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Revisiting the Concept of "Safe by Design"

By Eddy Ball
April 2010

Shape Changes Physico-Chemical Properties: Changes in shape, she explains, changes the products' physical and chemical properties in ways that impact their potential effects on human health.
This slide from a presentation by Tinkle illustrates the range of shapes found in ENMs. Changes in shape, she explains, alter the products' physical and chemical properties in ways that impact their potential effects on human health. (Graphic courtesy of Sally Tinkle)

In her recent editorial for the Web site Azonanotechnology, NIEHS Senior Science Advisor Sally Tinkle, Ph.D., offers a balanced critique of what she describes as "the Holy Grail of Nanotechnology" as she examines the assumptions behind the concept of Safe by Design (SxD).

As Tinkle explains in her article ( NIEHS, however intriguing SxD may be as a concept, it also presents several fundamental challenges for scientists. According to her, scientists still don't understand the wide range of variables that determine how elements behave well enough to determine whether SxD can actually be a feasible strategy for achieving product safety while maintaining the beneficial properties of ENMs, as the sizes, electrical charges, surface coatings, and shapes of elements are altered to produce engineered nanomaterials( (ENMs).

Like the Holy Grail's promise of salvation, the promise of SxD remains enticingly out of reach, according to Tinkle.

Sally Tinkle, Ph.D.
Tinkle describes SxD efforts as "a balancing act between benefit and risk." Tinkle cautioned that ENMs may be "so exquisitely sensitive" that engineering for safety could also alter their beneficial properties. (Photo courtesy of Steve McCaw)

Tinkle's caveats parallel concerns( expressed by NTP scientists Nigel Walker, Ph.D., and John Bucher, Ph.D., in a paper last year on applying a next-generation systematic biological-pathways testing approach to risk analysis of ENMs. Walker and Bucher pointed to the variability among products in terms of physical qualities, manufacturing practices, and microenvironmental interactions as reasons that high-throughput methods may not be appropriate for testing many ENMs.

Not surprisingly, Tinkle thinks "it's time to step back and re-examine what we've been saying [about SxD] with some research that asks the necessary questions, so we could perform meta-analysis on the data from those studies to take us in the direction we want to go." Tinkle added that NIEHS has taken the lead in funding research "to see how precisely we can identify the subset of physical and chemical properties of a material and relate them to biological response."

Even if scientists can discover systematic principles that govern the often contradictory behaviors of the broad spectrum of nanomaterials, Tinkle added, SxD may not be the most cost-effective approach for protecting public health. In the end, the most feasible approach may yet prove to be a time-consuming product-by-product approach to risk analysis and safety engineering similar to the one used in conventional toxicology.

Teasing out the Implications of SxD

Tinkle looked at three as yet unsubstantiated assumptions behind the concept of SxD:

  • That there exist subsets of physical and chemical properties, which consistently produce the same biological response in multiple microenvironments
  • That scientists will be able to isolate the physical and chemical properties that trigger individual adverse biological reactions that may affect an organism from the exponential number of combinations of physical and chemical properties possible for ENMs
  • That designers can maintain the beneficial properties of an ENM, such as targeted drug delivery, while engineering out those that cause an adverse effect

In support of the plausibility of SxD, Tinkle points to recent findings about what is called the "corona," a limited number of proteins that bind to the surface an ENM. "Because the number of proteins that bind to the surface of the material is more limited," Tinkle explained, "it may be that these proteins limit the biological responses than can occur."

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