News Column

Patent Issued for Nonlinear System Identification Techniques and Devices for Discovering Dynamic and Static Tissue Properties

July 8, 2014



By a News Reporter-Staff News Editor at Journal of Technology -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventors Hunter, Ian W. (Lincoln, MA); Chen, Yi (St. Charles, MO), filed on August 31, 2010, was published online on June 24, 2014.

The assignee for this patent, patent number 8758271, is Massachusetts Institute of Technology (Cambridge, MA).

Reporters obtained the following quote from the background information supplied by the inventors: "Identifying the mechanical properties of skin and other biological tissues is important for diagnosing healthy from damaged tissue, developing tissue vascularization therapies, and creating injury repair techniques. In addition, the ability to assess the mechanical properties of an individual's skin is essential to cosmetologists and dermatologists in their daily work. Today, the mechanical properties of skin are often assessed qualitatively using touch. This, however, presents a problem in terms of passing information between different individuals or comparing measurements from different clinical studies for the diagnosis of skin conditions.

"Studies have explored both the linear and nonlinear properties of biological materials. Testing methods used include suction (S. Diridollou, et al. 'An in vivo method for measuring the mechanical properties of the skin using ultrasound,' Ultrasound in Medicine and Biology, vol. 24, no. 2, pp. 215-224, 1998; F. M. Hendricks, et al., 'A numerical-experimental method to characterize the non-linear mechanical behavior of human skin,' Skin Research and Technology, vol. 9, pp. 274-283, 2003), torsion (C. Excoffier, et al., 'Age-related mechanical properties of human skin: An in vivo study,' Journal of Investigative Dermatology, vol. 93, pp. 353-357, 1989), extension (F, Khatyr, et al., 'Model of the viscoelastic behavior of skin in vivo and study of anisotropy,' Skin Research and Technology, vol. 10, pp, 96-103, 2004; C. Daly, et al., 'Age related changes in the mechanical properties of human skin.' The Journal of Investigative Dermatology, vol. 73, pp. 84-87, 1979), ballistometry (A. Tosti, et al., 'A ballistometer for the study of the plasto-elastic properties of skin,' The Journal of Investigative Dermatology, vol. 69, pp. 315-317, 1977), and wave propagation (R. O. Potts, et al., 'Changes with age in the moisture content of human skin,' The Journal of Investigative Dermatology, vol. 82, pp. 97-100, 1984).

"Commercial devices, such as the CUTOMETER.RTM. MPA580, DERMALFLEX, and DIA-STRON brand dermal torque meter, exist for some of these methods. Generally, these devices only provide information about limited aspects of skin behavior which may not be enough to properly diagnose disease. Many of these devices also focus on only linear properties such as skin elasticity,

"In another method known as indentometry, (F. J. Carter, et al., 'Measurements and modeling of human and porcine organs, ' Medical Image Analysis, vol. 5, pp. 231-236, 2001; M. P. Ottensmeyer, et al., 'In vivo data acquisition instrument for solid organ mechanical property measurement,' Lecture Notes in Computer Science, vol. 2208, pp. 975-982, 2001; G. Boyer, et al., 'Dynamic indentation of human skin in vivo: Aging effects, ' Skin Research and Technology, vol. 15, pp. 55-67, 2009) a probe tip is pushed orthogonally into the skin to discover tissue properties. If large enough forces are used, this method is capable of measuring the mechanical properties of not only the epithelial layer, but also the properties of the underlying connective tissue.

"The interaction between different tissue layers (C. Daly, et al., 'Age related changes in the mechanical properties of human skin.' The Journal of Investigative Dermatology, vol. 73, pp. 84-87, 1979; H. Oka, et al., 'Mechanical impedance of layered tissue,' Medical Progress through Technology, supplement to vol. 21, pp. 1-4, 1997) is important in applications like needle-free injection (B. D. Hemond, et al., 'A Lorentz-force actuated autoloading needle free injector,' in 28.sup.th Annual International Conference of the IEEE EMBS, pp. 679-682, 2006), where the dynamic response of skin to a perturbation is important in determining the required injection depth.

"Linear stochastic system identification techniques have been used to describe a variety of biological systems (M. P. Ottensmeyer, et al., 2001; G. Boyer, et al., 2009; M. Garcia-Webb, et al., 'A modular instrument for exploring the mechanics of cardiac myocytes,' American J. of Physiology: Heart and Circulatory Physiology, vol. 293, pp. H866-H874, 2007). However, many systems cannot be fully described by linear dynamic models. Investigators have also used nonlinear relationships to describe the stress strain relationship in skin (F. M. Hendricks, et al., 2003). However, most of this work has been done at low frequencies and therefore does not describe the dynamic properties of skin.

"Another problem with existing methods is that the dynamics of the testing device are often not characterized and are assumed to apply perfect forces to the tissue. For example, actuators are assumed to have perfect output impedance such that the dynamics of the system being tested do not affect the dynamics of the actuator. In addition, many existing methods and devices are limited to one test geometry and one perturbation scheme. Once a different geometry or testing direction is used, the measured results are not easily comparable.

"Trends in consumer skin care have shown the use of specific molecules and proteins, such as tensin, which are well known to cause collagen growth or increase skin suppleness in hydration and anti-aging products. Although standard testing devices for skin have been proposed, industry specialists have expressed dissatisfaction with existing devices."

In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventors' summary information for this patent: "The present invention generally is directed to devices and methods for measuring one or more mechanical properties of tissue, such as the skin of an animal, skin of a fruit or vegetable, plant tissue, or any other biological tissue.

"A device for measuring a mechanical property of a tissue includes a probe configured to perturb the tissue with lateral movement relative to a surface of the tissue, an actuator coupled to the probe to move the probe, a detector configured to measure a response of the tissue to the perturbation, and a controller coupled to the actuator and the detector. The controller drives the actuator using a stochastic sequence and determines the mechanical property of the tissue using the measured response received from the detector.

"The probe can be placed against the tissue surface and may be coupled to the tissue surface, for example using a static preload or an adhesive. The device can further include a reference surface configured to contact the tissue surface. The probe may include a set of interchangeable heads, the set including a head for lateral movement of the probe and a head for perpendicular movement of the probe.

"Lateral movement of the probe is movement directed across the surface of the tissue and may be used to extend the tissue with the probe or to slide the probe across the tissue surface to measure surface mechanics. Interchangeable heads for lateral movement may be configured differently for extension than for surface mechanics testing. Perpendicular movement is movement normal to the surface of the tissue and may be used to indent the tissue, which can include pushing and pulling on the tissue.

"In general, the perturbation can include indentation of the tissue with the probe, extension of the tissue with the probe, or sliding the probe across the tissue surface. In some embodiments, the actuator includes a Lorentz force linear actuator and perturbing the tissue can include using the Lorentz force linear actuator.

"The mechanical property may be determined using non-linear stochastic system identification. The mechanical properties may be indicative of, for example, tissue compliance and tissue elasticity.

"In some embodiments, the detector includes a force sensor detecting force of the perturbation, for example, using a current sensor detecting a current input to the actuator. The detector can include a position sensor detecting displacement of the tissue surface. The device can further include a handle for manual application of the probe to the surface of the tissue and may include an accelerometer detecting an orientation of the probe. Probe types can include indentation, extension and surface mechanics (sliding). Additional attachment methods may include twist-and-pull microhooks or suction.

"A method of measuring the mechanical properties of tissue includes placing a probe against a surface of the tissue, mechanically perturbing the tissue with lateral movement of the probe using a stochastic sequence, measuring a response of the tissue to the perturbation, and determining the mechanical properties of the tissue based on the measured response to the perturbation.

"Determining the mechanical properties can include using non-linear stochastic system identification and may further include modeling the probe and tissue as a system comprising a linear dynamic component and a non-linear static component. The non-linear component may include a Wiener static nonlinear system and the linear component may include a second order mechanical system. In some embodiments, using non-linear stochastic system identification includes using a Volterra Kernel method. Further, the method may include detecting force of the perturbation with respect to a reference surface, Measuring a response can include detecting displacement of the tissue surface with respect to a reference surface.

"A method of testing produce, e.g., fruits and vegetables, includes placing a probe against a skin of a piece of produce, mechanically perturbing the piece of produce with the probe, measuring a response of the piece of produce to the perturbation, and analyzing the measured response using non-linear stochastic system identification.

"Perturbing the piece of produce can include using a Lorentz force linear actuator and may include using a stochastic sequence. Analyzing can include determining the mechanical properties of the piece of produce. The mechanical property may be indicative of ripeness.

"A method of analyzing the mechanical properties of tissue includes mechanically perturbing the tissue using a stochastic input sequence, measuring a response of the tissue to the perturbation, partitioning the measured response, and generating a representation of the mechanical properties of the tissue based on the partitioned response.

"Measuring a response can include detecting position of the tissue. Partitioning can include grouping the measured response into position bins over which the measured response approximates a linear response to the perturbation. Generating a representation can include generating a time-domain representation of the partitioned response. Further, generating a representation can include using orthogonalization of the input sequence based on the position bins and the time-domain representation can include an impulse response for each position bin.

"A method of analyzing the mechanical properties of tissue includes mechanically perturbing the tissue with a probe using a stochastic input sequence, measuring a response of the tissue to the perturbation, analyzing the measured response, and, while perturbing, adjusting the input sequence based on the analysis.

"Analyzing can include using a non-linear stochastic system identification and may include obtaining a distribution, such as a probability density function, of the measured response. In an embodiment, analyzing includes determining a mechanical property of the tissue. Further, the method may include generating the stochastic input sequence.

"The present invention has several advantages. Embodiments of the invention are capable of measuring the mechanical properties of skin in a clinical setting because they are low cost and robust, because they enable the testing procedure to be fast and accurate, and because they can be implemented in a hand-held form factor. In addition, devices and methods disclosed herein are able to fully characterize the dynamic linear and nonlinear aspects of the mechanical behavior of skin.

"A benefit of using non-linear stochastic system identification to measure tissue properties is that measurements can be done in vivo. Another benefit is that tests can be conducted quickly and each test can obtain as much information as possible. For example, the devices and methods described herein can be used to characterize the parameters of human skin using nonlinear stochastic system identification, which can be completed within 2 to 4 seconds when perturbing the skin using indentation. As an additional benefit of using non-linear stochastic system identification, the data acquisition and analysis method is relatively immune to the movements of the patient during the test.

"Embodiments of the invention can provide quantitative measurements and may be used to standardize the qualitative measurements that physicians currently use to diagnose tissue diseases. A device with the ability to diagnose tissue diseases (e.g. Scleroderma, Myxoedema, or connective tissue diseases) or identify the presence of dehydration can have a large societal impact in healthcare and large market impact in terms of tools that are available to clinicians. Quantitative measurements in a clinical setting can advance the field of tissue mechanics by standardizing assessments made by different individuals. In addition, devices and methods disclosed herein can be used for understanding mechanics for manufacturing artificial prosthetic tissue, for determining mechanical properties in locations that are difficult to palpate (such as in the colon during endoscopy), and determining parameters needed for needle-free injection.

"While different types of tests and devices can be used to identify the anisotropic properties of skin, for in vivo testing, the contribution from directions outside the testing plane can affect the results. The disclosed devices methods are capable of testing multiple directions, by using different perturbation modes and interchangeable probe heads, which can be useful in determining these anisotropic material properties.

"Furthermore, embodiments of the invention can be used to quickly measure the mechanical properties of plant tissue, such as fruits and vegetables, which can be beneficial for harvesting, processing, and packaging applications in agricultural, commercial, or industrial environments. In addition, a consumer may use an embodiment of the invention, such as a handheld measuring device, to test fruits and vegetables for ripeness, crispness, or freshness prior to purchase.

"Another benefit is that embodiments of the invention can provide a standardized measurement technique designed to assess the effectiveness of skin care products. The disclosed method can be used to distinguish the change in skin properties after dehydration or after application of commercial products, such as lotions, creams, and anti-aging products."

For more information, see this patent: Hunter, Ian W.; Chen, Yi. Nonlinear System Identification Techniques and Devices for Discovering Dynamic and Static Tissue Properties. U.S. Patent Number 8758271, filed August 31, 2010, and published online on June 24, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8758271.PN.&OS=PN/8758271RS=PN/8758271

Keywords for this news article include: Skin Research And Technology, Massachusetts Institute of Technology.

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Source: Journal of Technology


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