IELTS Academic Reading Practice Test 194

Most manufacturing removes material: a block of metal is cut and ground until the wanted shape remains. Additive manufacturing does the opposite. A three-dimensional printer takes a design held as a computer file, divides it into thin horizontal slices, and builds the object one slice at a time, stacking the slices until the form is complete. Because each slice can differ from the one below, shapes that would be impossible to cut — a hollow lattice, a spiral channel inside a solid block — can be produced without a mould. The idea is now used in dentistry and aerospace, but it began with a researcher working with liquid plastic.

The first published account of such a process appeared in 1981, when Hideo Kodama, a researcher at the Nagoya Municipal Industrial Research Institute in Japan, described a method for making a solid model from a liquid that hardened when exposed to ultraviolet light. A thin layer of the liquid was spread across a plate, exposed to the light through a mask and allowed to set; the plate was then lowered slightly and the process repeated. Kodama filed a patent application, but it was never completed. His paper is nevertheless now credited as the earliest description of layer-by-layer manufacturing from a photopolymer, a plastic that sets when light falls on it.

Independently, an American engineer named Chuck Hull arrived at a similar principle. Hull worked for a company in California that made coatings which cured under ultraviolet lamps, and he knew such materials well. In 1983 he used a beam of ultraviolet light to harden a thin layer of liquid resin, and then built up a small cup from successive layers. Realising that this might allow complex parts to be made quickly, he filed a patent in 1984 and was granted it in 1986. He called the process stereolithography, and his work gave rise to the first widely used file format for describing a solid surface, still known today as the STL file.

In 1986 Hull founded 3D Systems to sell machines based on the technique. The first, the SLA-1, was sold in 1988. It was large and expensive, but it turned a computer model into a physical object overnight. Rival methods soon appeared. In 1988 Scott Crump developed fused deposition modelling, in which a thin filament of plastic is melted and laid down through a nozzle like a computer-controlled glue gun; he founded a company, Stratasys, the following year. Around the same time Carl Deckard, a student at the University of Texas at Austin, developed selective laser sintering, which uses a laser to fuse successive layers of powder, and patented it in 1989.

In stereolithography, a platform is lowered into a vat of liquid photopolymer so that its surface lies just below the liquid level. A computer-controlled mirror directs an ultraviolet laser across the surface, tracing the outline of the first cross-section; wherever the beam passes, the liquid sets in a chemical reaction called polymerisation. The platform then sinks by a fraction of a millimetre, a blade sweeps across to level the liquid, and the next cross-section is drawn on top of the first. When the object is complete it is lifted out, drained and cured in an oven. Overhanging parts must be supported by thin pillars that are cut away afterwards.

Other approaches broadened the range of materials. In the early 1990s, researchers at the Massachusetts Institute of Technology developed a technique in which an inkjet print head deposited a liquid binder onto a bed of powder, hardening it only where the two met. Their work did much to popularise the term three-dimensional printing, which gradually replaced the various trade names. For a long time, however, the machines remained too costly for anyone but a large company or a university laboratory.

The situation changed in 2005, when Adrian Bowyer, an engineer at the University of Bath in England, launched an open-source project called RepRap. Its aim was a machine that could reproduce a substantial proportion of its own components, so that anyone with one printer could make another. Designs were published freely, and enthusiasts built machines from kits. A crucial legal event followed in 2009, when the original patent covering fused deposition modelling expired; competitors could at last make extrusion printers without paying a licence. Desktop machines appeared in schools, workshops and homes, and the price of an entry-level printer fell sharply.

Industry adopted the technology where conventional methods were weakest. The great majority of custom-made hearing aids, whose shells must fit an individual ear exactly, are now printed from a digital scan. Dental laboratories routinely print models and crowns, and aircraft engines contain printed fuel nozzles: one designed by GE for its LEAP engine replaced about twenty separate components with a single piece that the company says is five times more durable. In 2015 the US Food and Drug Administration approved the first printed medicine, a tablet designed to dissolve rapidly in the mouth.

Printing is not yet a substitute for mass production. A single printed part may take hours to make, whereas a mould can turn out thousands in the same time, so printing pays only when numbers are small or the shape is complicated. The range of usable materials is narrower than in conventional engineering, and certification for aircraft or medical use is slow and costly. Even so, the technology has travelled a long way since 1983.

A Passive House is not a house without heating. It is a house designed so carefully that the heat given off by its occupants, its appliances and the sun is almost enough to keep it warm, and the small amount still needed can be delivered by warming the incoming ventilation air. The idea was developed in the late 1980s by Bo Adamson, a Swedish professor of building science, and Wolfgang Feist, a German physicist. The first homes built to the resulting standard were completed in Darmstadt in 1991, and the Passive House Institute, which certifies designs and trains designers, was founded in the same city five years later.

Every building loses heat in three ways. Heat conducts through solid materials; it is carried away by air that leaks through gaps and cracks; and it radiates through glass. The rate at which a material conducts heat is expressed as its U-value, measured in watts per square metre for each degree of temperature difference across it. A single-glazed window has a U-value of about 5, a good modern wall about 0.3, and the aim of the Passive House standard is to push walls, roofs and floors down to about 0.15. Heat always flows from warm to cold, so the only way to slow the flow is to put something resistant in its path.

That something is insulation, and a Passive House uses a great deal of it: typically between 25 and 40 centimetres of mineral wool, expanded polystyrene or wood fibre, wrapped around the whole building to form a continuous jacket. Just as important is the avoidance of thermal bridges — places where a material that conducts well, such as a steel beam or a concrete balcony, passes through the insulated layer and offers heat a shortcut. Each junction must be designed and drawn in detail, which is one reason why a certified designer is normally involved from the first sketch.

The second route for heat, leakage, is tackled by making the building almost airtight. A continuous membrane around the inside of the walls, sealed at every joint, window frame and pipe, prevents warm air from escaping and cold air from seeping in. Airtightness is measured by a blower-door test, in which a fan is fitted into a doorway and used to create a pressure difference of fifty pascals; the amount of air that then leaks in is calculated. The standard demands no more than 0.6 air changes per hour at that pressure, whereas an ordinary new house may reach three or four.

An airtight house must nevertheless be ventilated, because cooking, washing and breathing produce moisture and stale air. A Passive House therefore has a mechanical system that extracts air from kitchens and bathrooms and supplies fresh air to living rooms and bedrooms. The two streams pass through a heat exchanger, a device in which the warm outgoing air and the cold incoming air flow on either side of thin plates without mixing. Heat crosses the plates, so that most of the warmth is recovered; the standard requires at least 75 per cent, and good units achieve more. Filters remove pollen and dust, and the system runs continuously at a low rate.

Windows are the weakest element, since even a good wall is useless if the openings in it lose heat several times faster. Passive Houses therefore use triple glazing: three panes separated by spaces filled with argon or krypton, gases that conduct heat poorly, with thin transparent coatings that reduce radiation. Frames are insulated and the glass is set deep into them, and the whole window is required to reach a U-value of about 0.8. In the cool climates where the standard began, most windows face the sun so that winter sunlight provides free warmth.

Does it work? The standard sets a limit on the space-heating demand of 15 kilowatt-hours per square metre per year, or roughly a tenth of what the average existing home in Germany uses; a well-built Passive House typically measures less. For a family house of 100 square metres, that is about 1,500 kilowatt-hours a year, the energy contained in some 150 litres of heating oil. A European research project that monitored more than a hundred dwellings across five countries in the late 1990s found consumption close to the predicted figures.

The main obstacle is cost. Building to the standard is commonly estimated to add between five and ten per cent to construction costs, partly for insulation, triple glazing and the ventilation system, and partly for the extra design time. That money is recovered through lower fuel bills, but only over many years, which makes the standard a hard sell to a developer who intends to sell the building quickly. Retrofitting an old building is harder still, because insulation must be fitted inside or outside walls without destroying their appearance; the institute has created a separate standard, EnerPHit, for renovations.

Researchers are also studying warmer climates, where the problem is keeping buildings cool rather than warm. Heavy insulation helps there too, but large areas of glass can cause overheating, so shading and careful orientation become essential. Other work examines how far measured savings depend on the behaviour of occupants, who may open windows in winter or demand higher indoor temperatures than the designers assumed. As the standard spreads from Germany to Spain, Mexico and China, these questions are becoming more urgent.

Human enhancement is usually defined as the use of technology to improve capacities that already lie within the normal range, rather than to correct a defect. Read a little further, and the definition begins to wobble. A hearing aid restores a lost sense; a cochlear implant gives a deaf child access to speech; an athlete's carbon-fibre blade replaces a missing leg and may allow a faster run than two legs would. The line between repair and improvement is not a fact about the world; it is a judgement we make, and different societies draw it in different places.

The most common way of drawing the line appeals to disease. Medicine, on this view, treats illness and restores people to normal functioning; enhancement goes beyond that. The trouble is that 'normal' is statistical rather than moral. Being short is not a disease, yet growth hormone is prescribed for children whose expected adult height falls below a threshold that shifts as treatments improve. Nor is the concept of disease stable: mild hypertension, once regarded as a risk rather than an illness, is now treated with tablets. If the boundary moves whenever a new drug appears, it cannot by itself settle what should be permitted.

The debate is sharpest where the technology is already with us. Stimulants developed for attention disorders are widely used by healthy adults, and surveys on American campuses have repeatedly found that a substantial minority of students have taken them without a diagnosis. The drugs are not harmless: they raise blood pressure, disturb sleep and can cause anxiety, and their long-term effects on healthy brains have not been studied in any systematic way. The claim that they make their users genuinely cleverer is also uncertain; the evidence suggests that they mainly increase wakefulness and motivation.

Sport is often held up as a warning. Cheating is the word used in sport, and it is applied to students who take pills. I find this charge harder to sustain than it looks. A student who drinks coffee, sleeps eight hours and books a quiet room has an advantage too, and nobody calls it fraud. What makes doping in sport objectionable is not the desire for advantage but the breaking of an agreement that makes competition meaningful; in the classroom there is no such agreement. If we are worried about fairness among students, the answer is to examine the conditions under which they work, not to blame individuals for using what is available.

A deeper objection is that enhancements make achievements less valuable, because they are no longer fully our own. This is a common intuition, but it is hard to defend once it is examined. Nobody thinks a pianist's performance is diminished by years of practice with a metronome, or a mathematician's proof by the use of a calculator; we distinguish between tools that support effort and those that replace it, though the line is not drawn by the tools themselves.

The most serious concerns, however, are not about authenticity but about power. In 2018 a Chinese researcher who edited the genes of human embryos that were then carried to birth was condemned by scientists around the world, and he was later sentenced to imprisonment by a Chinese court. The outcry was not chiefly about tampering with nature. It was about the absence of any reliable evidence that the procedure was safe, and about the impossibility of obtaining consent from a person who did not yet exist.

Then there is inequality. If a pill or an implant can raise performance, it will go first to those who can pay for it, and the advantage will be passed on to their children. This matters less for trivial enhancements, such as a neighbour's coffee, than for expensive interventions that alter the course of a life. There is also a subtler pressure. Where some colleagues work late with the help of stimulants, others feel obliged to do the same, and a choice freely made by one person quietly removes the choice of another. That is the strongest argument for regulation, and it has nothing to do with what is natural.

My own position is therefore neither permissive nor prohibitive. I would allow competent adults to use enhancement technologies that have passed the ordinary tests of safety, and I would regulate the supply so that neither employers nor schools can compel their use. I would stop short of editing future generations until both the techniques and the systems of consent are far better than they now are. Above all, I would not let the debate about wonder drugs distract attention from the interventions we know work: decent schools, enough sleep, clean air and secure housing do more for mental performance, on the evidence available, than any drug now on the market.