Bioheat Transfer

Bio means Life. Bioengineering applies engineering principles, laws of physics and chemistry in a general sense, to the understanding and modeling of living systems. Biotechnology identifies methods, processes and techniques resulting from bioengineering. Such technology attempt control and duplication of bio-processes (chemical, mechanical) performed by living systems. Examples include the micro-scale (molecular level) recombination of DNA to the macro-scale functioning of an artificial lung [1].

Contributions from bioengineering has been to several conventional engineering categories including instrumentation and measurements, materials, analysis, and modeling. Biomedical engineering (BME) is the application of engineering principles and techniques to the medical field. In a way, any engineering invention, application or method applied to humans and their health related issues in particular can be considered as a contribution of bioengineering. For instance, as early as the 16th century Professor Sanctorius (1561-1636) of Padua, attempted developing a thermometer for comparing temperatures of different persons [3].

Biothermology or Bioheat transfer, the study of heat transfer in biological systems, can be seen as a subdivision of bioengineering.

The objectives of heat transfer are threefold: insulate, enhance or control temperature at a place. These objectives when applied in the context of biological systems such as mammalian and plants, seek better insight into the underlying biological processes by modeling them through mathematical statements and finding particular solutions for them. Biofluid dynamics forms an inherent part of bioheat transfer, when convection or conjugate heat transfer needs to be investigated.

Specific to humans several applications of bioheat transfer processes can be listed [2]:

Thermoregulation; metabolic heat generation, evaporation, convection and radiation to achieve steady state;
Effect of increased Metabolic Heat Generation; temperature rise during exercise
Bioheat transfer in muscles and tissues accompanied with effects due to blood flow (perfusion)
Burning; skin burning as transient heat transfer process
Fever and Hypothermia
Thermal Comfort; Convection, conduction heat transfer through clothing, optimum temperature, humidity, energy transfer in artificial fittings like contact lens

Similarly, biomass transfer processes can be identified within the human body:

Blood as oxygen carrier; equilibrium of oxygen in blood with inhaled air
Metabolism; diffusive oxygen transfer in a tissue
Membranes as barriers to bulk flow; diffusive and ionic flows through membrane channels; porous medium models of capillaries and tissues
Liquid Diffusion in tissues; drug delivery to local regions inside body; diffusion of gastric juice in the stomach

A simple illustration should highlight the importance of thermoregulation and the related bioheat transfer mechanisms inside human body. The human body is homeothermal with a core temperature of 37 °C. This is about 3 °C higher than the surface skin temperature. Within a reasonable extent the thermoregulation mechanisms of the body maintain the core temperature constant in spite of variations in the surrounding (environment) and also variations in human body activity like exercising and resting. This suggests the temperature gradients in the human body to be of the order of 0.1 °C/cm . Assuming the human tissue thermal conductivity as 0.6 W/m °C (same as that of water), the heat fluxes across the body only by diffusion (conduction) is ~ 6 W/m². Assuming man as a cylinder and estimating a body surface of 1.8 m², it is evident that heat diffusion alone is not efficient to release the basic metabolic rate of about 90 W to the surrounding. Every body (everybody) becomes a ‘hot body’ in time. Other heat transfer mechanisms (convection, radiation, evaporation) must be important for thermoregulation.

Major applications of bioheat transfer include cryosurgery (i.e. surgery at low temperatures), cryopreservation, therapeutic hyperthermia (healing at high temperatures), ablation of (usually, malignant) tissues using lasers and in general, surgical processes involving lasers (such as retinopathy).

Cryosugery, for instance, use low temperatures (or cryogenics) to destroy cells. Such cell death is caused by the physical or osmotic disruption of plasma membrane and proteins by freeze-growing ice crystals within the cytoplasm of the cell. On the other hand, in therapeutic hyperthermia, heat energy is used to recover cells and tissues. Bioheat transfer mathematical models of varying complexity can be used to predict particular solutions, thereby better understanding, of such processes. Read also the separate note discussing cryopreservation of cells by freezing that was published in the ASME heat transfer gallery in 2008.

The effects of blood flow on heat transfer in living tissue and the mathematical modeling of the complex thermal interaction between the vasculature and tissue has remained a topic of interest for more than a century. H. H. Pennes in 1948 proposed the first bioheat transfer model. The next fifty years many variants have been suggested for this model. A separate note shall discuss these bioheat transfer models. Although there are enough variants of the bioheat transfer model, the development and testing of bioheat transfer models on all scales from microscopic to the entire human body is expected to be continued.

Major challenges in biothermology arise due to several factors:

multi component (and sometimes multiphase) media; conjugate heat transfer mode is essential for thermal equilibrium
variability of blood flow rate in the internal removal of heat and related vasodilation and vasoconstriction phenomena
laminar blood flow with sporadic turbulent bursts and flow reversals due to pulsation
exhibition of both Newtonian and non-Newtonian behaviour of blood due to the presence of deformable bodies (red blood cells)
accurate determination of thermal properties of tissue become difficult due to heterogeneities, anisotropy and ageing
unusual range of system size from macroscopic to microscopic scales makes modeling and simulation of even local regions within the body a challenging task
measurement of temperature at small scales and in general, performing in vivo experiments within human body pose difficulties

Ongoing and future interest in bioheat transfer aim to investigate centrally (and perhaps, obviously) the inter-related diffusive phenomena of heat, mass, and momentum transfer. Also, basic anatomical studies with targeted experiments are needed of the vasculature, particularly in the 50- to 500-pm diameter range, in tissues, organs, and in tumors [3]. The development of improved and noninvasive measurement techniques and devices is always encouraged.

References

Heat Transfer in Biology and Medicine Course Notes, Jose Lage, 2002, (unpublished)
Biological and Bioenvironmental Heat and Mass Transfer, Ashim K. Dutta, Marcel Dekker Inc., NY, 2002
Bioengineering Heat Transfer, Advances in Heat Transfer, v. 22, Eds. Cho et al., Academic Press, 1992.
Heat Transfer in Energy and Biology, v.1 and 2, Eds. A.Shitzer and R.C.Eberhart, Plenum Press, 1985.
Cryopreservation of cells by freezing

Highly Recommended

Biothermal Fluid Sciences – Principles and Applications, W.-J. Yang, Hemisphere Pub., New York, 1989.
Visual Dictionary of the Human Body, DK Publishing, New York, 1991.
Microscale Heat Transfer in Biological Systems at Low Temperatures, B. Rubinsky, Experimental Heat Transfer 10, 1-29, 1997.
Human Physiology Series: Temperature Control, M. E. Armstrong and L. C. Parsons, S-3275A US National Library of Medicine, 1979.
Progress in Clinical and Biological Research, v. 107, Proc. Int. Symposium, Eds. M. Gautherie and E. Albert Alan R. Liss, Inc, New York, 1981.
Physical Biochemistry, Kensal E. van Holde, Prentice-Hall, New Jersey, 1992.
Development of Ni-4wt.% Si thermoseeds for hyperthermia cancer treatment, J.-S. Chen et al., J. Biomedical Materials Research, 22, 303-319, 1988.
Bioheat Transfer by Liang Zhu, Chapter 2 of Mechanics of Human Body in Standard Handbook of Biomedical and Engineering Design
Biofluid Dynamics, Principles and Selected Applications, Clement Kleinstreuer, Taylor and Francis, 2006
The role of porous media in modeling flow and heat transfer in biological tissues, International Journal of Heat and Mass Transfer 46 (2003) 4989-5003, doi:10.1016/S0017-9310(03)00301-6
Physics of Fluids, March 2005, Volume 17, Issue 3, SPECIAL TOPIC: BIOFLUID MECHANICS [link]
research papers of interest are collected from various journals under ‘Bioheat transfer’ in my monthly Read List at this blog.

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