing complications. Finite element (FE) simulations of skin tissues have been shown to be a reliable tool in preoperative planning, yet, a major obstacle in the creation of predictive software comes from the inherent uncertainty in material properties of biological materials, and the high computational cost of creating and calibrating virtual surgery models. In this study we build computationally inexpensive surrogates to easily predict stress profiles for arbitrary material parameters and a range of defect sizes in three reconstructive scenarios: advancement, transposition, and rotation flaps. The surrogates are built by first creating a training data set of FE simulations that cover the input space of experimentally-determined skin properties from the literature. A reduced order representation of the training data set is achieved via principal component analysis, and computationally efficient surrogates are then created through Gaussian Process (GP) regression. We show that the GP surrogates predict stress contours with relative errors that are on average 2% in the l2-norm compared to the high-fidelity FE models. We apply the GP surrogates to predict differences in the probability densities of stress contours between two different age groups undergoing the same procedure. By replacing nonlinear FE models with accurate yet inexpensive models that can be evaluated for any combination of human skin material parameters and a range of defect sizes, we aim to enable calibration and prediction of stress contours in individualized clinical cases in the near future.

Preparation and Luminescence Properties of Europium(III) Ternary Complex-modified Poplar Wood-based Materials

Inspired by the ultraviolet induced properties of biological materials, we developed fluorescent wood as a new type of functional material which can be applied to anti-counterfeit label technology and wood-emitting artwork. This study discusses the influence of temperature, time, ligand ratio, and reactant concentration on the structure and properties of fluorescent wood with europium(III) ternary complexes. The surface morphology and fluorescent properties of the modified wood were characterized by scanning electron microscopy (SEM) and fluorescence spectrometry. The results showed that a high reaction temperature of 75 oC was beneficial in obtaining stronger fluorescent wood; a reaction time of 8 h exerted a significant influence on fluorescent wood when the reactant concentration of the sample was 0.03 mol/L. In addition, the results of infrared spectroscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray analysis showed that modification of wood takes place through the Si-OH groups of the silane coupling agent, which link to the surface of wood. After 120 hours of UV light irradiation, the surface fluorescence intensity of the modified wood can reach 500 arb. units. It can be considered that the modified wood has better anti-ultraviolet aging properties, thereby prolonging the life of poplar.

Design and Evaluation of an Electrical Bioimpedance Device Based on DIBS for Myography during Isotonic Exercises

Electrical Bioimpedance Spectroscopy (EIS) is a technique used to assess passive electrical properties of biological materials. EIS detects physiological and pathological conditions in animal tissues. Recently, the introduction of broadband excitation signals has reduced the measuring time for application techniques such as Electrical Bioimpedance Myography. Therefore, this work is aimed at proposing a prototype by using discrete interval binary sequences (DIBS), which is based on a system that holds a current source, impedance acquisition system, microcontroller and graphical user interface. Measurements between 5 Ω to 5 kΩ had impedance acquisition and phase angle errors of aproximately 2% and were lower than 3 degrees, respectively. Based on a proposed circuit, bioimpedance of the chest muscle (Pectoralis Major) was measured during isotonic exercise (push-up). As a result, our analyses have detected tiredness and fatigue. We have explored and proposed new parameters which assess such conditions, as both the maximum magnitude and tiredness coefficient. These parameters decrease exponentially with consecutive push-ups and were convergent in the majority of the sixteen days of measurement.

Dye Aggregate-Mediated Self-Assembly of Bacteriophage Bioconjugates.

One of the central themes of biomolecular engineering is the challenge of exploiting the properties of biological materials. Part of this challenge has been uncovering and harnessing properties of biological components that only emerge following their ordered self-assembly. One biomolecular building block that has received significant interest in the past decade is the M13 bacteriophage. There have been a number of recent attempts to trigger the ordered assembly of M13 bacteriophage into multivirion structures, relying on the innate tendency of M13 to form liquid crystals at high concentrations. These, in general, yield planar two-dimensional materials. Presented here is the production of multivirion assemblies of M13 bacteriophage via the chemical modification of its surface by the covalent attachment of the xanthene-based dye tetramethylrhodamine (TMR) isothiocyanate (TRITC). We show that TMR induces the formation of three-dimensional aster-like assemblies of M13 by providing "adhesive" action between bacteriophage particles through the formation of H-aggregates (face-to-face stacking of dye molecules). We also show that the H-aggregation of TMR is greatly enhanced by covalent attachment to M13 and is enhanced further still upon the ordered self-assembly of M13, leading to the suggestion that M13 could be used to promote the self-assembly of dyes that form J-aggregates, a desirable arrangement of fluorescent dye, which has interesting optical properties and potential applications in the fields of medicine and light harvesting technology.

Review on Electromechanical Coupling Properties of Biomaterials.

Electromechanical coupling properties of biological materials, especially cellulose from plant cell walls and proteins from animals, are of great interest for applications in biocompatible sensors and actuators and ecofriendly energy harvesters. On the basis of their anisotropic nanostructures, cellulose and fibrous proteins such as collagen, silk, keratin, etc. are expected to be piezoelectric; however, this property does not necessarily translate to cellulose- or protein-containing bulk materials. In fact, the values of piezoelectric coefficients reported for cellulose and proteins in the literature vary over several orders of magnitude, which raises the question of whether these are truly intrinsic piezoelectric properties of biological materials or whether they are obscured with other electromechanical coupling processes such as electrostriction, flexoelectricity, electrochemical transport, or electrostatic deformation. This critical question about intrinsic and extrinsic electromechanical coupling mechanisms is reviewed in this article. The origin of piezoelectricity of cellulose and collagen (the most widely studied protein for piezoelectricity) is discussed based on their molecular structures. Key requirements to construct macroscopic piezoelectric biocomposites are addressed in terms of packing orders or arrangements of polar domains in composites. On the basis of this structural argument, truly piezoelectric responses of macroscopic materials fabricated with or containing cellulose and collagen are found to be extremely difficult to observe or quantify; most values reported in the literature as piezoelectric coefficients of such materials appear to originate from other electromechanical coupling mechanisms. Clarifying these mechanisms is important to properly design electromechanical devices using biobased materials.

Dehomogenized Elastic Properties of Heterogeneous Layered Materials in AFM Indentation Experiments

Atomic force microscopy (AFM) is used to study mechanical properties of biological materials at submicron length scales. However, such samples are often structurally heterogeneous even at the local level, with different regions having distinct mechanical properties. Physical or chemical disruption can isolate individual structural elements but may alter the properties being measured. Therefore, to determine the micromechanical properties of intact heterogeneous multilayered samples indented by AFM, we propose the Hybrid Eshelby Decomposition (HED) analysis, which combines a modified homogenization theory and finite element modeling to extract layer-specific elastic moduli of composite structures from single indentations, utilizing knowledge of the component distribution to achieve solution uniqueness. Using finite element model-simulated indentation of layered samples with micron-scale thickness dimensions, biologically relevant elastic properties for incompressible soft tissues, and layer-specific heterogeneity of an order of magnitude or less, HED analysis recovered the prescribed modulus values typically within 10% error. Experimental validation using bilayer spin-coated polydimethylsiloxane samples also yielded self-consistent layer-specific modulus values whether arranged as stiff layer on soft substrate or soft layer on stiff substrate. We further examined a biophysical application by characterizing layer-specific microelastic properties of full-thickness mouse aortic wall tissue, demonstrating that the HED-extracted modulus of the tunica media was more than fivefold stiffer than the intima and not significantly different from direct indentation of exposed media tissue. Our results show that the elastic properties of surface and subsurface layers of microscale synthetic and biological samples can be simultaneously extracted from the composite material response to AFM indentation. HED analysis offers a robust approach to studying regional micromechanics of heterogeneous multilayered samples without destructively separating individual components before testing.

Dielectric Spectroscopy of Biomolecules up to 110 GHz

Abstract Radio-frequency fields in the GHz range are increasingly applied in biotechnology and medicine. In order to fully exploit both their potential and their risks detailed information about the dielectric properties of biological material is needed. For this purpose a measuring system is presented that allows the acquisition of complex dielectric spectra over 4 frequency decade up to 110 GHz. Routines for calibration and for data evaluation according to physicochemical interaction models have been developed. The frequency dependent permittivity and dielectric loss of some proteins and nucleic acids, the main classes of biomolecules, and of their sub-units have been determined. Dielectric spectra are presented for the amino acid alanine, the proteins lysozyme and haemoglobin, the nucleotides AMP and ATP, and for the plasmid pET-21, which has been produced by bacterial culture. Characterisation of a variety of biomolecules is envisaged, as is the application to studies on protein structure and function.

Out-of-plane crashworthiness analysis of bio-inspired aluminum honeycomb patterned with horseshoe mesostructure

Numerous composite structures with excellent integrative performance that can replicate the mechanical properties of biological materials have been created to fill gaps in material-property charts, and these bio-inspired structures have crucial implications in a wide range of engineering communities. In this paper, a series of novel bio-inspired aluminum honeycombs consisting of horseshoe mesostructure have been proposed on the basis of triangular honeycomb, square honeycomb, hexagonal honeycomb and kagome honeycomb to improve the energy absorption capacity. The three-dimensional finite element models of the bio-inspired horseshoe-shaped aluminum honeycombs are developed in order to explore the mechanical behaviors under the out-of-plane uniform compression. The simulation results are validated based on the compression experiments of regular hexagonal honeycombs. Besides, parametric investigations are carried out to understand the influences of the wave amplitude, wave number and cell-wall thickness on the out-of-plane crashworthiness. The numerical results demonstrate that adding the horseshoe mesostructure to the regular honeycombs can increase the plateau force greatly compared with the traditional honeycomb structure, leading to the higher specific energy absorption although increasing the initial peak force as well. Finally, a multi-objective optimization is carried out to seek for the optimal honeycombs with the maximum specific energy absorption together with the minimum initial peak force simultaneously.

A two-step parameter optimization algorithm for improving estimation of optical properties using spatial frequency domain imaging

This research was aimed at optimizing the inverse algorithm for estimating the optical absorption (μa) and reduced scattering (μs′) coefficients from spatial frequency domain diffuse reflectance. Studies were first conducted to determine the optimal frequency resolution and start and end frequencies in terms of the reciprocal of mean free path (1/mfp′). The results showed that the optimal frequency resolution increased with μs′ and remained stable when μs′ was larger than 2 mm−1. The optimal end frequency decreased from 0.3/mfp′ to 0.16/mfp′ with μs′ ranging from 0.4 mm−1 to 3 mm−1, while the optimal start frequency remained at 0 mm−1. A two-step parameter estimation method was proposed based on the optimized frequency parameters, which improved estimation accuracies by 37.5% and 9.8% for μa and μs′, respectively, compared with the conventional one-step method. Experimental validations with seven liquid optical phantoms showed that the optimized algorithm resulted in the mean absolute errors of 15.4%, 7.6%, 5.0% for μa and 16.4%, 18.0%, 18.3% for μs′ at the wavelengths of 675 nm, 700 nm, and 715 nm, respectively. Hence, implementation of the optimized parameter estimation method should be considered in order to improve the measurement of optical properties of biological materials when using spatial frequency domain imaging technique.

Structurally Deformed MoS2 for Electrochemically Stable, Thermally Resistant, and Highly Efficient Hydrogen Evolution Reaction.

The emerging molybdenum disulfide (MoS2 ) offers intriguing possibilities for realizing a transformative new catalyst for driving the hydrogen evolution reaction (HER). However, the trade-off between catalytic activity and long-term stability represents a formidable challenge and has not been extensively addressed. This study reports that metastable and temperature-sensitive chemically exfoliated MoS2 (ce-MoS2 ) can be made into electrochemically stable (5000 cycles), and thermally robust (300 °C) while maintaining synthetic scalability and excellent catalytic activity through physical-transformation into 3D structurally deformed nanostructures. The dimensional transition enabled by a high throughput electrohydrodynamic process provides highly accessible, and electrochemically active surface area and facilitates efficient transport across various interfaces. Meanwhile, the hierarchically strained morphology is found to improve electronic coupling between active sites and current collecting substrates without the need for selective engineering the electronically heterogeneous interfaces. Specifically, the synergistic combination of high strain load stemmed from capillarity-induced-self-crumpling and sulfur (S) vacancies intrinsic to chemical exfoliation enables simultaneous modulation of active site density and intrinsic HER activity regardless of continuous operation or elevated temperature. These results provide new insights into how catalytic activity, electrochemical-, and thermal stability can be concurrently enhanced through the physical transformation that is reminiscent of nature, in which properties of biological materials emerge from evolved dimensional transitions.

Evaluation of a simple method for storage of blood samples that enables isolation of circulating tumor cells 96\xa0h after sample collection

BackgroundMinimizing the effects of transportation on the properties of biological material is a major challenge for the scientific community. The viability of cells is important in cases where their study is urgent for evaluation of treatment response or for the study of cancer progression. Circulating tumor cells (CTCs) constitute a cell subpopulation with great importance for oncologists, because of their prognostic value. Detection and isolation of CTCs from blood samples is a routine activity in many laboratories, but concerns exist with regard to the maintenance of the cells during transportation. In this study, experiments were conducted to determine the stability of gene and protein expression in CTCs over a period of 96 h.ResultsBlood samples collected from healthy individuals and patients with cancer were each divided into five aliquots, which were stored at 2–8 °C and analyzed after 0, 24, 48, 72 and 96 h of storage. CTCs from patients and CD45-negative cells from healthy individuals were isolated each day using enrichment protocols, and qPCR was performed to determine expression levels of genes encoding specific biological markers. In addition, cells from breast and colon cancer cell lines were spiked into blood samples from healthy individuals, and these samples were stored and analyzed over a period of 96 h by PCR and by flow cytometry. The markers that were studied included housekeeping genes and genes associated with the response to chemotherapy, as well as genes encoding transcription factors. The results demonstrated that the expression profiles of specific genes and proteins in CTCs were not significantly affected by 72 h of storage. After 96 h of storage, expression of some genes was altered.ConclusionThe transportation of blood at low temperature (2–8 °C) in the presence of the anticoagulant EDTA can protect CTCs from alteration of gene and protein expression for at least 72 h. Furthermore, under these conditions, CTCs can be detected and isolated 96 h after blood collection.

The Phe-Ile Zipper: A Specific Interaction Motif Drives Antiparallel Coiled-Coil Hexamer Formation.

Coiled coils are a robust motif for exploring amino acid interactions, generating unique supramolecular structures, and expanding the functional properties of biological materials. A recently discovered antiparallel coiled-coil hexamer (ACC-Hex, peptide 1) exhibits a unique interaction in which Phe and Ile residues from adjacent α-helices interact to form a Phe-Ile zipper within the hydrophobic core. Analysis of the X-ray crystallographic structure of ACC-Hex suggests that the stability of the six-helix bundle relies on specific interactions between the Phe and Ile residues. The Phe-Ile zipper is unprecedented and represents a powerful tool for utilizing the Phe-Ile interactions to direct supramolecular assembly. To further probe and understand the limits of the Phe-Ile zipper, we designed peptide sequences with natural and unnatural amino acids placed at the Phe and Ile residue positions. Using size exclusion chromatography and small-angle X-ray scattering, we found that the proper assembly of ACC-Hex from monomers is sensitive to subtle changes in side chain steric bulk and hydrophobicity introduced by mutations at the Phe and Ile residue positions. Of the sequence variants that formed ACC-Hex, mutations in the hydrophobic core significantly affected the stability of the hexamer, from a ΔGuw of 2-8 kcal mol-1. Additional sequences were designed to further probe and enhance the stability of the ACC-Hex system by maximizing salt bridging between the solvent-exposed residues. Finally, we expanded on the generality of the Phe-Ile zipper, creating a unique sequence that forms an antiparallel hexamer that is topologically similar to ACC-Hex but atomistically unique.

Optical property inversion of biological materials using Fourier series expansion and LS-SVM for hyperspectral imaging

Determination of the optical properties of biological materials based on steady-state spatially resolved diffuse reflectance imaging is a complicated inverse problem-solving process. An effective inverse algorithm is first necessary and validated. This article proposed a Fourier series expansion (FSE) coupled with least squares support vector machine (LS-SVM) as an inverse algorithm for determining the absorption coefficient (μa) and the reduced scattering coefficient () of biological materials. A hyperspectral imaging system was used to acquire scattering images and steady-state spatially resolved diffuse reflectance profiles of liquid phantoms. Experiment results demonstrated that the hyperspectral imaging system coupled with this inverse algorithm effectively improved prediction accuracy of both μa and of liquid phantoms. Tests on liquid phantoms showed that the mean relative errors of this inverse algorithm to be 11.03% for the absorption coefficient and 7.16% for the reduced scattering coefficient when corresponding Fourier coefficients of liquid phantoms were used to develop the prediction model. To further study the method, the Fourier coefficients calculated from normalized Monte Carlo simulation data were used to develop the prediction model for determining the optical properties of 36 liquid phantoms. The prediction errors were 15.96 and 10.91% for μa and , respectively. For all liquid phantoms, it was found that the prediction values of both μa and were generally in good agreement with their actual values. Therefore, the FSE–LS-SVM method provides an effectively means for improving the prediction accuracy of optical properties of biological materials.