Honoring the life and legacy of the Father of Modern Biomechanics whose work transformed our understanding of the human body
In September 2019, an extraordinary event unfolded at the Catamaran Resort in San Diego. More than 120 scientists and engineers from around the world gathered not merely for a conference, but to honor the 100th birthday of a man who had fundamentally reshaped our understanding of the human body. The guest of honor, Professor Yuan-Cheng Fung, sat at the pinnacle of a career that spanned two distinct scientific fields and revolutionized both. Known affectionately as the "Father of Modern Biomechanics," Fung had transitioned from being a pioneering aeronautical engineer to creating an entirely new scientific discipline that applied the principles of engineering to biological systems. This celebration was more than a birthday party; it was a tribute to a mind that could see the hidden mechanics within living tissues and taught generations of scientists to do the same 1 .
As Professor Shu Chien, a member of both the U.S. National Academy of Sciences and the Chinese Academy of Sciences, expressed in his tribute, Fung was a "renaissance man" – not only a great scientist and engineer but also an artist, as evidenced by his beautiful drawings and carved seals 1 .
Though Professor Fung would pass away just three months after this celebration, his centennial served as a powerful reminder of how one curious mind can transform science, medicine, and countless lives 1 2 .
Yuan-Cheng Fung's scientific journey reads like something from a novel, with two distinct acts that would each have constituted a complete career for most scientists.
Born in China's Jiangsu Province in 1919, Fung's early life was shaped by China's turbulent times. He once recalled childhood memories of "Christmas seasons being the time to seek refuge in the countryside" to escape warring factions, his family "crowded in a little boat eating cold chicken" – a memory so vivid he developed a lifelong preference for cold chicken 5 . This environment led him to study aeronautics, believing that "airplanes were needed most in China's fight for survival" 5 .
Excelling in his studies, Fung earned his bachelor's degree in 1941 and a master's in 1943 from China's National Central University 2 6 . In 1945, he seized an opportunity to continue his studies in the United States, arriving at the California Institute of Technology (Caltech) where he would earn his Ph.D. in aeronautics and mathematics in 1948 4 .
In 1957, while on sabbatical in Göttingen, Germany, Fung began a self-directed study of physiology 5 . The initial catalyst was personal – his mother's glaucoma sparked his interest in medical science 5 . He found the work on aeroelasticity at the institution "rather dull," but discovered an excellent physiology library next door 5 .
This casual interest soon blossomed into a profound realization: the principles of solid mechanics he had mastered with aircraft could be applied to living tissues. In 1966, in a move that astonished his colleagues, Fung resigned his prestigious chair at Caltech and moved to the University of California at San Diego to establish one of the world's first bioengineering programs 3 4 6 .
Professor Jay D. Humphrey of Yale University would later note with amazement that Fung's "highly cited papers on the residual stresses in blood vessels and the indicial functions of arterial remodeling were published when he was 72 and 77 years old, respectively" – a testament to his lifelong dedication to scientific discovery 1 .
When Fung began his work in biomechanics, the field barely existed. Medicine understood the chemistry of the body well, but the physics – particularly the mechanical properties of living tissues – remained largely unexplored. Fung brought an engineer's rigorous, quantitative approach to biology, asking fundamental questions: How do blood vessels withstand pressure? How do lungs expand and contract? What gives tissues their unique mechanical properties? 3
His work introduced totally new concepts, particularly in pulmonary mechanics, that motivated many new investigations 4 . He extended his research into microcirculation, blood cell rheology, and constitutive modeling of biologic tissues 4 . Perhaps most significantly, in 1988, Fung introduced the concept of "tissue engineering," which has since become incorporated into both biology and molecular biology 4 .
Professor Savio L-Y. Woo from the University of Pittsburgh would later describe Fung as "a consummate teacher, a distinguished scholar, a brilliant researcher, a quintessential leader, a visionary role model, and more personally, a revered mentor, a generous colleague, and a loving friend" 1 . He called Fung a "4G person: Genius, Gentle, Genuine and Generous" 1 .
Among his many contributions, Fung formulated a fundamental mathematical description of how soft tissues behave under stress, now known simply as "Fung's Law" 2 . This exponential strain constitutive equation for preconditioned soft tissues provided researchers with a powerful tool to predict how tissues would respond to mechanical forces 2 .
The equation expresses strain energy density (w) as:
\[ w = \frac{1}{2}\left[q + c\left(e^Q - 1\right)\right] \]
where \( q = a_{ijkl}E_{ij}E_{kl} \) and \( Q = b_{ijkl}E_{ij}E_{kl} \) are quadratic forms of Green-Lagrange strains \( E_{ij} \), with \( a_{ijkl} \), \( b_{ijkl} \), and \( c \) being material constants 2 .
This mathematical formulation may appear abstract, but its practical implications are profound. It helps researchers understand how arteries withstand blood pressure, how lungs expand with each breath, and even how skin stretches and recovers – knowledge essential for developing medical treatments and devices.
Fung's research, particularly his groundbreaking work on arterial mechanics, relied on several key methodologies and tools that became foundational to the field of biomechanics.
| Tool/Methodology | Function | Example in Fung's Research |
|---|---|---|
| Constitutive Modeling | Mathematical description of material behavior under stress | Developed "Fung's Law" to describe soft tissue mechanics 2 |
| Residual Stress Analysis | Measuring internal stress in absence of external loads | Radial cutting experiment to reveal arterial prestress 1 6 |
| Interference Microscopy | High-precision 3D imaging at microscopic level | Determination of red blood cell 3D geometry 1 |
| Biodyne | Custom apparatus for testing biological material properties | Invention of specialized equipment for tissue mechanical testing 1 |
| Continuum Mechanics Theory | Mathematical framework for analyzing continuous materials | Applied engineering principles to biological tissues 2 4 |
Among Fung's many groundbreaking investigations, his work on residual stress in arteries stands out for its elegant methodology and far-reaching implications.
Fung and his colleagues employed a remarkably straightforward yet powerful experimental approach to reveal the hidden stresses within arterial walls 1 6 :
This simple yet brilliant methodology revealed something astonishing: even when no external forces were applied, arteries contained built-in "residual stresses" that affected their mechanical behavior 1 6 .
The experiment yielded profound insights that transformed our understanding of cardiovascular mechanics:
When Fung's team made the radial cut in an artery, the vessel segment immediately sprang open, demonstrating that it had been under internal stress even in its supposedly "resting" state 1 6 . This residual stress, they discovered, served crucial physiological functions – it helped distribute stress more evenly across the arterial wall and likely played a role in how arteries remodel themselves in response to changing pressure conditions 1 .
| Observation | Interpretation | Physiological Significance |
|---|---|---|
| Arterial wall springs open upon radial cutting | Existence of significant residual stress in arteries | Revealed previously unknown mechanical pre-conditioning in living tissues |
| Stress distribution varies across arterial layers | Nature's engineering solution to optimize load-bearing | Explains how arteries withstand pulsatile blood pressure without damage |
| Opening angle differs by artery type and location | Tissue adaptation to local mechanical environment | Suggests arteries remodel based on their specific functional requirements |
| Changes in residual stress under hypertension | Tissue remodeling in response to disease | Provides insight into vascular pathology development |
The implications were immediately recognized as significant for understanding cardiovascular diseases. The research suggested that abnormalities in these residual stresses might contribute to conditions like hypertension and atherosclerosis 6 . The findings provided a new mechanical perspective on vascular health and disease, suggesting that the mechanical environment of blood vessels was as important as their biochemical one.
Yuan-Cheng Fung's century of life left an imprint on science that continues to pulse through laboratories and clinics worldwide. His honors speak to the breadth of his impact: the National Medal of Science (2000), the Timoshenko Medal (1991), the Fritz J. and Dolores H. Russ Prize (2007), and membership in all three U.S. national academies – Engineering (1979), Medicine (1991), and Sciences (1993) – an exceptionally rare triple crown of scientific achievement 2 4 .
But beyond the awards, Fung's true legacy lives on in the field he created and the minds he shaped. As Professor Shu Chien noted, Fung set "the highest standards" while "educating many students and their students" . His work "provided the foundation of research activities in many institutions in the United States and the world" .
Earned Ph.D. from Caltech
Launched his first career in aeronautical engineering
Moved to UC San Diego
Transition to biomechanics; established pioneering bioengineering program
Founded ASME Biomechanics Symposium
Created central platform for the emerging field
Published residual stress findings
Revolutionized understanding of arterial mechanics
Awarded Timoshenko Medal
Highest honor in applied mechanics
Received National Medal of Science
Highest U.S. scientific honor
Centennial Celebration
Global gathering to honor his lifetime of achievement
Perhaps most importantly, Fung demonstrated that scientific boundaries are meant to be crossed. He moved effortlessly between aeronautics and medicine, between mathematical theory and practical application, between the rigor of engineering and the wonder of biology. In doing so, he not only created a new science but provided a timeless example of how curiosity, coupled with analytical rigor, can reveal the hidden mechanics of life itself.
As Professor Andrew D. McCulloch of UC San Diego noted during Fung's centennial celebration, the great man "always kept a gentle smile on his face while taking photos with many friends, colleagues and junior researchers" 1 . Though he is no longer with us, "his character and his contributions shall forever be remembered" 1 .